A retrospective nationwide register-based study to evaluate the non-specific effects of first MMR vaccination among children in Finland.

Abstract

According to earlier studies, live vaccines like measles-mumps-rubella (MMR) vaccine could reduce also other infections than only the infections they are targeted against. This non-specific effect has been seen especially in studies in low-income countries and results from high-income countries have not been unambiguous. In 2011 Finland changed the recommended schedule for the first MMR vaccination from 18months to 12months of age. This change created a natural experiment for evaluating the potential non-specific effects. This is a retrospective nationwide register-based cohort study of Finnish children born between 2008 and 2012. Children were divided into two cohorts by age at MMR vaccination: children administered early MMR vaccination (11 through 12months of age) and late MMR vaccination (18 through 19months of age). Morbidity was evaluated during the main follow-up period (from 13 to 17months of age) and before any MMR vaccination (3 to 10months) and after all were vaccinated with MMR (20 to 35months) as control follow-up periods. We analyzed all infections and did additional analyzes for urinary tract infections (UTI) and bronchitis. Injuries were analyzed as a control outcome. Early MMR vaccinated children (N=79949) had fewer infections compared to late MMR vaccinated (N=60965) during the main follow-up period. The incidence rate ratio (IRR) was 0.84 (95% confidence interval (95% CI) 0.81-0.87). However, similar differences were also observed during the control follow-up periods. MMR vaccinated children had less UTI in the main follow-up period (IRR 0.73, 0.60-0.89) but not in the control follow-up periods. When stratified by sex, the difference was observed among girls but not in boys. No clear evidence was found for non-specific effects in infectious diseases morbidity. However, there could be a nonspecific effect on UTI. Confirmation is needed from other studies, especially from high-income countries.