Try Welltory
20 min read
5.0
7

Vaccine side effects: what's normal after shingles, flu, RSV, pneumonia, tetanus and MMR shots

Most reactions are a sore arm, tiredness or a low fever that pass in one to three days; here is what to expect after each vaccine, and the rare signs that need a doctor

Jane Smorodnikova
Founder & CEO
Tatsiana Yashyna
Deputy COO
A guide to vaccine side effects for adults and children, by vaccine: shingles (Shingrix, with trial rates of sore arm, aches and activity-limiting reactions and why the second dose can feel different), RSV (Arexvy, Abrysvo, mRESVIA, including the Guillain-Barré signal and RSV vaccination in pregnancy at 32–36 weeks), pneumococcal vaccines (Prevnar 20, Capvaxive, Pneumovax 23), tetanus (Td, Tdap and DTaP in children, including limb swelling), MMR (fever or rash 7–12 days later, febrile seizures, joint pain in adult women, and the evidence on autism), and a short section on flu shots. Covers how long side effects last, fever after vaccines in babies, why the arm hurts, what helps, anaphylaxis (about 1.3 per million doses) and when to call a doctor or 911, with US recommendations dated as of October 2026. Includes how tagging the shot day in Welltory lets adults look at their own sleep and resting heart rate afterward, citing wearable studies after COVID-19 and flu vaccines.

Short answer

Shingles vaccine side effects are common and can be strong: in the trials behind Shingrix, 78% of people had a sore arm and 45% had muscle aches or fatigue, and about one in six had a reaction bad enough to stop normal activities (2, 3). Across shingles, RSV, pneumonia, tetanus, MMR and flu shots, most reactions are a sore arm, tiredness, aches or a low fever that settle within one to three days.

MMR is the late one: fever or a rash can appear 7 to 12 days after the shot (26). Severe allergic reactions, the real emergency, usually start within minutes to hours and occurred about 1.3 times per million doses in a large US study (31).

If your first Shingrix dose flattened you, you're not imagining it, and dreading the second is not weakness.

Note: this article explains what guidelines and research say and is not medical advice; your doctor or pharmacist decides what fits you or your child. Call 911 for hives, swelling of the face or throat, trouble breathing, a fast heartbeat, dizziness or weakness after a shot, or for a seizure. Weakness or tingling that spreads up the legs or arms in the weeks after a vaccine needs care the same day. Any fever of 100.4°F or higher in a baby under 3 months means calling the pediatrician right away.

Vaccine side effects at a glance: normal, call a doctor, or call 911

Usually normal (1–3 days)Call your doctor or pediatricianCall 911
Sore, red or swollen arm or thighRedness or tenderness that grows after 24 hours, or arm pain and stiffness lasting more than a weekHives, swelling of face or throat, trouble breathing, fast heartbeat, dizziness, weakness
Tiredness, headache, muscle aches, chillsSymptoms not improving after a few days, or getting worseA seizure
Low fever; fussiness and sleepiness in babiesAny fever of 100.4°F or more under 3 months; fever above 104°F that keeps coming back; crying that won't stop for 3 hours or more
Fever or mild rash 7–12 days after MMRWhole arm or thigh swollen after a DTaP boosterWeakness that affects breathing or swallowing
Unusual bruising or bleeding in the weeks after MMR
Tingling or weakness in feet or hands that spreads, in the weeks after a shot (same day)

Why does my arm hurt after a vaccine?

Because your immune system is reacting where the vaccine went in. Side effects are "the physical manifestation of the inflammatory response to vaccination," in the words of a review by GSK vaccine scientists, and the adjuvant (an immune-boosting ingredient, as in Shingrix and Arexvy) is one factor in how strong they are (38).

Fast-onset pain that lasts for weeks with a stiff shoulder is different: SIRVA, shoulder injury related to vaccine administration, thought to result from vaccine reaching joint tissues instead of muscle (39). If you can't lift your arm normally a week after a shot, see your doctor.

How long do vaccine side effects last?

Usually one to three days, per the labels and CDC:

  • Shingrix (shingles) — Median 2–3 days (2)

  • RSV (Arexvy, Abrysvo, mRESVIA) — Median 1–2 days (8, 9, 10)

  • Pneumococcal — Usually gone within about 2 days (12)

  • Td, Tdap, DTaP (tetanus) — 1–3 days (23)

  • MMR — Fever starts 7–12 days after and lasts 1–2 days (26)

  • Flu — Within a few days (29)

Contact your doctor if symptoms are not improving or are getting worse (1).

Fever after vaccine: adults, children and babies

Common and usually short: 14% to 28% of adults had a fever after Shingrix, depending on age (2); 3% to 5% of children had 101°F or higher after DTaP (21); and 5% to 15% had 103°F or higher a week or more after MMR (26).

