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💉 When Vaccinated People Still Get the Infection — What Does It Mean? An important consideration for PLAB 2

One of the doctors recently asked a great question:

“If someone has already been vaccinated but still develops an infection, for example, mumps or measles, do we still recommend vaccination or boosters afterwards? Don’t they already have antibodies?”

It’s an excellent question and one that helps us understand how vaccines work and why they’re still important even in such situations.



🌍 The Purpose of Routine Vaccination Programs

Routine immunisation programmes (like MMR for measles, mumps and rubella, or DTaP/IPV for diphtheria/tetanus/pertussis/polio) are designed to protect the majority of the population from serious infections.


Vaccines work by training the immune system to recognise germs and respond quickly if exposed later. When most people are vaccinated, community transmission drops sharply, protecting even those who can’t be vaccinated (like newborns or immunocompromised people).


So while vaccines dramatically reduce infection rates and complications, no vaccine provides 100 % protection. There will always be a few “breakthrough” cases, people who develop the infection despite vaccination.



🧬 Why Can Breakthrough Infections Happen?

Breakthrough infections can occur for several reasons:

  • Waning immunity: Some vaccines, like MMR or pertussis, may provide strong protection initially, but the immune memory can fade over time.

  • Viral variation: Some viruses (or bacteria) may change or different strains may circulate, so the vaccine’s match may be less perfect.

  • Individual factors: Some people’s immune systems don’t produce as strong an antibody/t-cell response after vaccination (for example, if the first dose was given too early or during mild illness).

  • Natural infection vs vaccine immunity: Even if one has had the disease, the level and durability of immunity may vary.

Even then, infections in vaccinated people are usually milder, shorter, and less likely to cause complications.


If you’re fully vaccinated but still get the infection — what’s the follow-up plan (UK)?

Breakthrough infections are uncommon but possible. Here’s what UK guidance says to do after recovery, vaccine-by-vaccine:


MMR (Measles–Mumps–Rubella)

  • If you already had 2 documented MMR doses:There’s no routine extra booster just because you caught measles/mumps/rubella. Make sure your two doses are documented and you’re otherwise well. In certain outbreaks, local public health teams may give additional advice to specific groups (e.g., targeted vaccination to improve coverage), but this is not a standing booster for everyone.

  • If your record shows 0–1 dose(s):Complete the course to 2 doses total, spaced ≥4 weeks apart (adults and catch-up). This still helps, even after illness, because it standardises durable protection and covers all three viruses.

Why vaccinate after natural infection?Natural illness does provoke antibodies, but quality/duration is variable; MMR provides reliable, broad protection (including the components you didn’t get—e.g., rubella).


DTaP/IPV (Diphtheria–Tetanus–Pertussis/Polio) & boosters

Pertussis (whooping cough)

  • Fully vaccinated but still had pertussis:Immunity to pertussis wanes. UKHSA advises some groups (e.g., healthcare workers) who’ve recovered from a primary infection to receive a pertussis-containing booster if they’ve not had one in the last 5 years (and no Td-IPV in the previous month). Wider public recommendations focus on being up to date with the routine boosters and the pregnancy booster (20–32 weeks).


Tetanus (for completeness within DTaP)

  • Even if you developed tetanus disease:Illness does not confer reliable immunity. After you finish treatment and recover, re-immunisation is advised to complete/refresh the 5-dose UK schedule (specialist advice often needed).


BCG (Tuberculosis)

  • Breakthrough TB after prior BCG: Do not repeat BCG. Manage as TB (active or latent) per TB guidance. BCG is contraindicated in people with current or past TB, and is not given after a positive Mantoux/IGRA because it provides no added benefit and raises adverse-reaction risk.

  • General rule with BCG in the UK: targeted use only for higher-risk groups; single lifetime dose if eligible and tuberculin/IGRA negative.



Practical checklist after breakthrough (non-pregnant, community setting)

  • Check the person’s documented doses.

  • If missing doses: give catch-up to complete the standard schedule.

  • If fully up to date: no extra routine booster unless advised during an outbreak or due to occupation/exposure risk.


A quick “why this helps” recap

  • MMR: completing 2 doses ensures durable, predictable protection across three viruses; no routine third dose after illness; outbreak advice may vary locally.

  • Pertussis: protection wanes; targeted boosters (e.g., HCWs, pregnancy) reduce severe disease and transmission.

  • Tetanus: disease doesn’t immunise you—vaccination after recovery is essential.

  • BCG: not repeated after TB infection; follow TB diagnostic/treatment pathways instead.

Bottom line: After a breakthrough infection, UK practice is to verify doses, complete any missing doses, and use targeted boosters only where indicated (occupation, pregnancy, or public-health outbreak guidance). This approach keeps individual protection high and community spread low.

📚 References


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