Background During the recent years, a new theory postulating that lack of early childhood infections would increase the prevalence rate of allergies has rapidly gained momentum. This hygiene hypothesis has been widely disseminated to the general public and it has been suggested that vaccinations would accordingly indirectly increase rates of atopy. We thus investigated associations between acute infections, infection pressure (i.e. number of daily child contacts) and atopy in one of the largest population-based medical surveys ever published in the medical literature.
Methods Almost all Finns born between 1976 and 1984 and a sample of older teenagers aged up to 19 years (n=5 47 190) were vaccinated and questioned to establish clinical history of mumps and rubella and manifestations of atopy (rhinoconjunctivitis, eczema, and asthma) in 1982–1986. A subsample (n=37 733) including all those subjects who were vaccinated during the first 2 months of the measles, mumps, and rubella (MMR) programme were also queried information about upper respiratory infections (URIs) and infection pressure. Crude and adjusted prevalence ratios of atopy among those with infectious disease history compared with those without it were calculated.
Results The risk of URI and a history of mumps and rubella increased with the number of daily contacts. This association was apparent especially among the youngest subjects with regard to URIs whereas the proportion with histories of mumps and rubella increased with increasing infection pressure more clearly among the 6-year-olds. Atopy was not associated with daily child contacts among pre-schoolers. Children with histories of repeated URIs and MMR diseases had substantially more atopy than those with lower URI or MMR disease rates.
Conclusions Atopic subjects seem to be especially prone to clinically apparent acute respiratory tract infections, and might be in particular need of protection by immunizations. This study does not support the idea that the prevalence of atopy in affluent countries would be affected from disappearing respiratory tract infections.