Baby fever after vaccines

First shots come at 2 months, inside the age when any fever needs a call. The AAP says to call right away if a baby under 3 months (12 weeks) has a temperature of 100.4°F or higher, if a child has a seizure, or if the temperature rises above 104°F repeatedly (37). A seizure means 911.

What helps after a shot, and what to skip

Cool the arm, keep it moving, and treat symptoms once they appear. CDC's after-vaccination handout (written for COVID-19 vaccines) suggests a cool, wet washcloth on the area, using the arm, and fluids and light clothing for fever (34).

  • Pain relievers: after, not before. CDC's immunization guidelines say evidence does not support fever reducers "before or at the time of vaccination," though they can be used to treat fever and discomfort afterward (33). In infant trials, acetaminophen given at vaccination reduced fever but also lowered antibody responses to several vaccine components (3561208-3)). Ask your doctor or pharmacist what suits you or your child.

  • Never aspirin for children unless their own clinician says so, because of the risk of Reye syndrome (36).

  • For babies at the visit, CDC lists distraction, cooling the site, sweet liquids, breastfeeding and swaddling (33).

  • Sit for the shot if you've ever fainted; 80% of reported fainting happens within 15 minutes (32).

Shingles vaccine side effects (Shingrix)

Most people get a sore arm after Shingrix, and many feel unwell for a few days; CDC says side effects "may temporarily prevent a vaccinated person from doing regular activities" (1).

Who gets it. CDC recommends two doses, 2 to 6 months apart, for adults 50 and older and adults 19 and older with a weakened immune system, even after past shingles; wait if you have shingles now or are pregnant (1). It is not a children's vaccine.

Shingrix side effects by age

The manufacturer-funded ZOE-50 and ZOE-70 trials recorded reactions for 7 days after each dose (2):

ReactionAges 50–59Ages 60–69Ages 70+Placebo (range)
Arm pain88%83%69%9–14%
Pain that stops normal activities10%7%4%0.2–1%
Muscle aches57%49%35%10–15%
Fatigue57%46%37%14–20%
Headache51%40%29%12–22%
Shivering36%30%20%5–7%
Fever28%24%14%3%

In ZOE-50, activity-limiting (grade 3) symptoms were reported by 17.0% of vaccine and 3.2% of placebo recipients (3). Serious adverse events and deaths were similar in both groups in both trials (3, 4).

How soon do side effects start after Shingrix?

Within the first week: the trials looked for reactions on the day of the shot and the six days after, and most recipients reported them in that window (2).

Shingles vaccine side effects: how long do they last?

Usually 2 to 3 days, the median in the trials (2, 1).

Can Shingrix side effects last longer than a week?

Sometimes, mainly in the arm: post-approval reports include reduced movement of the injected arm "which may persist for 1 or more weeks" and a hardened area at the site (2). Fever or aches lasting more than a week are not typical, nor is a stiff, painful shoulder that doesn't ease; call your doctor (39).

How soon after the 2nd shingles vaccine do side effects start, and why can it feel worse?

On the same timeline as the first. In the trials, arm reactions were no more common after dose 2, but headache (28% vs 24%) and shivering (21% vs 14%) were, and activity-limiting headache, shivering, aches and fatigue rose but stayed at 4% or less each (2). After dose 1 your immune system has already met the vaccine, and pre-existing immunity is one factor that shapes reactions (38). CDC says to get the second dose "even if you had one of these reactions after the first dose" (1).

Rare: Guillain-Barré syndrome after Shingrix

Guillain-Barré syndrome (GBS) is a rare nerve disorder causing weakness. Two Medicare studies of adults 65 and older estimated 3 and 7 extra cases per million doses in the 42 days after Shingrix (6 and 12 after the first dose, none after the second); the label, updated February 2026, says the results "suggest a causal association" (2). CDC notes a very small increased risk of GBS after shingles itself as well (1).

Red flag: tingling in the feet or hands followed by weakness moving up the body over hours or days needs same-day care; trouble breathing or swallowing means 911 (40).

RSV vaccine side effects

A sore arm, fatigue, headache and muscle or joint aches, usually mild (5); rates depend on the product.

Who gets it. As of October 2026, CDC recommends one dose for everyone 75 and older and for adults 50 to 74 at increased risk, such as chronic heart or lung disease or a weakened immune system; it's not yearly, and CDC has no preference among the three vaccines (5). The thresholds moved in 2024 (75+, and at-risk 60–74) (50) and again on June 25, 2025, when the at-risk group was extended to 50–59 (11).

  • Arexvy (adjuvanted) — Arm pain 60.9%, fatigue 33.6%, muscle aches 28.9%, headache 27.2%, joint pain 18.1% (8)

  • Abrysvo — Fatigue 15.7%, headache 12.9%, arm pain 10.6%, muscle aches 10.2% (9)

  • mRESVIA (mRNA) — Arm pain 55.9%, fatigue 30.8%, headache 26.7%, muscle aches 25.6%, joint pain 21.7%, underarm swelling or tenderness 15.2% (10)

How long do RSV vaccine side effects last?

