Optimising the Timing of Whooping Cough Immunisation in MUMs

Pertussis is a highly infectious respiratory illness caused by Bordetella pertussis, which characteristically causes paroxysms of coughing with an associated inspiratory 'whoop'. In young infants the clinical presentation can be atypical, is frequently associated with apnoeic episodes and can cause significant morbidity and mortality. Following the introduction of pertussis vaccination into routine infant schedules worldwide there was a significant reduction in the incidence of pertussis. However, recently in a number of countries, despite high vaccine coverage, there has been a resurgence of pertussis disease associated with an increase in infant deaths. Various strategies to control the increasing burden of infant pertussis disease have been considered including cocooning, a strategy in which close contacts of an infant are vaccinated, a neonatal dose of a pertussis containing vaccine, the addition of an adolescent dose to the schedule and vaccination in pregnancy. Of these, the latter has gained most support.

Vaccination in pregnancy is a strategy which relies on effective transport of antibody across the placenta, a process which depends on the antibody available, the gestation of the pregnancy and the health of the placenta. In the case of pertussis, the main aim is to prevent disease in babies (prior to the age of routine vaccination) as they have a higher mortality than any other age group from pertussis disease.

It is now clear that pertussis vaccination in pregnancy can reduce the burden of disease in young infants in the period prior to them being fully immunised, and can do so safely. However, it is not established whether there is an optimal time to vaccinate in pregnancy to ensure maximal protection of the infant. This is reflected in the different guidelines currently in place in different countries: in the UK vaccination is advised from 16 weeks gestation, in Canada from 26 weeks, in the USA from 27 weeks and in Australia from 28 weeks.

While some studies have suggested that vaccination later in pregnancy can achieve higher antibody concentrations in the newborn, others have found earlier vaccination provides improved immunity compared to later vaccination. The significance of this is that if earlier vaccination is shown to be equivalent, there are clear logistical benefits in allowing the widest possible time window in order to maximise the opportunities for pregnant women to be vaccinated and thereby improve vaccine coverage.

When the pertussis vaccination in pregnancy programmes were first implemented, vaccination was recommended in the third trimester (USA 27-36 weeks, UK 28-32 weeks). This decision was based on the hypothesis that vaccination would be most effective if it was timed to allow the peak in maternal antibody levels after vaccination to coincide with the time of most efficient placental transport.