Fever after a vaccine: wait it out or see a doctor? Child and adult

Hello. I am Shunichi Gamo, MD, PhD, a board-certified pulmonologist and the director of Sendai Airport North Clinic.

Autumn is the season when more people get the influenza vaccine and the COVID-19 vaccine. Children’s routine vaccinations also continue all year round. At this time of year, a question we often hear at the fever clinic goes like this.

“I got a fever the night I had the vaccine. Is it a side effect, or is it some other illness?”

Let me start with the conclusion: most fevers after a vaccination start soon after the shot and go down within a few days (usually 2 to 3 days with the influenza vaccine[1]). However, if the fever lasts, there are symptoms that seem unrelated to the shot, or the person is limp, think of other illnesses and see a doctor. Some vaccines, such as the MR vaccine (measles-rubella combined vaccine), tend to cause a fever around 5 to 14 days after the shot[2].

This article explains how likely each vaccine is to cause a fever, how to tell a side effect from another illness, and when to see a doctor or call 119. It also covers how to use fever reducers and the support systems for the rare case when something goes wrong, summarized in plain terms for children and adults alike.

At our fever clinic, we give you the result of the PCR test (a test that copies the virus’s genetic material to detect it) on the same day.

Since October 4, 2026, our fever clinic is also open on Sundays and public holidays
10:00 to 15:00 (no lunch break). No appointment is needed; you are seen by checking in on the day. We give you the PCR test result on the same day. On Sundays and public holidays we run the fever clinic only, with no general outpatient care or IV drips. We are closed on Sunday, October 25, and from November we are in principle open every Sunday and public holiday. Announcement about the Sunday and public holiday fever clinic

Conclusion: judge a fever after vaccination by “when, how many days, and other symptoms”

If you get a fever after a vaccination, just check these 3 things.

If all 3 look “as usual,” watching at home is often fine. The dividing lines are summarized below.

What to do	Guide
Call 119	After the shot: trouble breathing or wheezing, swelling of the lips or face, hives all over the body with limpness, limpness with an odd response when called, or a seizure lasting 5 minutes or more
Go right away, even at night or on a holiday	Under 3 months old with a temperature of 38°C (100.4°F) or higher, cannot keep any fluids down, a first seizure, chest pain or trouble breathing, or looks miserable with a high fever 5 to 14 days after the MR vaccine
Go during clinic hours	The fever lasts (with the influenza vaccine, it has not gone down after 2 to 3 days), a fever that had gone down rises again, or there are symptoms besides fever such as cough, sore throat, or diarrhea
Fine to watch	A fever that began soon after the shot, with the person drinking fluids and mood and responsiveness as usual. Only pain at the injection site or tiredness

*For details on the signs, see the section below on when to see a doctor right away or call 119.

How likely is a fever with each vaccine? Data from Japan and abroad

How likely a fever is differs quite a lot by vaccine type, age, and which dose it is. Studies also differ in how they define fever (37.5°C or 38°C and above) and how many days they watched, so please treat the numbers only as a rough guide.

Influenza vaccine: fever and similar reactions in 5 to 10%, usually gone in 2 to 3 days

According to the Ministry of Health, Labour and Welfare, with the seasonal influenza vaccine, redness, swelling, or pain at the injection site occurs in 10 to 20% of people vaccinated[1]. Whole-body reactions such as fever, headache, chills, and tiredness occur in 5 to 10%, and both are said to disappear in usually 2 to 3 days[1].

This is often misunderstood, but an injected inactivated vaccine (one in which the virus has been made unable to function as a virus) cannot cause influenza. On the other hand, the live attenuated (weakened) vaccine given as a nasal spray can cause fever and other reactions like those of having influenza[1].

Put the other way, if cough or sore throat appears along with fever after an injected vaccine, consider that the person may have caught real influenza or COVID-19. During an epidemic, the day of the shot and the day of infection can happen to overlap.

COVID-19 vaccine: fever was more common after the second dose

There is a 2021 survey of staff at a university hospital in Tokyo who received two doses of the COVID-19 vaccine (Pfizer’s mRNA vaccine). An mRNA (messenger RNA) vaccine is a type that puts a piece of the virus’s blueprint into the body to build immunity. Using questionnaires covering the day of the shot through day 8, a fever of 37.5°C or higher occurred in 2.9% of 3,254 people after the first dose[3]. After the second dose, however, it was 42.8% of 3,165 people, and 38°C or higher was seen in 597 people (about 19%)[3].