One to two days at the median for all three vaccines (8, 9, 10).

The GBS question

In January 2025, FDA added a Guillain-Barré warning to Arexvy and Abrysvo after a Medicare study estimated 7 and 9 extra GBS cases per million doses within 42 days in adults 65 and older; FDA called the evidence "insufficient to establish a causal relationship" and said benefits outweigh risks (7). CDC reached the same benefit judgment (6). Same red flags: spreading tingling or weakness means same-day care; trouble breathing means 911.

RSV vaccine in pregnancy and the preterm-birth question

Abrysvo is the only RSV vaccine for pregnancy, given at 32 through 36 weeks, September through January in most of the US (11, 6). In pregnant women, the most common reactions were arm pain (40.6%), headache (31.0%), muscle pain (26.5%) and nausea (20.0%) (9).

The open question is preterm birth: in the main trial, 5.7% of babies in the vaccine group and 4.7% with placebo were born early, a difference that was not statistically significant, and FDA limited use to 32 weeks and later to avoid a potential earlier risk (9). A preliminary US study of the first season found no increased preterm-birth risk at 32 to 36 weeks; studies of preterm birth and high blood pressure in pregnancy continue (6). Your obstetric clinician decides what fits you.

Babies get an antibody, not a vaccine

Infants are protected by the mother's vaccine or by a monoclonal antibody such as nirsevimab given to the baby; most don't need both. Nirsevimab's trials found no safety concerns, and serious allergic reactions have been reported since approval (6).

Pneumonia vaccine side effects

Pneumococcal ("pneumonia") vaccines usually cause a sore, red or swollen arm, tiredness, headache, muscle aches and sometimes fever or chills (12).

Who gets it. CDC recommends it for all children under 5, all adults 50 and older, and people 5 to 49 with certain risk conditions (12). The adult age dropped from 65 to 50 after an October 23, 2024 ACIP vote; adults get a conjugate vaccine, PCV20 or PCV21 alone, or PCV15 followed by PPSV23 (14).

What are the side effects of pneumonia vaccine? Prevnar 20 and Capvaxive

  • Prevnar 20 (PCV20), adults 60+: arm pain in over 50%, muscle pain and fatigue in over 30%, headache in over 20%; younger adults reported more (15).

  • Capvaxive (PCV21), adults 50+: arm pain 41.2%, fatigue 19.7%, headache 11.0% (16).

  • Pneumovax 23 (PPSV23), first dose: arm pain or soreness 60.0%, swelling 20.3%, headache 17.6%, redness 16.4% (17). Repeat doses react more: in adults 65 and older, whole-body reactions were 33.1% after revaccination versus 21.7% after a first dose (17).

Pneumonia vaccine side effects: how long do they last?

Usually 1 or 2 days (12, 13).

In babies and toddlers

Children get four doses of conjugate vaccine at 2, 4, 6 and 12 to 15 months (15). After Prevnar 20 in infants, fussiness (over 60%), injection-site pain and drowsiness (over 30%), reduced appetite and redness (over 20%), and fever (over 10%) were most common (15). A 2012 study found a small extra febrile-seizure risk when the older PCV13 and a flu shot were given at one visit (13); ask your pediatrician if that worries you.

Tetanus shot side effects

Mostly the arm: pain in 66% to 75% of Tdap recipients, redness in 25%, swelling in 21%, and fever of 100.4°F or more in 1.4% to 5% (20). Headache, tiredness and an upset stomach also occur (18).

Who gets it. Children get five doses of DTaP at 2, 4 and 6 months, 15 to 18 months and 4 to 6 years (19). Adolescents get Tdap at 11 or 12; pregnant women get Tdap in every pregnancy, preferably early in the third trimester; adults get a Td or Tdap booster every 10 years, or after 5 for a severe or dirty wound (18).

Tdap side effects, and how long tetanus shot side effects last in adults

Usually 1 to 3 days (23). CDC's safety reviews found no unexpected concerns, including in pregnant women and adults over 65, and no association with GBS (23).

What to avoid after a tetanus injection

No guideline lists foods or activities to avoid; what to avoid is tetanus shots too often. A rare exaggerated local swelling, an Arthus-type reaction, can follow tetanus- or diphtheria-containing vaccines; after one, CDC advises waiting at least 10 years since the last tetanus-containing dose (20). If you're treated for a cut, say when your last tetanus shot was.

DTaP side effects in children

Soreness, swelling, fever, fussiness, tiredness and reduced appetite (19). Local reactions follow 20% to 40% of each of the first three doses and are more frequent after the 4th and 5th (21). Swelling of the entire upper arm or thigh after a booster has been estimated at 2% to 6%; it subsides without lasting harm but can be mistaken for infection (2200033-5)). Fever of 105°F or more, seizures and crying for 3 hours or longer generally occur in fewer than 1 in 10,000 doses (21). Call the pediatrician for crying that won't stop; a seizure means 911.

