Key Takeaways

After the MenB vaccine alongside the other infant shots, more than half of babies develop a temperature without paracetamol. It peaks around 6 hours and is nearly always gone within 2 days ¹.

Fever is the immune response doing its job. It does not show how well the vaccine worked, and no fever does not mean it failed ¹².

38°C is the line, and age changes what it means. Under 3 months, any fever needs same-day advice, even on vaccination day ³⁴.

Paracetamol is for a distressed baby, not for the number on the thermometer. The exception is UK guidance: three doses after the first MenB vaccines, because fever is so common ¹⁴.

A fever that starts more than 48 hours after the injection is more likely to be an infection than the vaccine. That is the point to check in ¹.

The short answer

A fever after a vaccination is common, expected, and usually short. The useful skill is telling the vaccine pattern apart from the infection pattern, because the two need different responses.

What you see	What it probably is	What to do
38 to 38.9°C within 24 hours, feeding and alert	Expected vaccine reaction	Fluids, fewer layers, paracetamol if distressed
Fever after MenB at the first infant appointments	Expected, and the most common of all	UK schedule: three paracetamol doses over the first day, then reassess
39°C or higher, baby otherwise themselves	Often still the vaccine	Check the age thresholds below before deciding where to go
Fever starting 6 to 10 days after a measles-containing vaccine	The measles component working	Usually 1 to 2 days; a rash may follow
Fever starting more than 48 hours after the injection	Not assumed to be the vaccine	Same-day advice

The rest of this article is about the two questions behind that table: how high is too high, and how long is too long.

How common it actually is

More common than most parents expect, and the numbers depend on which vaccine.

The clearest figures come from the MenB vaccine, because it is the one that reliably produces fever in infancy. In the phase 3 trial of 4CMenB, 77% of infants had a temperature of 38.5°C or higher after at least one dose, against 45% who received the routine vaccines alone ⁵. UK guidance puts it more simply: without paracetamol, more than half of babies develop a temperature after the MenB vaccine is given with the other routine vaccines at 8 and 12 weeks ¹.

With paracetamol, that changes. Fewer than 1 in 5 babies develop a fever, and nearly all of those stay below 39°C ¹. In the United States, MenB is not part of the routine infant schedule, so this reaction is mostly a UK, Irish and Australian experience.

The measles component of MMR runs on a different clock entirely: about 1 in 10 children develop a fever, but not until 6 to 10 days later ².

Parents often ask about the 4-month and 6-month appointments specifically. The schedule matters here. In the UK, MenB is given at 8 and 12 weeks, so later visits are the combined vaccines and pneumococcal vaccine alone ¹. In the US, the 4-month and 6-month visits repeat the same combination without a new MenB dose. A fever can still follow, and the same rules apply, but the very high rates described above belong to the MenB doses.

The pattern is not a defect in the vaccine. It is a signal that the immune system noticed it.

Why a vaccine causes a fever

Fever is a controlled rise in the body’s set point, triggered by chemicals the immune system releases when it recognizes something foreign. A vaccine produces a small, deliberate version of that response, without the infection.

That has three consequences worth holding on to.

The fever is not the harm. It is downstream of the response you want. Suppressing it does not improve the immune result, and in some cases the opposite is true, which is covered below.

The height of the fever is not a measure of protection. UK guidance says this explicitly in both directions. Fever shows the body is responding, but the level of fever does not indicate how well the vaccine worked. Not getting a fever does not mean it did not work ¹².

The timing is a fingerprint. Each vaccine type produces fever on its own schedule. That is what makes the timing useful when you are trying to work out whether a fever is the vaccine or the beginning of a cold ⁶.

The timing tells you what it is

Vaccine type	When fever appears	How long it usually lasts
Inactivated and conjugate vaccines (DTaP, polio, Hib, PCV, MenB)	Within 24 hours, peaking around 6 hours	Nearly always gone within 48 hours ¹
Measles-containing vaccines (MMR, MMRV)	6 to 10 days after the dose	A day or two; a measles-like rash may follow ²
Any vaccine, fever starting after 48 hours	Unlikely to be the vaccine	Treat it as a possible infection ¹

The practical version: if the temperature comes up the same evening as the injection, the vaccine is the likely cause. If your baby is fine for two days and then spikes a fever, you are probably looking at something else, and the usual rules for a fever in a baby apply.

The 48-hour mark is not arbitrary. UK guidance for parents says that if a baby still has a fever more than 48 hours after vaccination, they should be seen ¹. That is the point where “expected reaction” stops being the safe default.

38°C is the line, and age changes what it means

A fever is a temperature of 38°C (100.4°F) or higher ³. How much it matters depends almost entirely on how old the baby is.