After the second dose, 19.3% of people took a day or more off work[3]. The number of doses and the target groups differ from today’s vaccinations, so the numbers do not apply as they are, but it is reassuring to be prepared for the possibility of staying home with a fever the next day.

Note that serious side effects listed for mRNA vaccines include myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the membrane around the heart), although how often they occur is not known[1]. If you have chest pain, trouble breathing, or palpitations after the shot, see a doctor early, even without a fever.

Children getting several vaccines at once (pneumococcal, 5-in-1, and others): fever is common

In infancy, children often get the pneumococcal vaccine and the 5-in-1 vaccine (diphtheria, pertussis, tetanus, polio, Hib) on the same day.

In a study at 10 centers in the Czech Republic, 459 healthy infants received a pneumococcal vaccine, a 6-in-1 vaccine, and a rotavirus vaccine together. In the group that did not receive a preventive fever reducer, 66% (154 of 233) had a temperature of 38°C or higher after at least one of the first series of shots[4]. On the other hand, a temperature above 39.5°C was rare, at 1% (3 of 233)[4].

In a US study of 221 children aged about 1 year to 1 year 4 months, a temperature of 38°C or higher on days 1 to 2 after the pneumococcal and DTaP (diphtheria-tetanus-pertussis) vaccines occurred in about 8 to 9%[5]. The rate was about the same whether the influenza vaccine was given on the same day or on a different day[5]. In that study, 9 in 10 children also received other vaccines on the same day[5].

The numbers vary from study to study, but a fever after several vaccines at once is not unusual in itself.

MR vaccine (measles-rubella): a fever can appear about 5 to 14 days after the shot

The MR vaccine is a live vaccine that uses weakened live viruses. For this reason, a fever may appear a little later rather than on the day of the shot. According to the package insert for the MR vaccine in Japan (the official drug information sheet), a fever lasting about 1 to 3 days may be seen 5 to 14 days after the shot. Centered on days 7 to 12, about 20% have a temperature of 37.5°C or higher and 10% or fewer have 38.5°C or higher[2].

The official guide for patients also says to watch your health right after the shot and during the 5 to 14 days after it. If anything abnormal such as a high fever or seizure occurs during this period, it says to see a doctor right away[6]. If you are unsure at night, you can call #8000 (a phone line for advice on children’s illnesses).

A study in 4 large US health insurance organizations examined about 680,000 children. On days 8 to 14 after the MMR (measles-mumps-rubella) vaccine, febrile seizures (seizures with fever) were more likely. The relative risk (how much more likely than in a period without the shot) was 2.83 (95% confidence interval, the range the true value probably falls within: 1.44 to 5.55)[7].

Febrile seizures thought to be caused by the vaccine occurred in 25 to 34 per 100,000 children. Compared with febrile seizures from other causes, there were no more later seizures or developmental problems[7]. Please do not think, “It was weeks ago, so it can’t be the vaccine.” When you see a doctor, tell them the date of the shot.

A side effect, or another illness? Points for telling them apart

In fact, a fever after a vaccination is not always a side effect. In the following cases, also consider another illness, such as an infection caught around the same time.

For babies under 3 months, do not assume it is “because of the vaccine”

A study at the emergency departments of 2 US military hospitals examined 508 infants 6 to 12 weeks old with a temperature of 38°C or higher. Among the 114 who were within 72 hours of a vaccination, serious bacterial infection was found in 3.5%, fewer than the 13.7% of infants who had not been vaccinated[8].

However, even among vaccinated babies, bacterial infection was not zero, and most of it was urinary tract infection (a bacterial infection of the urinary passages)[8]. The researchers also recommend a urine test even for babies who have just been vaccinated. If a baby under 3 months has a temperature of 38°C or higher, go to a clinic even right after a shot and even at night or on a holiday[9].

Which signs after a vaccination mean going right away or calling 119?

The signs after a vaccination that call for going right away or calling 119 are trouble breathing, a swollen face, hives all over the body with limpness, and a long seizure. Anaphylaxis (a sudden, severe allergic reaction) most often occurs within 30 minutes of the shot[6]. That is why you are asked to stay for 30 minutes at the clinic that gave the shot[1]. Even after you get home, if even one of the following applies, call 119 without hesitation.

In the following cases, even if you do not need to call 119, go right away, even at night or on a holiday.

In the following cases, you can wait through the night, but see a doctor during clinic hours.