MMR vaccine side effects

A sore arm, fever and a mild rash, with the fever and rash arriving later than with other shots (24).

Who gets it. Children get two doses, at 12 to 15 months and 4 to 6 years; teens and adults who aren't immune need one or two doses (24).

MMR side effects in children: the delayed fever

After MMR, 5% to 15% of people who weren't already immune develop a fever of 103°F or higher, usually 7 to 12 days after the shot and lasting 1 or 2 days; about 5% get a rash 7 to 10 days after (26).

Febrile seizures occur in about 1 in 3,000 to 4,000 children 6 to 14 days after MMR and are not associated with long-term effects (25). For a first dose at 12 to 23 months, the combined MMRV vaccine (with chickenpox) about doubles that risk, so CDC's guidance favors separate MMR and varicella shots unless parents prefer MMRV (26). A seizure means 911.

MMR vaccine side effects in adults

Joint pain or stiffness occurs in up to 1 in 4 women past puberty who weren't already immune to rubella; it usually begins 1 to 3 weeks after vaccination, is mild and lasts about 2 days (25). It's linked to the rubella component.

Rare MMR reactions

A temporary low platelet count (immune thrombocytopenic purpura), estimated at 1 case per 40,000 vaccinated children within six weeks (25); call your doctor for unusual bruising or bleeding. MMR is a live vaccine, so people with serious immune problems should not get it (24).

MMR and autism: what the evidence shows

A Danish study of 657,461 children found no increased autism risk after MMR, including in children with an autistic sibling (27). CDC's autism page, revised July 22, 2026, questions the evidence on other infant vaccines but states that reviews by the Institute of Medicine and the Agency for Healthcare Research and Quality find, with "high strength of evidence," no association between MMR and autism, while criticizing those observational studies' methods (28).

Flu shot side effects (briefly)

Flu shot side effects are usually a sore arm, sometimes headache, fever, nausea or muscle aches, generally gone within a few days (29):

  • Severe allergic reactions are rare and most likely within minutes to hours (29).

  • If flu shots raise GBS risk, it's on the order of one to two extra cases per million doses (30).

  • "Flu vaccines cannot cause flu," CDC states; feeling achy for a day is the immune response (29).

For timing and "does the flu shot make you sick," see when to get a flu shot; for children's reactions, see the flu shot for kids.

Rare serious reactions, in real numbers

EventEstimated rateSource
Anaphylaxis, all vaccines1.31 per million dosesUS Vaccine Safety Datalink (31)
GBS after Shingrix, 65+3 to 7 extra per million dosesFDA label (2)
GBS after Arexvy / Abrysvo, 65+7 / 9 extra per million dosesFDA (7)
GBS after flu shot1 to 2 extra per million, if anyCDC (30)
Febrile seizure after MMRabout 1 in 3,000–4,000 childrenCDC (25)
Low platelets after MMRabout 1 in 40,000 childrenCDC (25)

For scale, GBS strikes about 1 to 2 in 100,000 Americans a year, mostly unrelated to vaccines (30).

Anyone can report a reaction to VAERS, the federal Vaccine Adverse Event Reporting System, online or at 1-800-822-7967 (1). A report doesn't prove the vaccine caused the problem; it helps spot patterns (32).

Who recommends what in October 2026

As of October 11, 2026, CDC's schedules in effect are the July 2, 2025 versions (43): on March 16, 2026 a federal court stayed a January 2026 childhood schedule, the appointments of 13 ACIP members and their 2025 votes, and CDC restored the July 2025 schedules (44, 45); the appeals court heard the government's appeal on October 6, 2026 and had not ruled as of October 10 (46, 51). The AAP published its own 2026 childhood schedule in January (49). In September 2026, four medical societies issued aligned guidance: for RSV, the AAFP matches CDC's adult ages, the IDSA adds immunocompromised adults 18 and older, and the AAP recommends immunizing infants under 8 months unless maternal vaccination is documented (47). An August 10, 2026 executive order asked HHS for plans to split MMR into three shots (52); CIDRAP notes that CDC sets federal recommendations and that single measles, mumps and rubella vaccines are not licensed in the US (48). Your doctor or pharmacist decides what fits you.

Vaccine side effects: the questions people ask next

Can I exercise after a vaccine?

If you feel fine, light activity is reasonable; CDC even suggests using the arm to ease soreness (34). With fever or body aches, skip hard training until they pass; working out while sick explains how to judge it. Chest pain, fainting or severe breathlessness means 911.

Should I take a day off work after Shingrix?

