Black Death’s rapid spread in 2nd outbreak could have lessons for COVID-19, study says

A new study of thousands of personal wills, parish registers and other documents on deaths over a 300-year span reveals that plague outbreaks in England spread four times faster in the 17th century than they did in the 14th century.

The number of people infected by the Black Death of 1348 doubled about every 43 days, but during the second outbreak, coined the Great Plague of 1665, the number doubled every 11 days, according researchers with McMaster University in Ontario, Canada.

Both diseases — characterized by black boils that oozed blood and pus — were caused by the same bacterium, Y. pestis.

Why the plague epidemics’ growth rate increased fourfold over time is not clear, but the researchers speculate several different reasons: bacterial mutations, dramatic climate differences, growing population densities or different modes of transmission from one century to the next.

The team says records of epidemics past can offer lessons for understanding the transmission patterns of COVID-19 and other modern outbreaks, and how factors that drive their spread have changed over time. Diving deep into epidemic growth rates, such as those explored in the Canadian study, have been at the center of public health policy and discussion for the current pandemic.

The study was published Monday in the journal Proceedings of the National Academy of Sciences.

“It is an astounding difference in how fast plague epidemics grew,” study lead author David Earn, a statistics professor at McMaster University, said in a news release. “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.”

“No one living in London in the 14th or 17th century could have imagined how these records might be used hundreds of years later to understand the spread of disease,” Earn added.

The concept of a disease getting worse the second time around mirrors that of the current pandemic, but on a much smaller time frame. Experts have been saying for months that the introduction of cooler temperatures and resulting changed behaviors could lead to a more intense wave of coronavirus cases and deaths.

The team of statisticians, biologists and evolutionary geneticists says the plagues likely spread via bites from fleas (bubonic plague) that got infected from rats, rather than human-to-human contact (pneumonic transmission).

However, the researchers admit they cannot confirm or deny this hypothesis. But it does offer an explanation behind the expedited spread in the 17th century.

Disease spread in the 14th century could have been driven by rats and fleas, “with human infection and mortality as a secondary consequence.” Then in the 17th century when the plague spread to people’s lungs, the resulting pneumonia could have made human-to-human spread easier and faster via coughing, like the novel coronavirus today.

Dr. William Schaffner, an epidemiologist at Vanderbilt University in Tennessee, told CNBC that he expects the U.S. to experience “a substantial third wave” of infections complicated by winter spread of the seasonal flu and COVID-19 fatigue.

“I’m pretty glum at the moment because it does look as though in the majority of states there’s an increasing number of cases,” Schaffner told the outlet. “There’s a growing sense of coronavirus fatigue out there. People really want to get back to the old normal.”

It’s important to note that COVID-19 has an “infection fatality ratio” that’s “substantially lower than for the 1918 influenza and much lower than for the historical plague epidemics studied here,” the researchers wrote in their study.

“Nevertheless, COVID-19 has had a much greater impact than seasonal influenza epidemics, which cause of order 500,000 deaths worldwide annually,” they continued.

Other explanations behind the plague’s expedited spread

The researchers also hypothesize that the bacterium that caused the Black Death outbreaks could have evolved to become more infectious. But they said they aren’t sure how the “evolution of resistance” could have accelerated epidemics over centuries.

Another idea is that human population size and density increased “enormously” from the 14th to 17th centuries, which could have aided the spread of infectious diseases because of crowded living conditions.

Lastly, the researchers cite a wave of cooler temperatures that struck Northern Europe in the 17th century that could have affected transmission. But again, the team notes it’s hard to make that conclusion “due to the lack of consensus about climate and weather variations in Medieval/Renaissance Europe.”