maternal immunisation

Maternal immunisation protects a baby indirectly. The vaccine is given to the pregnant woman, her immune system responds by making antibodies, and those antibodies cross the placenta to the baby before birth. The infant is therefore born already carrying protection it did not have to generate itself. UKHSA's immunisation handbook, the Green Book, calls this transplacental passive immunisation, and names it as one of only two ways to protect a newborn against respiratory syncytial virus, the other being a monoclonal antibody injected into the baby directly.

The approach exists because of a timing problem. A newborn's own immune system is too immature to respond well to a vaccine during the very weeks it is most at risk, so the Green Book notes there are no approved RSV vaccines for infants and that such an approach is unlikely to protect during those first months. Vaccinating the mother moves the protection earlier. Timing still matters on the mother's side: the UK licence for the RSV vaccine in pregnancy runs from 28 to 36 weeks of gestation, and antibody transfer improves the longer the interval between the injection and the birth.

Maternal immunisation is not new to RSV. Vaccines against influenza, tetanus and whooping cough have been offered in pregnancy for the same reason, and the protection they confer is temporary in every case: maternal antibodies fade over the baby's first months, covering the window until the child's own immunisations take over.