Baby · Vaccines
Vaccine Reaction Guide
Common reactions: site soreness, mild fever 24-72h. MenB needs 3 prophylactic paracetamol doses. URGENT: anaphylaxis, persistent high fever, prolonged seizure. UK Green Book.
Last reviewed 2 June 2026
Vaccine reaction check
Is this vaccine reaction normal — or do I need to call?
⚠️ Concerning signs — same-day medical review
✅ Common, expected reactions
Assessment
Within 1 hour — normal observation
Most vaccine reactions appear in the first 24-48 hours. The earliest concerning reaction (anaphylaxis) usually appears within 30 minutes — which is why surgeries ask you to wait 15 minutes after vaccinations. If signs develop in the next few hours, recheck this tool.
Normal vs concerning at a glance
Expected: sore injection site (24-48 h), mild fever < 39 °C, fussiness or drowsiness for a day, reduced feeds for a day, mild measles-like rash 7-10 days after MMR.
Concerning: anaphylaxis (within 30 min), high-pitched inconsolable cry > 3 h, pale-floppy episode, seizure, fever ≥ 40 °C or > 48 h, swelling > 10 cm or spreading, non-blanching rash.
Common vaccine reaction questions
- "Should I give paracetamol routinely after vaccinations?" Routinely, NO — for most vaccines, give only if your baby is uncomfortable. EXCEPTION: MenB vaccine. NICE / Green Book recommend prophylactic paracetamol after MenB because high fever is very common — three doses at 0, 4-6 h, and 8-12 h after. Specific dosing per the leaflet.
- "Why do I wait 15 minutes after the jab?" Anaphylaxis (severe allergic reaction) is rare (~1 in a million doses) but almost always appears in the first 15-30 minutes. The wait lets staff manage it immediately if it happens. After 30 min, anaphylaxis becomes vanishingly unlikely.
- "MMR rash and fever 10 days later — is that normal?" Yes — MMR is a live attenuated vaccine, so a mild measles-like illness 7-10 days after is expected (~5-15% of children). NOT infectious to others. The fever from MMR is also when febrile convulsions are slightly more likely; preventive paracetamol is sometimes advised in high-risk children but isn't routine.
- "My baby developed eczema after vaccines — connected?" No. Multiple large studies have shown no causal link between routine vaccines and eczema, asthma, autism, or other chronic conditions. Eczema typically emerges in the same first-year window vaccines are given, but the timing is coincidental.
- "What about autism and MMR?" The Wakefield 1998 paper was fraudulent and retracted; the author was struck off the medical register. Subsequent studies in millions of children worldwide have shown NO link between MMR and autism (CDC, NICE, WHO, Cochrane). The original concern came from a discredited study and has been thoroughly examined.
- "My baby is unwell — should I delay the vaccine?" Mild illness (cold, runny nose, mild cough, low-grade temperature) is NOT a contraindication. Defer only for: significant febrile illness, recent severe immune-system event, intercurrent illness needing hospitalisation. Asthma, eczema, antibiotics, breastfeeding — all fine.
- "Can I bath my baby after vaccinations?" Yes — bathing has no effect on the vaccine. Some babies prefer not to put the injected limb underwater for 24 hours if it’s sore, but it’s not medically necessary.
- "Lump at the injection site weeks later — is it normal?" A small pea-sized firm lump after some vaccines can persist for weeks or months — usually a localised inflammatory reaction. Painless lumps that aren’t growing don’t need treatment. Growing, painful, red, hot, or pus-discharging lumps = GP review.
- "What if my baby missed a vaccine?" See /calculators/vaccine-catchup. The rule is catch up, don’t restart. Most vaccines can be caught up flexibly; rotavirus has age limits (start < 15 weeks, last < 8 months) so missing the window means missing the vaccine.
- "Egg allergy and MMR?" Even severe egg allergy is NOT a contraindication to MMR (Green Book / AAP). MMR is grown in chick embryo fibroblasts which contain minimal egg protein. Some flu vaccines (particularly nasal sprays) have egg-protein traces and may need precautions.
- "Should I report a reaction?" Significant reactions should be reported to MHRA Yellow Card (UK) or VAERS (US). Your healthcare provider will usually report on your behalf. This tracking informs vaccine safety monitoring. For minor expected reactions, no need to report.
- "Should I separate or delay vaccines?" Standard schedules combine multiple vaccines because they’re safe and effective together — and because separated schedules leave babies vulnerable longer. The standard schedule is endorsed by AAP, NICE, WHO. Delayed / spread-out schedules are not evidence-based and increase risk windows.
- "Travel vaccines — are they safe with routine ones?" Usually fine to give the same day at different sites. Discuss with your travel-medicine clinic; live vaccines may need spacing 4 weeks apart if not given together.
Educational tool only — not medical advice. Concerning reactions need same-day clinical assessment and reporting. Suspected anaphylaxis: 999 / 911 immediately.
What does this mean?