Consider planning for one. Pain that stopped normal activities hit 10% of trial participants aged 50 to 59 and 4% of those 70 and older, and activity-stopping fatigue or aches up to 9% (2). Older adults reacted less, yet protection stayed high: 91% at 70 and older versus 97% at 50 to 69 (1), so feeling fine doesn't mean the shot failed.

Can I get shingles and RSV or flu vaccines at the same visit?

Usually yes; CDC says Shingrix and RSV vaccines can be given with other adult vaccines, at different injection sites (1, 6). Fewer visits can mean fewer missed doses. If you'd rather have one rough day at a time, or reacted strongly to a past dose, ask your pharmacist about spacing them; they decide what fits your schedule.

Why is my resting heart rate higher after a vaccine?

A brief rise is common after reactogenic shots. In a smartwatch study of 5,674 people after mRNA COVID-19 vaccines, resting heart rate rose from each person's baseline, peaked on day 2 and was back to normal by day 6, more after dose 2 (41). After flu shots, a Garmin study found no statistically significant change (42). We found no wearable data for Shingrix. If it stays up for days, or comes with palpitations, dizziness or breathlessness, call your doctor; high resting heart rate causes covers other reasons it climbs.

I had a strong reaction last time. Should I skip the next dose?

Not on your own. CDC says to get the second Shingrix dose even after a reaction to the first, but a severe allergic reaction to a previous dose is a reason not to repeat that vaccine (1, 12). An Arthus-type reaction after a tetanus shot means waiting 10 years before the next one (20). Tell your vaccinator what happened; they decide.

When to seek care after a vaccine

Call 911 for signs of a severe allergic reaction, even after you've left the clinic: hives, swelling of the face and throat, difficulty breathing, a fast heartbeat, dizziness or weakness (24). Also call 911 for a seizure, or for weakness that affects breathing or swallowing (40).

Get care the same day for tingling or weakness in the feet or hands that spreads in the weeks after a vaccine (40); for a baby under 3 months with a temperature of 100.4°F or higher; for a fever above 104°F that keeps coming back; or for a child with a fever who looks very ill, is unusually drowsy or very fussy (37).

Call your doctor or pediatrician if redness or tenderness grows after 24 hours or side effects don't go away after a few days (34), for crying lasting 3 hours or more after DTaP (19), unusual bruising after MMR, or a stiff, painful shoulder after a week.

What to say. "I had [symptom] starting [day] after my [vaccine], lasting [days]." Mention past severe allergies, fainting with shots, GBS, an Arthus-type reaction, pregnancy, a weakened immune system and your last tetanus shot.

What to ask. "Is this a reason to change or delay my next dose?" and "Should I report this to VAERS, or will you?"

Start understanding your body

Learn what affects your energy, stress, sleep, and daily state. Get the app.

Explore your data

How Welltory helps — and what it cannot do

The limits first. Welltory is a general wellness product, not a medical device. It does not diagnose, detect, predict, monitor, prevent, treat or mitigate any condition, including vaccine reactions or Guillain-Barré syndrome. It can't tell you whether a symptom is a normal side effect or whether a vaccine worked, and it is not for tracking children.

You note the shot; the app records the rest. Add a tag from the plus sign at the top of the feed ("shingles shot dose 1", "sore arm", "chills"). When the Today screen marks a stress or rest stretch, it asks "What happened?"; tap a suggested tag, type or just talk, and Welltory picks out the tags, which you can correct before saving.

Look at the next nights against your own history. Sleep analysis shows how your heart rate changed overnight and whether it ran higher than usual; resting heart rate, HRV, stress minutes and Battery are recorded the same way every day (Battery and stress minutes need iOS with an Apple Watch or Oura; detailed sleep analysis needs an Apple Watch). Some people see a change: after mRNA COVID-19 vaccines, resting heart rate rose for a few days and people slept longer after dose 2 (41); after flu shots, a Garmin study found no significant change (42); for Shingrix we found no data. Your numbers sit next to your own history, not a population range and not a line marked normal.

Over a season, My Patterns shows what your tags have in common. My Patterns collects tags from stress and rest stretches (iOS with an Apple Watch or Oura); patterns appear at around seven tagged events in a month, so two shots alone won't make one.

A record you made, for an appointment. The Journal (Premium) keeps your measurements, tags and notes; from the web app you can export a CSV (Dashboard → choose a chart → Export). A dated "Shingrix dose 1, chills for two days" helps when you're asked before dose 2 how the first went. Any pattern is an association to discuss, not a cause.

The same red flags apply here, in the same type size: hives, swelling of the face or throat, trouble breathing, a fast heartbeat or a seizure after a shot means 911; tingling or weakness that spreads in the weeks after a vaccine needs care the same day (24, 40).

