Developing a better mathematical understanding of historical smallpox epidemics in Denmark

The work being done by Nordemics (NordForsk) and PandemiX (Carlsberg Foundation) members on historical Danish epidemics has implications that reach well beyond the borders of Denmark. The Danish national and local archives contain rich and understudied data on past epidemics which may teach us new things about diseases in general, past and present, locally and globally.

Therefore, we collaborate closely with epidemiologists at Princeton University in the US, where Professor Bryan Grenfell is a Scientific Advisory Board member of the NordForsk-funded Nordemics-project. In April and May 2023, Nordemics postdoc Andreas Eilersen has been visiting the Grenfell Lab at Princeton with the goal of developing a better mathematical understanding of historical smallpox epidemics in Denmark in the early days of vaccination around 1800.

The first vaccine against smallpox was invented by the English physician Edward Jenner in 1798 and was then swiftly rolled out internationally, with the Danish vaccination program picking up speed from 1801 onwards. Vaccination drove smallpox to the brink of extinction in Denmark shortly after the introduction of the vaccine, although vaccination only became universal and mandatory in 1810. This means that there must have been a generation of people who were young children in the smallpox-free ”honeymoon period” between 1801 and 1810 who were thus neither vaccinated, nor immune from contracting smallpox. When smallpox returned in 1824 and 1835, we would therefore expect to see this generation being particularly afflicted by the disease. This is indeed confirmed by hospital records from the period, uncovered by Nordemics historian Søren K. Poder. At the same time, we clearly observe that a lacking vaccination coverage is not the only reason for the return of smallpox, as the effect of the vaccine appears to decrease over time.

Together with Bryan Grenfell, Andreas Eilersen is therefore developing a model which attempts to shed light on the relative importance of the ”honeymoon generation” and the waning of vaccine immunity in the smallpox epidemics of the post-vaccination era. The results of this work may tell us more about the lifelong effects of smallpox vaccination. This may in turn prove important in a time when related viruses such as monkeypox are once again public health threats. Thus, the local history of smallpox in Denmark may turn out to be of global importance.