Vaccines are one of the most-studied medical interventions in history, and the safety data is robust. The vast majority of post-vaccine reactions are mild, expected, and self-limiting: soreness at the injection site for 24–48 hours, mild fever < 39 °C, a day of fussiness or drowsiness, briefly reduced feeds. These signs indicate the immune system is doing its job — making antibodies. Paracetamol if your baby is uncomfortable; ibuprofen if ≥ 3 months and > 5 kg. One specific exception: the MenB vaccine is the only vaccine where NICE / UK Green Book routinely recommends prophylactic paracetamol — high fever after MenB is so common that pre-emptive treatment is standard (three doses at 0, 4–6 h, and 8–12 h). MMR is a live attenuated vaccine, so a mild measles-like rash and fever 7–10 days afterwards is expected (and not infectious to others). A similar mild illness ~10 days after MMR catches parents out who expect immediate reactions only. The genuinely concerning reactions that need same-day medical review or 999 are: anaphylaxis (severe swelling of face/lips/throat + breathing difficulty + collapse, usually within 30 minutes — which is why surgeries ask you to wait 15 minutes after vaccinations); high-pitched inconsolable cry > 3 hours; hypotonic-hyporesponsive episode (pale, floppy, unresponsive within 48 h); seizure within 72 hours; fever ≥ 40 °C or persisting > 48 hours; spreading redness / swelling > 10 cm; non-blanching rash or widespread urticaria. These are rare but need clinical assessment and reporting to MHRA Yellow Card (UK) or VAERS (US) for vaccine-safety monitoring. A few common misconceptions worth dispelling clearly: the supposed MMR–autism link originated in a fraudulent 1998 paper that was retracted; the author was struck off; subsequent studies in millions of children worldwide show NO link. Mild illness (cold, runny nose, low-grade temp) is NOT a contraindication to vaccination. Even severe egg allergy is NOT a contraindication to MMR (Green Book / AAP). Spread-out schedules are not evidence-based and leave babies vulnerable longer than the standard schedule.
Common reactions (normal)
- Site soreness, redness, mild swelling 1-2 days.
- Mild fever (≤38.5°C) 24-72h.
- Irritability.
- Drowsiness / fatigue.
- Reduced feeding.
- Mild rash (some vaccines).
Peak 6-24h after vaccination.
MenB vaccine — paracetamol protocol
- Dose 1: 2.5 ml infant paracetamol immediately after vaccination.
- Dose 2: 4-6h later.
- Dose 3: another 4-6h later.
Reduces fever incidence. NHS-recommended.
Managing fever generally
- Paracetamol if >38.5°C or uncomfortable.
- Adequate fluids.
- Light dressing.
- Rest + cuddles.
Local site reaction
- Cool compress.
- Paracetamol if painful.
- Normal bathing + movement.
- No restriction needed.
When to seek help
URGENT (999 / A&E):
- Anaphylaxis: breathing difficulty, face/throat swelling.
- Seizure.
- High-pitched unusual cry.
URGENT GP / 111:
- High fever not settling.
- Prolonged inconsolable crying.
- Worrying signs.
- Reactions >72h.
Rare reactions
- Anaphylaxis: 1 in 1M. Minutes-hours post. Adrenaline.
- Febrile seizure post-MMR: ~5-12d, 1 in 2,000-3,000.
- HHE: 1 in 1,000-10,000 in 48h. Self-limiting.
Live vaccines reactions
- MMR: 5-12d post: mild fever, rash, glands.
- Rotavirus: mild diarrhoea, vomiting 1-7d.
- Varicella: mild rash 1-3 wk.
- BCG: site reaction healing 2-3 months.
Subsequent doses after reaction
- Most reactions don’t contraindicate further doses.
- Anaphylaxis: avoid same vaccine.
- HHE: usually omit pertussis next dose.
- Component allergy: alternative.
Reactions vs illness coincidence
Children get viral illnesses anyway. Vaccine reactions typically settle 72h. Separate illness possible. Evaluate clinically.
Different scenarios
Scenario 1: 38.2°C fever after MenB at 8 weeks
Normal. Paracetamol per protocol. Fluids + cuddles. Should settle 24-48h.
Scenario 2: Red sore swollen leg after 12-wk vaccines
Normal local reaction. Cool compress. Resolves 24-48h.
Scenario 3: Mild rash 7 days after MMR
Live vaccine reaction. Normal. Not contagious. No treatment needed.
Scenario 4: Anaphylaxis suspected within 15 min
999 immediately. Adrenaline. Hospital admission. Specialist allergy follow-up.
Scenario 5: Febrile seizure 10 days post-MMR
999 / A&E for assessment. Usually benign. Future vaccines fine.
Care guidance
- Expect mild reactions 24-72h.
- MenB needs paracetamol protocol.
- Cool compress + comfort.
- Severe reactions rare but treatable.
- Most can have next dose normally.
- Vaccines protect against worse diseases.
Sources
- UK Green Book (Immunisation against infectious disease).
- NHS. Side effects of vaccines.
- RCPCH guidance.
Frequently asked questions
What reactions are normal after vaccination?
COMMON (1 in 10): (1) SITE SORENESS, redness, mild swelling 1-2 days. (2) MILD FEVER (≤38.5°C) 24-72h. (3) IRRITABILITY. (4) DROWSINESS / fatigue. (5) REDUCED FEEDING. (6) MILD RASH (some vaccines). USUALLY mild + transient. PEAK 6-24h after vaccination.