Where to find other people in the same situation. If you are also living with an energy-limiting condition, Welltory runs a paid, moderated community called Energy Lab for women aged 18 to 65 living with ME/CFS, long COVID, fibromyalgia, POTS, MCAS and similar. It runs alongside the app: you keep collecting your own data, and the Lab is where people learn to read it together, with a medical board answering the science. It is education and peer support, not medical care, it does not replace your clinician, and there is a 14-day money-back guarantee on a first purchase.

How we made it

Made with AI tools, then edited and fact-checked by the Welltory team. See our Editorial & AI policy.

Data analysis by Jane Smorodnikova, co-founder of Welltory and the person who built the methodology behind how we read physiological data.

Written by Tatsiana Yashyna.

Discounts for blog readers: up to 36% off

See what affects your energy, stress, sleep, and daily state with Welltory

Explore your data

This article is for educational purposes only and is not medical advice; your doctor or pharmacist decides what fits you or your child. Call 911 for hives, swelling of the face or throat, trouble breathing, a fast heartbeat, dizziness or a seizure after a shot. Weakness or tingling that spreads in the weeks after a vaccine needs care the same day, and any fever of 100.4°F or higher in a baby under 3 months means calling the pediatrician right away. Welltory is a general wellness product and does not diagnose, detect or predict any condition.

Was this helpful?

Ask AI for a summary of page

ChatGPTGeminiClaudePerplexityGrok

Written by Jane Smorodnikova

The founder and CEO of Welltory. A recognized tech leader with two Master's degrees and experience at MIT, she has scaled Welltory to over 17 million users.

LINKEDIN

Written by Tatsiana Yashyna

Deputy COO at Welltory. With a background in medicine and years of working with health data, she translates research and real physiological signals — sleep, stress, heart rate, and hormones — into clear, evidence-based explanations that help people understand what their bodies are telling them.

