From the 14th to the 17th centuries, Britain was struck by frequent plagues. Although such outbreaks would be expected to get slower and less deadly with time, new research shows the disease actually spread more rapidly in later outbreaks, even as the death rate fell. The reasons are not entirely clear, but they could have disturbing implications for controlling Covid-19.

An estimated 30 to 60 percent of the population of Britain died within three years of the Black Death's arrival in 1348. According to McMaster University’s Professor David Earn, the rate of death doubled every 43 days from its beginnings to the peak.

Although the plague never killed as many people again, subsequent outbreaks were actually faster, Earn concludes in the Proceedings of the National Academy of Sciences. By the Great Plague of 1665, the doubling rate was every 11 days. "It is an astounding difference in how fast plague epidemics grew," Earn said in a statement. All before planes, trains, or automobiles.

"From genetic evidence, we have good reason to believe that the strains of bacterium responsible for plague changed very little over this time period, so this is a fascinating result," said co-author Professor Hendrik Poinar. 

Early plague outbreaks were so widespread that many survivors are thought to have carried partial genetic resistance that they passed on to their children, possibly explaining why subsequent outbreaks affected and killed fewer people. The invention of quarantine also probably helped, making the more rapid spread of later outbreaks a puzzle.

Earn and Poinar drew on published death records for London after 1538 but had to rely on less direct evidence for earlier eras. “At that time, people typically wrote wills because they were dying or they feared they might die imminently, so we hypothesized that the dates of wills would be a good proxy for the spread of fear, and of death itself,” Earn said. 

The authors compare the idea with studies tracking disease spread through Google searches of symptoms. Although wills were largely restricted to property-owning men, the authors validated their methods, confirming that the death rate as measured by will-writing closely matched the official records for the second half of their study period.

The reasons are unclear. Britain became colder through this period, and after London's population rebounded from the first plague, the greater density of humans or rats may have accelerated transmission. Whether such differences could account for such a large difference is another matter.

Earn and Poinar work supports the claim that in the 14th century, the plague spread via fleas infesting rats or other animals. One possible explanation for the faster subsequent spread would be if human-to-human transmission became more common. 

How much relevance an understanding of a bacterial pandemic with fatality rates above 50 percent has for a much less lethal virus is uncertain. Nevertheless, the paper does cast doubt on the idea that Covid-19 will spread more slowly through a previously exposed population in future waves.