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 country’s 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. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement We thank the funding from the Academy of Finland (TK: 278751; VL: 292368), NordForsk (104910), the Finnish Cultural Foundation (TK) and the Ella & Georg Ehrnrooth Foundation (MB). ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The study used (or will use) ONLY openly available human data that were originally located at: I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes All data produced are available online at: