Timing of routine immunisations and subsequent hay fever risk

Background: Suggestions that immunisation influences allergic disease risk, either positively ( pertussis) or negatively ( BCG) are of concern for vaccination policy. Aims: To determine whether DTP, MMR, and BCG vaccination in infancy influenced hay fever risk. Methods: Case-control study of 7098 hay fever cases and controls, within two primary care databases. One control per case was matched for practice, age, and sex. Odds ratios ( OR) were derived using conditional logistic regression. Results: Compared to those completing in month 5 ( base group) (39.3%), DTP unvaccinated children (4.3%) had a similar risk of hay fever ( OR = 0.94, 95% CI 0.73 to 1.23). However, those completing after 12 months (4.2%) had a reduced risk ( OR = 0.60, 95% CI 0.45 to 0.76) compared to the base group. Compared to those vaccinated in month 14 ( base group) (29.5%), MMR unvaccinated children (2.3%) had an OR of 0.79 ( 95% CI 0.58 to 1.08). Completion of MMR after two years was associated with reduced hay fever risk ( OR = 0.62, 95% CI 0.48 to 0.80) compared to the base group. The effects of late immunisation with DTP and MMR were independent. Those vaccinated with BCG by age 2 (2.4%) had an odds ratio of 1.28 ( 95% CI 0.96 to 1.70). Adjustment for consulting behaviour, social factors, or sibship size did not alter these associations. Conclusions: Immunisation against DTP or MMR does not increase the risk of hay fever. The lower confidence limit for BCG vaccination contradicts the hypothesised protective effect. The reduced risk of hay fever among children immunised late may be explained by a third factor causing both postponement and reduced risk such as intercurrent febrile illnesses.