Epidemic patterns revealed through three centuries of smallpox death records

HAMILTON, Ontario — The eradication of smallpox in 1980 is one of the greatest public health successes of all time. “The greatest killer,” as the virus behind this disease is nicknamed, causes death in about three of every ten people it infects. Disability and disfigurement occur in a majority of those who survive. Although it is believed that the only remaining viral samples of smallpox are contained in laboratories in the United States and Russia, the potential for this virus to be used as a bioterrorist agent in the future remains a possibility.

To better understand how intentional interventions and historical events can influence the dynamics of a disease like smallpox, a team of researchers from McMaster University analyzed nearly three centuries of records documenting deaths from smallpox in London. The study, published in the journal PLOS Biology, provides important insight into the changing patterns of infectious disease transmission over time.

“Our goal was to describe and make publicly available the weekly time series of smallpox mortality in London and to identify historical events that might have influenced smallpox dynamics over the centuries,” study author David Earn explains in a statement.

Epidemic patterns uncovered using nearly 300 years’ worth of data

For their analysis, the authors digitized more than 13,000 weekly smallpox death records published in the London Bills of Mortality (LBoM) and the Registrar General’s Weekly Returns (RGWRs) between 1664 and 1930. Since 1930 was the last year when more than one smallpox death was reported in a single week in London, the authors do not present data or analyses after that year.

The researchers used a variety of methods to uncover patterns in their data. One method, called spectral analysis, involves computational techniques to accurately determine the periodicity, or recurrence, of smallpox deaths at regular intervals during the target time period. Another method used in the study is called a seasonal heat map. This technique produces a visual representation of how outbreak peaks change over time with the seasons.

In the study, the authors use a combination of the two techniques to hone in on specific patterns in smallpox evolution over the centuries, specifically with regard to the time between epidemics, the size of outbreaks, and the season during which epidemics occurred. By further superimposing the data onto a timeline of historical events and uptake of smallpox interventions, the authors identify how these changes in epidemic patterns are correlated with changes in control interventions and public health policies.

An interesting feature of the data analyzed by this group is the dramatic change that occurred during the time range studied. The earliest data were collected from an era before any public health practices to prevent the spread of disease were in place. On the other hand, the most recent data reflect a time after the introduction of the smallpox vaccine, as the number of deaths from smallpox declined and ultimately disappeared.

“During the time period covered by the data, smallpox changed from a terrifying and unavoidable danger to an easily preventable infection. Introduction of better control measures, especially vaccination, naturally led to decreased smallpox mortality and eventually eradication,” notes Earn.

A number of major demographic and social changes also occurred in London during this time period. The authors suggest that major historical events, such as the Industrial Revolution, could potentially have had an impact on smallpox dynamics due to increases in population size and density, which could affect virus transmission and epidemic patterns.

How the smallpox epidemic ties into the COVID-19 crisis

Study authors emphasize that the introduction of the smallpox vaccine was a key factor for the successful eradication of smallpox. They also note other factors that made eradication possible including the absence of an animal reservoir, relatively low infectivity, and the presence of visible symptoms with each infection.

Smallpox is one of only two infectious diseases to have been completely wiped out through human efforts.

More broadly, the authors remark on the importance of considering the impact of intentional interventions and coincident events on disease dynamics when attempting to control the spread of any infection. This advice is especially relevant for the current COVID-19 pandemic, which the authors say has caused a surge of interest in the study of how public health interventions can influence the course of disease evolution.