Which side effects are normal, and when should I call a doctor?
Here is what the checked sources say. Each title links to the original.
Vaccine safety: Fever and vaccines
Fever is one of the most common vaccine side effects and a normal sign the immune system is responding; no fever does not mean the vaccine failed. Most fevers come within a week of the shot. Rates vary by vaccine, for example 6 to 7 in 100 after HPV vaccine. CHOP notes that fever can lead to a febrile seizure in some young children; it says these are frightening but usually cause no lasting harm, and to tell your child's doctor if one happens.
What to expect after vaccinations
UK guidance for parents of children under 5. Normal reactions include crying, and swelling, redness, a small hard lump or soreness where the shot went, usually lasting 2 to 3 days. Mild fever over 38°C is common, especially after MenB vaccine. Fever or a rash can appear 6 to 10 days after the MMR or MMRV vaccine. The UK guidance says to call a doctor right away if a child under 5 has a temperature of 39 to 40°C or higher, and to get urgent help if a child has a fit (seizure).
Vaccine Side Effects
US guidance for adults and families. Common side effects are mild and go away on their own: pain, swelling or redness where the shot went, mild fever, chills, tiredness, headache, and muscle and joint aches. Fainting can happen after any procedure. Severe allergic reactions affect about 1 to 2 people per million doses; signs include trouble breathing, face or throat swelling, a fast heartbeat, a bad all-over rash, dizziness and weakness. For those, call 911. Call your provider about side effects that bother you or don't go away. Reviewed August 2026.