Using mortality data collected over the course of 300 years, a pair of researchers have traced the history of smallpox epidemics in London between 1664 and 1930.

Their data analysis, detailed Monday in the PLOS Biology, revealed links between several historical events and smallpox outbreaks, as well as the effects of various control interventions and public health policies on epidemic patterns.

Smallpox was one of history's deadliest viruses, dating to Ancient Egypt. It is one of only two viruses that have been entirely eradicated as a result of human intervention. Before its eradication, the virus is estimated to have killed as many as 300 million people during the 20th century.

For the new study, researchers digitized weekly mortality rates recorded in two invaluable sources, the London Bills of Mortality, which provided data through 1842, and the Registrar General's Weekly Returns, which provided data after 1842.

"Our study shows very clearly the value of disease surveillance," study co-author David Earn, a professor of mathematics and statistics at McMaster University in Canada, told UPI in an email.

"We are now gaining insights about disease transmission patterns, because of the systematic record keeping by the city of London over hundreds of years."

"If these records had been lost, we would not have been able to discover how the shapes and sizes of epidemics changed over time, and would not have been able to identify associations with historical and environmental events," said Earn, who specializes in the modeling of infectious disease transmission.

The newly digitized datasets revealed the deadly impacts of smallpox prior to the advent of any kind of coordinated public health policies.

During the 18th century, It's estimated that 400,000 Europeans died from the disease each year. The virus killed three in every 10 infected. Those who survived often were left crippled and blind.

"Before the vaccine era, smallpox accounted for a substantial proportion of deaths," Earn said.

In addition to digitizing weekly smallpox mortality data, Earn and his research partner, Olga Krylova, a former doctoral student at McMaster, compiled a timeline of historical events that could have influenced the dynamics of specific smallpox outbreaks.

Their efforts showed an especially large outbreak in London was likely triggered or accelerated by activities related to the Franco-Prussian War.

The study co-authors also produced a timeline of public health interventions, including variolation and vaccination efforts.

Variolation, which preceded the development of vaccines, involved purposefully infecting people with small amounts of virus to elicit a protective immune response.

The introduction of variolation had a significant impact on the scope smallpox outbreaks in London, and according to the study's authors, the immunization technique set the stage for vaccination efforts, and ultimately, the eradication of the deadly disease.

Researchers hope their work will inspired other researchers to take a closer look at the changes in the dynamics of smallpox outbreaks over the course of the 19th and 20th centuries. The co-authors also hope their analysis will also serve as a reminder of the value of comprehensive record keeping.

"Much more detailed data have been, and continue to be, collected during the present COVID-19 pandemic," Earn said. "In the coming years, as we look back at these data, we will learn a great deal."

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