Background It has been hypothesized that early-life exposure to vaccinations, infections or antibacterials influence allergic disease development. Concurrent exposure to grass pollens may alter any effect.
Objective To test the hypothesis that exposure to antibacterials, vaccinations (DTP or MMR) or specific infections during the first grass pollen seasons of life influences the risk of hayfever more than at any other time of the year.
Methods Nested case–control studies were based on birth cohorts within two large databases of computerized patient records from UK general practices: the General Practice Research Database (GPRD) and Doctors' Independent Network (DIN). Seven thousand ninety-eight hayfever cases, diagnosed after age 2, were matched to controls for practice, age, sex and follow-up of control to case ascertainment date. Conditional logistic regression was used to compare exposure by age 1 (age 2 for MMR) inside vs. outside the grass pollen season (May, June, July). Odds ratios (ORs) were pooled across databases.
Results There were no associations in either database between MMR during vs. outside the grass pollen season and later hayfever. Of 23 infections studied, none were statistically significant; although analyses for the less common conditions were limited by low statistical power. The pooled OR for hayfever comparing exposure to antibacterials only in the grass pollen season with only outside it was 1.20 (95% CI 0.98–1.47) and for DTP was 0.84 (95% CI 0.72–0.98).
Conclusion Although an interaction between early exposure to microbial agents and concurrent grass pollen exposure on hayfever risk seemed plausible, there was little evidence to support it across a range of analyses. However, the effect of DTP though weak deserves further study.