The first round of jabs at eight weeks is rough — for the baby, who's just been stuck three or four times, and for you, watching it. The next 24–48 hours can be uncomfortable for the baby, and the worry afterwards is one of the most common reasons parents ring 111 in the first year.
Most reactions are entirely expected and mean the vaccine is working. A small number need a closer look. Knowing the difference saves a lot of unnecessary panic.
Healthbooq walks you through each appointment in the UK childhood schedule with what to expect before, during, and after.
Why Reactions Happen
A vaccine is meant to provoke an immune response. An immune response involves inflammation. The redness, warmth, and swelling at the injection site, and the fever and grumpiness that may follow, are the visible bits of that immune response. They are signs the vaccine is doing what it was designed to do — not signs that something has gone wrong.
Most reactions resolve in 24–72 hours.
A specific note on MenB (meningococcal B vaccine), given at 8 and 16 weeks: it runs hotter than most other vaccines because of how the immune response to it is shaped. NHS guidance recommends giving paracetamol prophylactically — three doses of infant paracetamol suspension (120 mg/5 ml strength, 2.5 ml per dose for an under-12-week baby of typical weight, but follow the dose on the bottle for your baby's weight):
- First dose: as soon as you get home after the jabs
- Second dose: 4–6 hours later
- Third dose: 4–6 hours after that
This is a specific, evidence-based recommendation for MenB, not a general rule for all vaccines. Routine paracetamol around other jabs isn't recommended unless the baby is symptomatic — there's some evidence that prophylactic paracetamol can blunt the immune response to a few vaccines, so save it for when it's needed.
Normal Reactions, in the First 24–48 Hours
Expect any of the following:
- Local: redness, warmth, mild swelling, or a small firm lump at the injection site. Usually fades over a few days; a small lump can occasionally persist for a couple of weeks.
- Fever: typically up to 38.5°C, sometimes higher with MenB. Babies often feel hot to touch and are flushed.
- Fussiness or crying more than usual.
- Sleeping more than usual. Some babies do the opposite — wake more, settle less.
- Eating less for a feed or two. As long as wet nappies stay regular, this is fine.
Management: paracetamol at the dose on the bottle for your baby's weight handles fever and discomfort. From three months, ibuprofen can be added if paracetamol alone isn't doing enough. Keep the baby hydrated — frequent shorter feeds if they're not interested in a full one. A cool room, light clothing, plenty of cuddles. Don't wrap the baby up to "sweat the fever out" — overheating doesn't help.
The injection-site reaction can be uncomfortable. A cool flannel briefly on the area can help; avoid pressing or rubbing it. The leg or arm may seem sore to move for a day or two — gentle nappy changes, no overdressing.
Red Flags
These need attention — call NHS 111 (or 999 if severe):
- Temperature above 39°C — particularly if it's not coming down with paracetamol, or if it persists past 48 hours
- Crying inconsolably for more than 3 hours — particularly a high-pitched, unusual cry
- Pale, mottled, or blue around the lips
- Floppy, limp, or very difficult to rouse — not just sleepy, but unresponsive
- A seizure (febrile convulsion or otherwise) — call 999 if it lasts more than 5 minutes or it's the first one
- Injection-site reaction getting worse after 48 hours — spreading redness, swelling growing, the area feeling hot and tender beyond what is expected
- A widespread rash, not just at the injection site
- Vomiting that won't settle, or signs of dehydration (dry mouth, sunken fontanelle, no wet nappies for 6+ hours, drowsiness)
A baby who just doesn't seem right to you is worth a call. Parents are usually correct when they think something is off.
Anaphylaxis: Rare, Fast, Treated on the Spot
Anaphylaxis to vaccines is genuinely rare — around 1–2 cases per million doses. It happens within 15 minutes of vaccination, which is why the clinic asks you to wait.
The signs:
- Rapid widespread hives or rash all over the body
- Swelling of the face, lips, or tongue
- Difficulty breathing, wheezing, or a persistent cough
- Pale, floppy, unresponsive
- Collapse
Vaccination clinics carry adrenaline and are trained to manage this. If anaphylaxis happens at the clinic, you're in the right place. If it appears (very unusually) in the car park or on the way home, call 999 immediately.
For perspective: the conditions vaccines prevent — meningitis B, pneumococcal disease, whooping cough, measles — carry vastly higher risks than the vaccines themselves. The serious-reaction rate is at one-in-a-million scale; the disease rates without vaccination are much higher and the consequences much worse.
When to Call NHS 111 or Your GP
- Temperature above 39°C, or a fever that hasn't settled by 48 hours
- Inconsolable crying for more than 3 hours
- Baby seems unusually unwell, very pale, very floppy, or hard to rouse
- Injection-site reaction that is getting bigger or hotter after 48 hours
- A widespread rash
- Signs of dehydration
- You are worried — a healthcare professional would rather have a quick call that turns out fine than a delayed one that didn't
When to Call 999
- The baby is having a seizure, especially if the first one or longer than 5 minutes
- The baby is blue around the lips, mottled, or extremely pale
- Difficulty breathing — fast, laboured, grunting, or chest sucking in
- Floppy, limp, unresponsive
- A bulging fontanelle (the soft spot) along with fever
- Any sign of anaphylaxis (face/lip/tongue swelling, widespread hives, breathing difficulty)
Key Takeaways
Normal post-vaccine reactions — sore, red, swollen leg or arm at the injection site, low-grade fever (under 38.5°C), fussiness, sleeping more, eating a bit less — are signs the immune system is doing its job. They settle within 24–72 hours. The MenB vaccine at 8 and 16 weeks runs hotter than most; NHS guidance is three prophylactic doses of paracetamol at 2.5 ml (60 mg) infant suspension, given immediately after the jab and again at 4–6 hour intervals. Red flags that need action: temperature above 39°C, persistent crying for more than 3 hours, very pale or floppy baby, or a high-pitched unusual cry. Anaphylaxis is rare (around 1–2 per million doses), happens within 15 minutes, and is why clinics ask you to wait.
