Should I give acetaminophen before the appointment?

AAP advises against medicating a low-grade fever (100-102F) at all, because it may reduce the immune response. Pre-dosing is common advice and runs the same risk. Above 102F, medicine for discomfort is fine. If your own doctor has advised otherwise for your child specifically, follow them.

How long should the fever last?

Beginning within 12 hours and lasting 1 to 2 days is the usual pattern. More than 3 days, or a fever that goes and comes back after 24 hours clear, is worth a call.

Their leg is red and swollen. Is that infected?

Redness and swelling at the site are expected, starting within 24 hours and lasting 3 to 5 days — up to 7 with DTaP. What is not expected is redness getting LARGER and more painful after day 3. That timing is the thing to watch.

They came out in a rash two weeks after the MMR. Is that related?

Possibly, yes. MMR and chickenpox reactions are delayed, typically 1 to 4 weeks after the shot, and a measles-vaccine rash usually starts day 6 to 12. Lasting more than 4 days is worth a call.

When it happens · 4 min read

Fever after vaccines

They had their shots this morning and they are hot, miserable and refusing to be put down. You are trying to work out whether this is the vaccine doing its job or the start of something.

This is the most predictable 2am of the whole first year — you know the date in advance — and it is worth knowing the numbers before you are in it.

Last updated August 19, 2026

What is expected, and when

AAP is unusually direct: fever with vaccines is normal, harmless and probably helpful, because fever speeds up the immune system. Redness and fever starting on day 1 or 2 mean the vaccine is working.

Fever with most vaccines begins within 12 hours and lasts 1 to 2 days.

Pain, redness and swelling at the injection site start within 24 hours and most often last 3 to 5 days — with DTaP, up to 7 days. A sore leg for most of a week is inside normal.

MMR and chickenpox run on a different clock entirely, and this is the one that catches people out: their fever and rash are DELAYED, usually starting between 1 and 4 weeks after the shot. A rash ten days later is easy to read as an unrelated illness.

The advice that surprises people

For a low-grade fever of 100-102F (37.8-39C), AAP says do not give fever medicines — because they may reduce the body's normal immune response. The fever is part of the mechanism you are paying for.

Above 102F (39C), medicine may be given for discomfort, and acetaminophen if needed. Note the framing: for DISCOMFORT, not to bring a number down.

This runs against the instinct to pre-dose before an appointment, and against a lot of well-meaning advice. If your own pediatrician has told you otherwise for a specific reason, follow them — they know the child.

What actually helps

Cool fluids in unlimited amounts, to prevent dehydration in a child who is hot and not much interested in feeding.

A cool, damp cloth on the injection site for redness, soreness and swelling.

Expect fussiness, crying and restless sleep — AAP lists all three as things every vaccine can cause. Plan the evening accordingly rather than treating a bad night as evidence something went wrong.

When to call

Within 24 hours: fever lasting more than 3 days; fever that returns after being gone more than 24 hours; redness around the shot becoming larger and more painful AFTER day 3; or a measles-vaccine rash (day 6 to 12) lasting more than 4 days.

The pattern worth noticing in that list is TIMING rather than intensity. Redness on day 1 is expected; redness worsening on day 4 is not. A fever on day 1 is the vaccine; a fever arriving on day 4 is probably something else that happened to follow it.

Straight to 911: trouble breathing or swallowing, not moving or very weak, or a child you cannot wake. Anaphylaxis is rare and fast, and it is not something to observe for a while.

And the general rule that overrides all of the above — if you think your child needs to be seen and it feels urgent, that is reason enough. None of these numbers replace looking at your own child.

Common questions

Should I give acetaminophen before the appointment?: AAP advises against medicating a low-grade fever (100-102F) at all, because it may reduce the immune response. Pre-dosing is common advice and runs the same risk. Above 102F, medicine for discomfort is fine. If your own doctor has advised otherwise for your child specifically, follow them.
How long should the fever last?: Beginning within 12 hours and lasting 1 to 2 days is the usual pattern. More than 3 days, or a fever that goes and comes back after 24 hours clear, is worth a call.
Their leg is red and swollen. Is that infected?: Redness and swelling at the site are expected, starting within 24 hours and lasting 3 to 5 days — up to 7 with DTaP. What is not expected is redness getting LARGER and more painful after day 3. That timing is the thing to watch.
They came out in a rash two weeks after the MMR. Is that related?: Possibly, yes. MMR and chickenpox reactions are delayed, typically 1 to 4 weeks after the shot, and a measles-vaccine rash usually starts day 6 to 12. Lasting more than 4 days is worth a call.

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