Age	Temperature	What to do
Under 3 months	38°C or higher	Same-day advice from a doctor or urgent care line ³⁴
3 to 6 months	39°C or higher	Same-day advice ³⁴
Over 6 months	Height alone is less useful	Look at the baby, not just the number ⁴

The under-3-months rule survives vaccination day. This is the part parents most often get wrong, and it is not their fault. NICE notes that some vaccines do cause fever in babies under 3 months, which means the fever may well be the vaccine, and you still cannot assume it ⁴. A baby that young with a fever is assessed, not observed at home. The same temperature in a 10-week-old can be a vaccine reaction or a serious infection, and the two are not distinguishable from the outside.

That is also why the age cut-offs matter more than the exact number. A 39°C fever in a 9-month-old who is drinking and playing is a different problem from 38.2°C in a 7-week-old who is sleepy.

What to do at home

Most post-vaccination fevers are managed at home, and the management is unglamorous.

Fluids first. Offer feeds more often. If your baby is breastfed, breast milk is the best fluid available ¹³.
Fewer layers, not cold. Remove a blanket or a layer of clothing. Do not undress the baby or sponge them with water ²³⁴.
No bath, no fan. UK guidance for post-vaccination fever is explicit on this: no bath, no sponging, no fan ². It is not that a bath is dangerous in itself; it is that cooling the skin does not fix the set point and adds a shivering, miserable baby to the problem.
Paracetamol or ibuprofen, for distress. The threshold is behavior, not temperature. A baby who is feeding and settled does not need medicine for a number ³⁴.
Check overnight. Fever after vaccination tends to peak in the first evening, so one check before bed is worth more than a monitor you keep refreshing.

The age limits on the medicines are not optional:

Paracetamol is not given under 2 months ³.
Ibuprofen is not given under 3 months or under 5 kg, or to a dehydrated baby, or one with chickenpox ³.
No aspirin under 16 ²³.
Do not alternate the two unless a clinician tells you to ³. NICE allows alternating only if distress returns before the next dose is due ⁴.

Should you give paracetamol before the appointment?

This is where the evidence gets genuinely interesting, because the answer moved twice.

The first change came in 2009. A randomized trial of 459 infants compared three prophylactic paracetamol doses over the 24 hours after each vaccination with no paracetamol. Fever of 38°C or higher fell from 66% to 42% after the primary series. But antibody concentrations were significantly lower in the paracetamol group, for all ten pneumococcal serotypes, Hib, diphtheria, tetanus and pertactin, and some of that gap persisted after the booster ⁷.

The trial’s own conclusion was that prophylactic paracetamol should not be routinely recommended, because the antibody reduction outweighed the fever reduction.

The second change is more recent, and it is a refinement rather than a reversal. A 2026 review of the evidence concluded that prophylactic paracetamol consistently lowers fever but blunts antibody titers, particularly against pneumococcal conjugate vaccine, while ibuprofen does not show that suppression. Therapeutic use, given only after symptoms appear, showed no negative effect on immune responses. The review’s recommendation: avoid routine pre-dosing, with prophylactic paracetamol justified only where reactogenicity is unusually high, such as 4CMenB, or where the child has a febrile seizure history ⁸.

That is exactly where UK practice landed. The MenB schedule is the exception that proves the rule. Because more than half of babies would otherwise develop a fever, UK guidance recommends three paracetamol doses starting at the appointment. That advice is specific to MenB, and it does not apply to the other infant vaccines ¹.

So the practical answer is:

MenB, in the UK schedule: give paracetamol as the leaflet directs, starting soon after the appointment.
Everything else: wait for symptoms, then treat distress. Do not pre-dose.
If your baby has had a febrile seizure: this is a conversation with your clinician before the appointment, not a decision to make in the parking lot.

The fever that arrives a week later

Measles-containing vaccines behave differently enough to catch parents out, because the fever does not come the same day.

The measles component starts working 6 to 10 days after the injection. About 1 in 10 children develop a fever and may go off their food, and some develop a measles-like rash that is not infectious ². By the time it happens, the appointment feels like history, and the fever gets attributed to a cold, or to teething.

Two things are worth knowing. First, this late fever is expected, and it is not a reason to worry more than you would about any other fever. Second, it is the window in which febrile seizures cluster: around 1 in 1,000 children have a fit after the first dose of MMRV, usually in the second week ². A meta-analysis of post-marketing data found roughly a two-fold increase in febrile seizure risk 5 to 12 days after MMRV in children aged 10 to 24 months. The absolute incidence stayed under 3 per 1,000 ⁹.

That sounds alarming, and mostly is not.

Febrile seizures, without the panic

Febrile seizures affect 2% to 4% of children, and about 30% of those children have a second one ¹⁰. They usually last 2 to 3 minutes, and it is rare for them to last more than 10 ¹¹. They look far worse than they are: most children recover fully, and the seizure itself does not cause brain damage.

