Learning from the spread of measles

Researchers in the US and the UK have modelled the dynamics of measles to try to better understand how epidemics spread and persist before and after introduction of a vaccine.

Analysing 40 years of data collected in England and Wales quantified the relative influence of different sources of infection, including major cities, spread among smaller towns, and unidentifiable outside sources. 

It also provided data on the importance of spatial modelling for the long-term control of global epidemics, the researchers say, and could help inform the long-term public health response to the current COVID-19 pandemic.

The findings are published in the journal Nature Ecology & Evolution.

Prior to the introduction of a vaccine in the 1960s, 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 say. 

They sought to locate the reservoirs where the virus persists in the dips between epidemics, which are the sources for the 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 team combined spatial modelling with the detailed historical data of measles cases in England and Wales to address these questions. The detailed dataset includes weekly reports from almost a thousand locations beginning in 1944 and continuing until the disease was all but locally eliminated by vaccination in the 1990s.

“Previous work stressed the importance of large centres as sources of infection,” said Bryan Grenfell from Princeton University, US, one of the research leaders. 

“However, our new modelling shows that local spread among smaller towns can also contribute to persistence of the virus.”

Grenfell says measles has always been the “model organism” of epidemic dynamics and, with influenza, a paradigm for understanding herd immunity. As such, the modelling may have value in understanding the impact the eventual development of a vaccine for COVID-19 might have on its dynamics.

More broadly, suggests Princeton’s Jessica Metcalf, the models could help scientists understand how diseases survive and spread at a time when a portion of the public is opposed to vaccines

“Understanding the drivers of persistence is also of growing importance in a context of growing vaccine hesitancy, which further complicates dynamics and amplifies the challenges of control,” she says.

The researchers stress, however, that a wide perspective should be taken when applying the results to other diseases.

“Our model and previous experience highlights the complexity of globally eradicating a virus,” says Ottar Bjørnstad, from Penn State University, US

“Smallpox was eradicated by 1977 through a massive global effort of mass-vaccination of all children, followed by targeted efforts in regional hotspots and finally local quarantining and ring vaccination to squash the scourge. 

“Polio, in contrast, while also targeted through vaccination for more than 50 years keeps escaping ‘the final blow’ as it successfully shifts and diffuses across regional pockets of susceptible individuals to evade eradication.”