Clinical overview
Vaccination in pregnancy is one of the highest-yield, lowest-cost interventions in the entire antenatal package, yet it is repeatedly missed at booking and underused in South African clinics. The logic is deceptively simple: the pregnant woman is herself more vulnerable to several infections (influenza, COVID-19, varicella, hepatitis E), and her newborn — who cannot mount a protective response to most pathogens for weeks to months — depends entirely on antibody transferred across the placenta before birth. Maternal immunisation is therefore a two-patient intervention: we protect the mother during a uniquely immunologically and physiologically altered state, and we protect the infant during the dangerous window before the infant's own primary vaccine series takes effect.
The registrar must hold two ideas in tension. First, the dominant principle is that inactivated vaccines, toxoids and most subunit/recombinant vaccines are safe at any stage of pregnancy and should be given when indicated — there is no plausible mechanism by which a non-replicating antigen harms the fetus, and decades of surveillance support this. Second, live attenuated vaccines are generally contraindicated because of a theoretical risk of transplacental fetal infection, although inadvertent administration has not been shown to cause fetal harm and is never on its own an indication for termination. Getting this dichotomy right — and being able to defend it from mechanism — is the examinable core of this objective. South African practice layers two further realities on top: a high background HIV prevalence that modifies both vaccine response and indication, and a vaccine schedule and access that differ from European or American guidance. This chapter discusses the safety, the mechanisms and the practical SA use, weighting the discussion as the HOTS verb "discuss" demands.
Core knowledge
The immunological rationale
Two physiological facts drive maternal immunisation. The first is transplacental IgG transfer. Maternal IgG (predominantly IgG1) is actively transported across the syncytiotrophoblast by the neonatal Fc receptor (FcRn), a process that accelerates markedly in the third trimester. Transfer is most efficient after roughly 32–34 weeks, which is why the timing of certain vaccines (notably tetanus-containing and any pertussis-containing vaccine) is chosen to maximise the antibody pool available for transfer before delivery. Vaccinating too late leaves insufficient time for maternal seroconversion and transfer; vaccinating very early may mean antibody titres have waned by term. The second fact is the relative immunomodulation of pregnancy — a shift that increases susceptibility to severe disease from influenza and varicella in particular, justifying maternal protection in its own right.
Figure I9.1 — Transplacental IgG transfer via FcRn and the timing logic for transfer-dependent maternal vaccines.
Classifying vaccines by safety in pregnancy
The single most important framework is the live-versus-non-live distinction.
| Category | Examples | Use in pregnancy |
|---|---|---|
| Inactivated / toxoid / subunit / recombinant | Tetanus toxoid (TT/Td), inactivated influenza, hepatitis B, COVID-19 (mRNA and protein subunit), pneumococcal, meningococcal, rabies (inactivated), inactivated polio (IPV) | Safe — give when indicated at any gestation |
| Live attenuated | MMR (measles, mumps, rubella), varicella, live attenuated influenza (LAIV, intranasal), oral polio (OPV), BCG, yellow fever, oral typhoid (Ty21a) | Generally contraindicated (theoretical fetal-infection risk) — defer to postpartum |
| Special-situation live | Yellow fever | Avoid unless unavoidable travel to an endemic high-risk area where exposure risk outweighs theoretical vaccine risk |
The reason for caution with live vaccines is the theoretical possibility that an attenuated replicating organism crosses the placenta and infects the fetus. It is crucial to be precise: this is a theoretical/precautionary concern. Inadvertent administration of MMR, varicella or even yellow-fever vaccine in early pregnancy has not been associated with the congenital syndromes caused by the wild-type infections, and inadvertent vaccination is not an indication for termination of pregnancy — see Termination of pregnancy. This nuance is a classic exam discriminator.
Figure I9.2 — The live-versus-non-live vaccine safety framework, including postpartum deferral and yellow-fever exceptions.