Black Death spread four times faster in second London outbreak

The plague spread across London around four times faster in the 17th Century than it had in the 14th Century, a new study suggests.

Researchers found an acceleration in transmission between the Black Death of 1348 – estimated to have wiped out more than one-third of the population of Europe – and later epidemics, which culminated in the Great Plague of 1665.

As well as the epidemics in the 14th and 17th Centuries, there were 18 other plague epidemics in London between 1563 and 1666, though these were all much smaller outbreaks.

They analysed thousands of documents covering a 300-year span of plague outbreaks in the city, and found that, in the 14th Century, the number of people infected during an outbreak doubled approximately every 43 days.

By the 17th Century, the number was doubling every 11 days, according to the study published in the Proceedings of the National Academy of Sciences journal.

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David Earn, a professor in the department of mathematics and statistics at McMaster University, Canada, and investigator with the Michael G DeGroote Institute for Infectious Disease Research, led the study.

“It is an astounding difference in how fast plague epidemics grew," he said.

The researchers, including statisticians, biologists and evolutionary geneticists estimated death rates by analysing historical, demographic and epidemiological data from three sources – personal wills and testaments, parish registers, and the London Bills of Mortality.

“At that time, people typically wrote wills because they were dying or they feared they might die imminently, so we hypothesised that the dates of wills would be a good proxy for the spread of fear, and of death itself," said Prof Earn.

“For the 17th Century, when both wills and mortality were recorded, we compared what we can infer from each source, and we found the same growth rates.

“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.”

“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 study co-author Hendrik Poinar, a professor in the department of anthropology at McMaster.

Researchers suggest the estimated speed of the 14th Century epidemics, along with other information about the biology of plague, indicates that during the plague bacterium did not spread primarily through human-to-human contact, known as pneumonic transmission, in this outbreak.

The scientists suggest that the early epidemic was primarily driven by bubonic transmission – that is, through bites from infected insects – and the later epidemic was primarily driven by pneumonic transmission.

They believe population density, living conditions and cooler temperatures could potentially explain the acceleration, and that the transmission patterns of historical plague epidemics offer lessons for understanding COVID-19 and other modern pandemics.