Vaccines

References

  1. Centers for Disease Control and Prevention. Shingles vaccination. August 19, 2025. https://www.cdc.gov/shingles/vaccines/index.html
  2. GlaxoSmithKline. SHINGRIX (zoster vaccine recombinant, adjuvanted): prescribing information. Revised February 2026. https://www.fda.gov/media/108597/download
  3. Lal H, Cunningham AL, Godeaux O, Chlibek R, Diez-Domingo J, Hwang SJ, et al. Efficacy of an adjuvanted herpes zoster subunit vaccine in older adults. New England Journal of Medicine 2015;372(22):2087–2096. https://doi.org/10.1056/NEJMoa1501184
  4. Cunningham AL, Lal H, Kovac M, Chlibek R, Hwang SJ, Díez-Domingo J, et al. Efficacy of the herpes zoster subunit vaccine in adults 70 years of age or older. New England Journal of Medicine 2016;375(11):1019–1032. https://doi.org/10.1056/NEJMoa1603800
  5. Centers for Disease Control and Prevention. RSV vaccine guidance for adults. February 20, 2026. https://www.cdc.gov/rsv/vaccines/adults.html
  6. Centers for Disease Control and Prevention. Respiratory syncytial virus (RSV) vaccine safety. July 11, 2025. https://www.cdc.gov/vaccine-safety/vaccines/rsv.html
  7. US Food and Drug Administration. FDA requires Guillain-Barré syndrome (GBS) warning in the prescribing information for RSV vaccines Abrysvo and Arexvy. January 7, 2025. https://www.fda.gov/vaccines-blood-biologics/safety-availability-biologics/fda-requires-guillain-barre-syndrome-gbs-warning-prescribing-information-rsv-vaccines-abrysvo-and
  8. GlaxoSmithKline. AREXVY (respiratory syncytial virus vaccine, adjuvanted): prescribing information. Revised July 2026. https://www.fda.gov/media/167805/download
  9. Pfizer. ABRYSVO (respiratory syncytial virus vaccine): prescribing information. Revised December 2025. https://www.fda.gov/media/168889/download
  10. Moderna. MRESVIA (respiratory syncytial virus vaccine): prescribing information. Revised August 2026. https://www.fda.gov/media/179005/download
  11. Centers for Disease Control and Prevention. RSV (respiratory syncytial virus) vaccine: what you need to know (VIS). Page updated February 28, 2025. https://www.cdc.gov/vaccines/hcp/current-vis/rsv.html
  12. Centers for Disease Control and Prevention. Pneumococcal vaccination. February 25, 2026. https://www.cdc.gov/pneumococcal/vaccines/index.html
  13. Centers for Disease Control and Prevention. Pneumococcal vaccine safety. December 20, 2024. https://www.cdc.gov/vaccine-safety/vaccines/pneumococcal.html
  14. Kobayashi M, Leidner AJ, Gierke R, Xing W, Accorsi E, Moro P, et al. Expanded recommendations for use of pneumococcal conjugate vaccines among adults aged ≥50 years: recommendations of the Advisory Committee on Immunization Practices — United States, 2024. MMWR 2025;74(1):1–8. https://doi.org/10.15585/mmwr.mm7401a1
  15. Pfizer. PREVNAR 20 (pneumococcal 20-valent conjugate vaccine): prescribing information. Accessed October 11, 2026. https://www.fda.gov/media/149987/download
  16. Merck Sharp & Dohme. CAPVAXIVE (pneumococcal 21-valent conjugate vaccine): prescribing information. Revised June 2026. https://www.merck.com/product/usa/pi_circulars/c/capvaxive/capvaxive_pi.pdf
  17. Merck Sharp & Dohme. PNEUMOVAX 23 (pneumococcal vaccine polyvalent): prescribing information. Revised April 2023. https://www.merck.com/product/usa/pi_circulars/p/pneumovax_23/pneumovax_pi.pdf
  18. Centers for Disease Control and Prevention. Tdap (tetanus, diphtheria, pertussis) vaccine: what you need to know (VIS). Page updated February 28, 2025. https://www.cdc.gov/vaccines/hcp/current-vis/tdap.html
  19. Centers for Disease Control and Prevention. DTaP (diphtheria, tetanus, pertussis) vaccine: what you need to know (VIS). August 6, 2021. https://www.cdc.gov/vaccines/hcp/current-vis/dtap.html
  20. Centers for Disease Control and Prevention. Epidemiology and prevention of vaccine-preventable diseases (Pink Book): tetanus. Updated April 25, 2024. https://www.cdc.gov/pinkbook/hcp/table-of-contents/chapter-21-tetanus.html
  21. Centers for Disease Control and Prevention. Epidemiology and prevention of vaccine-preventable diseases (Pink Book): pertussis. Updated April 12, 2024. https://www.cdc.gov/pinkbook/hcp/table-of-contents/chapter-16-pertussis.html
  22. Rennels MB. Extensive swelling reactions occurring after booster doses of diphtheria-tetanus-acellular pertussis vaccines. Seminars in Pediatric Infectious Diseases 2003;14(3):196–198. https://doi.org/10.1016/s1045-1870(03)00033-5
  23. Centers for Disease Control and Prevention. Diphtheria, tetanus, and whooping cough vaccine safety. January 31, 2025. https://www.cdc.gov/vaccine-safety/vaccines/dtap-tdap.html
  24. Centers for Disease Control and Prevention. MMR (measles, mumps, and rubella) vaccine: what you need to know (VIS). Page updated February 28, 2025. https://www.cdc.gov/vaccines/hcp/current-vis/mmr.html
  25. Centers for Disease Control and Prevention. Measles, mumps, rubella (MMR) vaccine safety. July 31, 2024. https://www.cdc.gov/vaccine-safety/vaccines/mmr.html
  26. Centers for Disease Control and Prevention. Epidemiology and prevention of vaccine-preventable diseases (Pink Book): measles. Updated April 11, 2024. https://www.cdc.gov/pinkbook/hcp/table-of-contents/chapter-13-measles.html
  27. Hviid A, Hansen JV, Frisch M, Melbye M. Measles, mumps, rubella vaccination and autism: a nationwide cohort study. Annals of Internal Medicine 2019;170(8):513–520. https://doi.org/10.7326/M18-2101
  28. Centers for Disease Control and Prevention. Autism and vaccines. Updated July 22, 2026. https://www.cdc.gov/vaccine-safety/about/autism.html
  29. Centers for Disease Control and Prevention. Flu vaccine safety information. August 25, 2023. https://www.cdc.gov/flu/vaccine-safety/index.html