If you cannot decide whether a child can wait, you can call the children’s medical consultation line (#8000). If your child is vomiting and cannot keep fluids down, please also see our article on children who cannot drink because of fever and vomiting.

How to use fever reducers after a vaccination: do not give them in advance as prevention

After a vaccination, the basic approach is to use a fever reducer when the person has a fever and looks miserable, and not to give it in advance as prevention. We are often asked, “Should I give a fever reducer before the fever starts?”

The Czech study mentioned earlier[4] looked at exactly this. It compared a group that used a fever reducer (acetaminophen) preventively, 3 times in 24 hours at each vaccination, with a group that did not. The preventive group had a temperature of 38°C or higher in 42%, fewer than the 66% in the group that did not use it[4]. However, the amount of antibodies (proteins that fight germs) made in response to the vaccines was lower for several of the vaccines[4].

The researchers concluded that routine preventive use of a fever reducer at every vaccination is not recommended[4]. What the study measured was the amount of antibodies, and it is not known how far protection against disease falls. Nor was it a study of using a fever reducer after a fever has appeared. Even so, there is no need to give one “just in case” before a fever starts.

Support systems for the rare case: the compensation system for health injury and suspected side effect reports

Serious health injury from a vaccine side effect is extremely rare, but it is not zero. For that reason, the national government has relief systems in place.

If you became so unwell after a vaccination that you were admitted to hospital, be sure to tell the medical institution you visit when you were vaccinated and with what. It goes more smoothly if you bring your Maternal and Child Health Handbook or vaccination record.

If you are worried about a fever after a shot, come to our fever clinic

Our clinic does not see patients at night (we are closed on Thursdays). If dangerous signs appear during the night, please use #8000, a night and holiday clinic, an emergency department, or 119. With that in mind,

“I think it’s the vaccine fever, but it’s lasting” or “A cough has appeared too, so it might be another illness”

in those cases, please come to our fever clinic. Our policy is not to turn anyone away from the fever clinic (depending on the symptoms, we may direct you to a more suitable medical institution). We also see fever in small children.

Day	Hours
Mon, Tue, Wed, Fri, Sat	9:00-12:00 / 14:00-18:00
Sundays and public holidays (from October 4, 2026)	10:00-15:00 (no lunch break, fever clinic only)
Thursday	Closed

On Sundays and public holidays we are open from 10:00 to 15:00. We are closed on Sunday, October 25, and from November we are in principle open every Sunday and public holiday. On Sundays and public holidays we run the fever clinic only; general outpatient care and IV drips are not available. For details, see the announcement about the Sunday and public holiday fever clinic.

How a visit works (no appointment needed)

You check in with the QR code (a square pattern read with a smartphone camera) at the fever clinic entrance. When your turn is near, we call you by SMS (a mobile text message) or by phone.

When you come, please tell us the name of the vaccine, the date of the shot, and when the fever began. Your Maternal and Child Health Handbook makes this reliable. For detailed steps, see About the fever clinic pre-visit questionnaire and same-day check-in.

Please note that we cannot give IV drips on Sundays and public holidays, so if the person cannot keep down any fluids and is limp, go to a medical institution that can give a drip.

Frequently asked questions

I got a temperature of 38°C after a vaccination. Should I contact the clinic that gave the shot?

If the fever began soon after the shot and the person is drinking fluids with mood and responsiveness as usual, watching at home is often fine. If the fever lasts (with the influenza vaccine, it has not gone down after 2 to 3 days), there are other symptoms, or you are worried, consult the clinic that gave the shot or your family doctor. For a baby under 3 months with a temperature of 38°C or higher, go right away, even at night.

Can I give a fever reducer before the shot so that no fever comes?

I do not recommend it. In the Czech study, using a fever reducer as prevention reduced fever, but the amount of antibodies made in response to the vaccines was lower. Use a fever reducer when the person looks miserable with the fever, or cannot sleep or drink.

I got a fever after the influenza vaccine. Did I catch influenza?

An injected inactivated vaccine cannot cause influenza. Fever and similar reactions after the shot occur in 5 to 10% of people and usually disappear in 2 to 3 days[1]. However, if cough or sore throat appears, you may have happened to catch the influenza or COVID-19 that is going around, so please see a doctor.

I got a fever about 10 days after the MR vaccine. Is it a side effect?

The MR vaccine is a live vaccine, so a fever lasting about 1 to 3 days can appear 5 to 14 days after the shot. A side effect is quite possible. However, it can also be another illness. If there is a seizure or the person looks miserable with a high fever, see a doctor right away and be sure to tell them the date of the shot.

