How does maternal pertussis immunisation effect infant risk for pertussis in first six months of life accounting for early primary immunisation series

Problem

Maternal pertussis vaccination attenuates the primary infant antibody response to acellular pertussis vaccine. We aimed to determine the impact of fetal exposure to a maternal dose of aPertussis vaccine on pertussis disease during infants’ first 6-months of life, accounting for receipt of the primary aPertussis series commencing at 6-weeks of age.

Approach

We conducted aretrospective cohort study linking administrative datasets (including maternal and infant immunisation, pertussis notification and hospitalisation) through an encrypted personal identifier. The study population was all infants born alive in New Zealand to mothers 28–38 week’s gestation 1 January - 31 December 2013.

Findings

A total of 101,460 pregnancies were eligible for nationally-funded Tdap during our study period. 8,298 (12%) infants were born to vaccinated and 61,069 (88%) to unvaccinated mothers. Controlling for sociodemographic, maternity variables, and infant pertussis-containing doses, receipt of <2 first primary dose of pertussis-containing vaccine in infants of vaccinated mothers increased the infant’s risk for pertussis: OR 2.49 (95% CI [1.32, 4.70], p = 0.005). This effect was attenuated (trend only, p-value not significant at 5%) when excluding infants who received their first dose prior to 6-weeks. The finding was supported in subgroup analysis by infants who received 0/1 dose and who received at least 2 doses in first six months of age. There was a small additional protective effect of maternal vaccination among infants who received at least two doses of routine pertussis vaccine (trend reversed, p-value non-significant). Protective effect of infant routine dose on pertussis disease remained significant in all models.

Consequences

Our findings suggest when the first infant pertussis dose is given at 6-weeks there may be a greater risk of pertussis prior to the infant completing the primary series. This finding supports a significant inhibition when the first infant dose is given at an early age, which translates to clinical outcomes. While the first dose could be moved to 8-weeks of age, in a population with very low maternal pertussis vaccine uptake this would result in increased infant morbidity. Delaying vaccination in only those infants born to vaccinated mothers would likely be programmatically challenging. This study needs urgent follow-up using more data, and immunogenicity studies to determine if using a different vaccine to boost mothers mitigates the effect.