Study Examines Spread of Epidemics Before and After Vaccine

An international team of researchers recently published a study that shows that before the introduction of a vaccine, measles could persist in large population centers and spread among sets of smaller towns. Max Lau, assistant professor at the Rollins School of Public Health, was first author of the article, published in the journal Nature Ecology & Evolution.

The model details measles dynamics on over 40 years of data collected in England and Wales and provides data on the importance of spatial modeling for the long-term control of global epidemics. This could help inform the long-term public health response to the current COVID-19 pandemic.

“Innovating new models for understanding the balance between different sources of measles spread is critical for designing control strategies and having access to this unique dataset allowed us to test our new model of measles dynamics with unprecedented rigor,” said Lau. “Going forward, we can apply what we learn from this test case to understand disease spread beyond measles.”

Prior to the introduction of a vaccine, the number of measles cases in England and Wales would undergo periodic (often biennial) epidemics. This pattern, driven by herd immunity, is common among a number of diseases and in other locales. The researchers sought to identify the causes of these periodic spikes and locate the reservoirs where the virus is maintained in the dips after major epidemics, which are the sources for reintroduction of the virus into the general populace in the next major epidemic. This persistence question is central to understanding the dynamics of measles and other viral diseases and for coordinating public health interventions.

The research team combined spatial modeling with the detailed historical data of measles cases in England and Wales to address these questions. The uniquely detailed dataset includes weekly measles reports from almost a thousand locations across England and Wales beginning in 1944 and continuing until the disease was all but locally eliminated by vaccination in the 1990s. 

The researchers’ new model quantifies the relative influence of different sources of infection, including major cities, spread among smaller towns, and unidentifiable outside sources. Following the introduction of vaccination, the source of reintroduction shifted from a combination of large centers and local metapopulation spread to mainly unidentifiable sources possibly outside of England and Wales. Beyond the COVID-19 pandemic, the models also could help scientists understand how diseases could survive and spread at a time when a portion of the public is opposed to vaccines, according to co-author C. Jessica E. Metcalf, assistant professor of ecology and evolutionary biology and public affairs at Princeton University. 

The researchers added that wide perspective should be taken when applying the results to other diseases.

In addition to Lau and Metcalf, the research team included Ottar N. Bjørnstad, distinguished professor of entomology and biology at Penn State; Bryan T. Grenfell, the Kathryn Briger and Sarah Fenton Professor of Ecology and Evolutionary Biology and of Public Affairs at Princeton University; Princeton PhD candidates Alexander Becker and Hannah M. Korevaar and postdoctoral researcher Quentin Caudron; and Darren J. Shaw from the University of Edinburgh. 

The research was supported by the RAPIDD Program of the US Department of Homeland Security and the Fogarty International Centre, the US National Institutes of Health, the Eunice Kennedy Shriver National Institute of Child Health & Human Development of the National Institutes of Health, and the US National Science Foundation.