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

Background: 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 18 months to 12 months of age. This change created a natural experiment for evaluating the potential non-specific effects.

Methods: 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 12 months of age) and late MMR vaccination (18 through 19 months of age). Morbidity was evaluated during the main follow-up period (from 13 to 17 months of age) and before any MMR vaccination (3 to 10 months) and after all were vaccinated with MMR (20 to 35 months) 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.

Results: 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.

Conclusion: 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.