Does getting several vaccines at once make a fever more likely?

A fever after a baby gets several vaccines at once is not unusual, but one study found that a temperature above 39.5°C was rare. In a US study, adding the influenza vaccine to the pneumococcal and DTaP vaccines on the same day or spacing it to a different day gave about the same share with a fever.

If I am hospitalized because of a vaccine side effect, is there any support system?

For routine vaccinations, there is the health injury relief system under the Preventive Vaccination Act, and the contact point is the municipality where you were registered as a resident at the time of the shot. For optional vaccinations, you may be covered by the PMDA relief system for adverse drug reactions, among others. Please also consult the medical institution that treated you.

Summary: a fever after a vaccination is judged by “when, how many days, and other symptoms”

A fever after a vaccine is, in most cases, a short-lived and normal reaction of the body. Even so, it is also important not to assume it is “because of the vaccine” and miss another illness. If you are unsure, please do not hesitate to consult us. Leave the medical care to us.

That’s all.

Disclaimer

This article is intended to provide general medical information and does not replace an examination, diagnosis, or treatment by a doctor. The content is based on medical knowledge at the time of publication, but the right response differs from person to person. If you are worried about your symptoms, please consult a medical institution.

References

1. Ministry of Health, Labour and Welfare. Q&A on the comprehensive measures against acute respiratory infections (ARI), winter 2026 (Reiwa 8) season (in Japanese). Ministry of Health, Labour and Welfare website. Accessed October 2026. https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/kenkou_iryou/kenkou/kekkaku-kansenshou/infulenza/QA2026.html
2. Takeda Pharmaceutical Company Limited. Package insert for Freeze-dried Live Attenuated Measles and Rubella Combined Vaccine “Takeda” (revised January 2026, 3rd edition) (in Japanese). PMDA search for prescription drug information. Accessed October 2026. https://www.pmda.go.jp/PmdaSearch/iyakuDetail/ResultDataSetPDF/400256_636940CD2025_1_23
3. Saita M, Yan Y, Ito K, Sasano H, Seyama K, Naito T. Reactogenicity following two doses of the BNT162b2 mRNA COVID-19 vaccine: Real-world evidence from healthcare workers in Japan. J Infect Chemother. 2022 Jan;28(1):116-119. doi:10.1016/j.jiac.2021.09.009 PMID: 34580011
4. 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. Lancet. 2009 Oct;374(9698):1339-50. doi:10.1016/S0140-6736(09)61208-3 PMID: 19837254
5. Walter EB, Klein NP, Wodi AP, Rountree W, Todd CA, Wiesner A, et al. Fever After Influenza, Diphtheria-Tetanus-Acellular Pertussis, and Pneumococcal Vaccinations. Pediatrics. 2020 Mar;145(3):e20191909. doi:10.1542/peds.2019-1909 PMID: 32029684
6. Pharmaceuticals and Medical Devices Agency (PMDA). Guide for people receiving the vaccine: Freeze-dried Live Attenuated Measles and Rubella Combined Vaccine “Takeda” (updated January 2026) (in Japanese). PMDA website. Accessed October 2026. https://www.info.pmda.go.jp/downfiles/ph/GUI/400256_636940CD2025_1_01G.pdf
7. Barlow WE, Davis RL, Glasser JW, Rhodes PH, Thompson RS, Mullooly JP, et al. The risk of seizures after receipt of whole-cell pertussis or measles, mumps, and rubella vaccine. N Engl J Med. 2001 Aug;345(9):656-61. doi:10.1056/NEJMoa003077 PMID: 11547719
8. Casey K, Reilly ER, Biggs K, Caskey M, Auten JD, Sullivan K, et al. Serious bacterial infection risk in recently immunized febrile infants in the emergency department. Am J Emerg Med. 2024 Jun;80:138-142. doi:10.1016/j.ajem.2024.03.025 PMID: 38583343
9. Japan Pediatric Society. Children’s Emergency (ONLINE-QQ): fever (38°C or higher) (in Japanese). Children’s Emergency website. Accessed October 2026. https://kodomo-qq.jp/index.php?pname=hatsunetsu

More articles to read about fever

For the Sunday and public holiday fever clinic, see the announcement about the Sunday and public holiday fever clinic; for how to come in, see About the fever clinic pre-visit questionnaire and same-day check-in.