The epidemic dynamics of three childhood infections and the impact of first vaccination in 18th and 19th century Finland

Childhood infectious such as smallpox or measles have devasted human populations, but our knowledge on the history of public health interventions remains limited. Here, we use 100 years of newly available records in 18th and 19th century Finland to investigate the epidemic dynamics of three infections, smallpox, pertussis and measles and the impact of the countrys first nationwide vaccination programme. Between 1750 and 1850, we found over 40 epidemics of smallpox, pertussis and measles, which together were responsible for almost 20% of all registered deaths under age 10. The start of the first vaccination programme against smallpox in 1802 promptly triggered five major changes in smallpox epidemiology: (i) decreasing mortality, (ii) increasing age at infection, (iii) increasing time between epidemics, (iv) increasing fade-outs, which all result from (v) decreased pathogen transmission. In contrast, most epidemic characteristics of the childhood infections without vaccines, pertussis and measles, changed in the opposite direction to that of smallpox: their death toll almost doubled, their reproduction number increased and pertussis, but not measles, showed a decreasing age at infection and accelerating epidemics. Our study captures a rare glimpse of the epidemiology of childhood infections in historical populations and the possible long-term impact of major public health interventions.