  30. Centers for Disease Control and Prevention. Guillain-Barré syndrome and flu vaccine. September 17, 2024. https://www.cdc.gov/flu/vaccine-safety/guillainbarre.html
  31. McNeil MM, Weintraub ES, Duffy J, Sukumaran L, Jacobsen SJ, Klein NP, et al. Risk of anaphylaxis after vaccination in children and adults. Journal of Allergy and Clinical Immunology 2016;137(3):868–878. https://doi.org/10.1016/j.jaci.2015.07.048
  32. Centers for Disease Control and Prevention. General best practice guidelines for immunization: preventing and managing adverse reactions. July 25, 2024. https://www.cdc.gov/vaccines/hcp/imz-best-practices/preventing-managing-adverse-reactions.html
  33. Centers for Disease Control and Prevention. General best practice guidelines for immunization: vaccine administration. June 18, 2024. https://www.cdc.gov/vaccines/hcp/imz-best-practices/vaccine-administration.html
  34. Centers for Disease Control and Prevention. What to expect after getting a COVID-19 vaccine (CS 321466-A). December 13, 2020. https://stacks.cdc.gov/view/cdc/98825/cdc_98825_DS1.pdf
  35. Prymula R, Siegrist CA, Chlibek R, Zemlickova H, Vackova M, Smetana J, et al. Effect of prophylactic paracetamol administration at time of vaccination on febrile reactions and antibody responses in children: two open-label, randomised controlled trials. The Lancet 2009;374(9698):1339–1350. https://doi.org/10.1016/S0140-6736(09)61208-3
  36. MedlinePlus (US National Library of Medicine). Reye syndrome. Reviewed July 16, 2024. https://medlineplus.gov/ency/article/001565.htm
  37. American Academy of Pediatrics, HealthyChildren.org. Fever: when to call the pediatrician. Updated December 2, 2022. https://www.healthychildren.org/English/health-issues/conditions/fever/Pages/When-to-Call-the-Pediatrician.aspx
  38. Hervé C, Laupèze B, Del Giudice G, Didierlaurent AM, Tavares Da Silva F. The how's and what's of vaccine reactogenicity. npj Vaccines 2019;4:39. https://doi.org/10.1038/s41541-019-0132-6
  39. Atanasoff S, Ryan T, Lightfoot R, Johann-Liang R. Shoulder injury related to vaccine administration (SIRVA). Vaccine 2010;28(51):8049–8052. https://doi.org/10.1016/j.vaccine.2010.10.005
  40. National Institute of Neurological Disorders and Stroke. Guillain-Barré syndrome. Last reviewed March 13, 2026. https://www.ninds.nih.gov/health-information/disorders/guillain-barre-syndrome
  41. Quer G, Gadaleta M, Radin JM, Andersen KG, Baca-Motes K, Ramos E, et al. Inter-individual variation in objective measure of reactogenicity following COVID-19 vaccination via smartwatches and fitness bands. npj Digital Medicine 2022;5:49. https://doi.org/10.1038/s41746-022-00591-z
  42. Yechezkel M, Qian G, Levi Y, Davidovitch N, Shmueli E, Yamin D, Brandeau ML. Comparison of physiological and clinical reactions to COVID-19 and influenza vaccination. Communications Medicine 2024;4:169. https://doi.org/10.1038/s43856-024-00588-7
  43. Centers for Disease Control and Prevention. Adult immunization schedule by age (addendum updated July 2, 2025). Accessed October 11, 2026. https://www.cdc.gov/vaccines/hcp/imz-schedules/adult-age.html
  44. Congressional Research Service. The 2026 childhood immunization schedule. CRS Report R48982. June 11, 2026. https://www.everycrsreport.com/reports/R48982.html
  45. Immunize.org. CDC restores official U.S. immunization schedules to July 2, 2025, versions to comply with March 16 court order. IZ Express, issue 1871, April 29, 2026. https://www.immunize.org/news/iz-express/issue/1871/
  46. Georgetown Law Litigation Tracker. American Academy of Pediatrics et al. v. Kennedy et al. Updated October 7, 2026. https://litigationtracker.law.georgetown.edu/litigation/american-academy-of-pediatrics-et-al-v-kennedy-et-al/
  47. American Medical Association. Doctors united on respiratory virus vaccine recommendations. September 2, 2026. https://www.ama-assn.org/public-health/prevention-wellness/doctors-united-respiratory-virus-vaccine-recommendations
  48. CIDRAP. Trump signs executive order to break up measles, mumps, rubella vaccine. August 10, 2026. https://www.cidrap.umn.edu/childhood-vaccines/trump-signs-executive-order-break-measles-mumps-rubella-vaccine
  49. Contemporary Pediatrics. AAP publishes 2026 childhood and adolescent immunization schedule. January 26, 2026. https://www.contemporarypediatrics.com/view/aap-publishes-2026-childhood-and-adolescent-immunization-schedule
  50. Britton A, Roper LE, Kotton CN, Hutton DW, Fleming-Dutra KE, Godfrey M, et al. Use of respiratory syncytial virus vaccines in adults aged ≥60 years: updated recommendations of the Advisory Committee on Immunization Practices — United States, 2024. MMWR 2024;73(32):696–702. https://doi.org/10.15585/mmwr.mm7332e1
  51. Courthouse News Service. First Circuit urged to unblock RFK Jr.'s vaccine policy makeover. October 6, 2026. https://www.courthousenews.com/first-circuit-urged-to-unblock-rjk-jr-maha-vaccine-policy-makeover/
  52. Scientific American. Trump issues executive order aimed at rewriting childhood vaccine schedule. August 10, 2026. https://www.scientificamerican.com/article/trump-signs-executive-order-aimed-at-rewriting-childhood-vaccine-schedule/

FAQ

When to get a flu shot in 2026: the best time, side effects, and myths vs facts

September or October is the sweet spot for most adults, a later shot still helps while flu circulates, and the shot cannot give you the flu

Vaccines

Flu shot for kids: a 2026–27 guide by age, from babies to teens

What the AAP and CDC advise for every age from babies to teens, one dose or two, shot or nasal spray, and what the research says about the common worries

Vaccines