What to do is narrow and specific ¹¹:

Note the time it starts and ends.
Protect the head with your hands or something soft.
Move dangerous objects away.
After it stops, put the child in the recovery position.
Do not hold the child down, and do not put anything in their mouth.

Call emergency services for a first seizure, one lasting more than 5 minutes, one affecting only one side, or a child who is unusually sleepy more than an hour afterwards ¹¹.

One myth is worth killing here, because it drives a lot of unnecessary medicine: antipyretics do not prevent febrile seizures. NICE states it directly, and the Cochrane review of prophylactic treatment found no significant benefit from intermittent ibuprofen or paracetamol for preventing recurrence ⁴¹⁰. Giving paracetamol to prevent a fit does not work. Giving it because the baby is miserable does.

When a fever is not the vaccine

The hard part of post-vaccination fever is that the vaccine gives you a reason to under-react. These are the features that override that reason.

Call your doctor or urgent care line	Call emergency services
Under 3 months with 38°C or higher	Stiff neck
3 to 6 months with 39°C or higher	A rash that does not fade under a glass
Fever lasting 5 days or more	A first febrile seizure, or one over 5 minutes
Not feeding, fewer wet diapers, sunken eyes	Difficulty breathing, grunting, chest sucking in
A rash alongside the fever	Drowsy and hard to wake, or inconsolable
Fever still present more than 48 hours after vaccination	Blue, gray or blotchy skin, lips or tongue
You are worried and cannot say why	Unusually cold hands and feet

Those lists are the NHS thresholds for children with fever, and they apply whether or not there was a vaccine that week ³⁴. The vaccination explains the fever; it does not exempt the baby from being assessed.

Two specific traps:

The rash that does not fade. A non-blanching rash is a red flag for meningococcal disease, and it is the one thing in this article worth memorizing. Press a clear glass against it. If the spots stay visible, call emergency services.

The baby who is “just sleepy”. Drowsiness after a vaccine is common. A baby who cannot be roused, or who does not stay awake, is not the same thing.

What a fever does not tell you

Two things parents reasonably assume, and the evidence does not support.

It is not a score of vaccine performance. UK guidance is explicit that the level of fever does not indicate how well the vaccine worked, and that no fever does not mean it failed ¹².

It is not a reason to skip the next dose. Bring it up at the next appointment, especially if the fever was high or came with a seizure, so the plan is written down in advance. But the protection depends on the series being completed, and a previous fever is a reason to plan, not a reason to stop.

The bath is a smaller question than it feels. Skip it that day, for the reasons in the bathing guide, and go back to the normal routine the next.

Track the fever, not the anxiety

The useful record after a vaccination is short: the time of the dose, the temperature readings with times, what you gave and when, and whether diapers stayed wet.

That is what a clinician asks for, and it is impossible to reconstruct from memory at 2 a.m. Our vaccine tracker keeps the dose and the date, the doctor tracker holds the appointment and what was advised, and a temperature log sits beside them, so the next conversation starts with a timeline instead of a guess.

If the fever turns out to be an ear infection rather than the vaccine, the ear infection guide covers what antibiotics do and do not change. The teething versus ear infection versus cold comparison covers the differential that keeps parents up at night.

Frequently asked questions

How long does a fever after vaccination last?

Fever after the MenB vaccine peaks around 6 hours and is nearly always gone within 2 days ¹. If it is still there after 48 hours, or it started more than 48 hours after the injection, get advice rather than waiting it out ¹.

What temperature counts as a fever in a baby?

38°C (100.4°F) or higher, measured with a digital thermometer in the armpit ³. If your baby has just had a bath or been wrapped up, wait a few minutes and take it again ³.

Can I bathe my baby with a fever after vaccines?

No. UK guidance for post-vaccination fever is to avoid a bath, sponging and fans ². Offer fluids, remove a layer, and use paracetamol or ibuprofen if the baby is distressed ³.

Is a fever normal after the 2-month vaccines?

Yes, particularly with MenB. More than half of babies develop a temperature without paracetamol, and fewer than 1 in 5 do with it ¹. A fever in a baby under 3 months still needs same-day advice, even when the timing fits the vaccine ³⁴.

Should I wake my baby to give paracetamol?

No. UK guidance says not to wake a sleeping baby for a dose; give it when they next wake, as long as at least 4 hours have passed since the last one ¹.

Can I give ibuprofen instead of paracetamol?

Sometimes, but not for a young baby. Ibuprofen is not given under 3 months or under 5 kg, or to a dehydrated baby ³. Do not give both at once, and do not alternate them unless a clinician has told you to ³⁴.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Fever after vaccination is common, but it does not rule out other illness. Follow the leaflet supplied at your appointment, and seek same-day advice for a fever in a baby under 3 months, a fever lasting more than 48 hours after vaccination, or any of the red-flag features listed above.

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