How to manage fever after vaccination?
(1) PARACETAMOL (15 mg/kg) if uncomfortable or temperature >38.5°C; (2) ENSURE FLUID intake; (3) DRESS LIGHTLY; (4) REST + cuddles; (5) IBUPROFEN avoided if available paracetamol (unless ibuprofen specifically advised). PARACETAMOL after MEN-B specifically given prophylactically per NHS (3 doses post-vaccination).
MenB vaccine + paracetamol — protocol?
NHS recommends PROPHYLACTIC PARACETAMOL after MenB vaccine due to high rate of fever: (1) FIRST DOSE: 2.5 ml infant paracetamol immediately after vaccination; (2) 4-6 HOURS LATER: 2.5 ml; (3) 4-6 HOURS AFTER THAT: 2.5 ml. TOTAL 3 doses. Reduces fever incidence. Continue normal infant paracetamol if needed beyond.
What about local site reactions?
RED, sore, swollen at injection site — common, peaks 24-48h. (1) COOL compress; (2) PARACETAMOL if painful; (3) NORMAL bathing + movement; (4) NO need to restrict arm / leg movement. RARELY: extensive redness (>5cm) — document, monitor, GP if concerned.
When to call doctor?
URGENT (999 / A&E): (1) ANAPHYLAXIS — difficulty breathing, swelling of face/throat, severe rash, collapse (within minutes-hours); (2) SEIZURE; (3) HIGH-PITCHED unusual cry persistent. URGENT GP / 111: (1) HIGH FEVER not settling with paracetamol; (2) PROLONGED inconsolable crying >3h (rare); (3) UNUSUAL or worrying signs; (4) REACTIONS that persist beyond 72h.
Anaphylaxis — very rare
1 in 1,000,000 vaccinations. PRESENTATION: minutes to hours after; difficulty breathing, throat swelling, hives all over, hypotension, vomiting, collapse. MANAGEMENT: 999, adrenaline (clinic / hospital). PROGNOSIS: excellent with prompt treatment. PARENTS observed 15 min post-vaccination at GP/nurse clinic.
Febrile seizure after MMR?
MMR can cause febrile seizure ~5-12 days post-vaccination (correlates with mild measles-like illness). RISK ~1 in 2,000-3,000. SHORT GENERALISED seizure usually <5 min. PROGNOSIS excellent; no long-term harm. CALL 999 if first seizure or >5 min; otherwise GP for review. /calculators/febrile-seizure.
Hypotonic-hyporesponsive episode (HHE)?
RARE reaction (~1 in 1,000-10,000) within 48h: SUDDEN paleness, floppiness, lack of response. CONCERNING but usually SELF-LIMITING within minutes. CALL 999 or A&E. MOST CHILDREN recover fully + can have subsequent vaccinations safely (specialist input). MAY DECIDE to switch to DT/IPV-only (omit pertussis) for next dose.
Are vaccine reactions a sign vaccine isn’t safe?
NO — mild reactions = IMMUNE RESPONSE forming; vaccine working. SEVERE reactions are RARE + outweighed by disease protection. VACCINE-PREVENTABLE DISEASES have MUCH worse outcomes (measles encephalitis, meningitis death, polio paralysis). RISK/BENEFIT strongly favours vaccination.
Can my child have the next dose if reacted?
USUALLY YES — most reactions don’t contraindicate further doses. EXCEPTIONS (rare): (1) ANAPHYLAXIS to that vaccine (avoid same vaccine; can use different formulation if specialist agrees); (2) PROLONGED seizures with developmental concern; (3) HHE: usually omit pertussis next dose; (4) ALLERGY to component (egg in flu — alternative available).
Live vaccines — specific reactions?
MMR: 5-12 days post: mild fever, rash, swollen glands, brief joint aches. ROTAVIRUS (oral): mild diarrhoea, vomiting, irritability 1-7 days. VARICELLA: mild rash 1-3 weeks post (small chickenpox-like spots). NORMAL; not contagious or rarely so. BCG: small site reaction healing 2-3 months.
Reactions vs viral illness coincidence?
Children often get viral illnesses anyway. POST-VACCINATION illness MAY BE COINCIDENCE. EVALUATE: (1) FEVER pattern; (2) OTHER symptoms (cough, rash, diarrhoea, ear symptoms); (3) TIMING (vaccine reactions usually settle 72h; viral illness may continue or differ). UNRELATED illness common.
What if my child gets really sick after vaccination?
EVALUATE NORMAL vs concerning signs. RED FLAGS: high fever not settling, severe lethargy, breathing problems, persistent crying, decreased feeding markedly, signs of sepsis. CALL 111 / A&E. MOST cases: vaccine reaction settling 72h. SEPARATE illness possible. ASSESS clinically.
How does this relate to other calculators on BumpBites?
Companion: /calculators/vaccine-catchup; /calculators/baby-fever; /calculators/baby-age; /calculators/febrile-seizure; /calculators/anaphylaxis; /calculators/sepsis-newborn.