Statistics show major pandemics are more likely than we thought

The COVID-19 pandemic is perhaps the deadliest viral outbreak the world has seen in over a century. But statistically, such extreme events are not as rare as one might think, says a new analysis of new disease outbreaks over the past 400 years.

The study, published the week of August 23 in the Proceedings of the National Academy of Sciences, used a new record of past epidemics to estimate the intensity of these events and the annual likelihood of recurrence.

He found that the likelihood of a pandemic having an impact similar to COVID-19 is around 2% each year, meaning that a person born in the year 2000 would have about a 38% chance of undergo one to date. And that likelihood is only increasing, which the authors say underscores the need to adjust pandemic risk perceptions and preparedness expectations.

“The most important takeaway is that major pandemics like COVID-19 and the Spanish flu are relatively likely,” said William Pan, Ph.D., associate professor of global environmental health at Duke and one of the co -authors of the article. Understanding that pandemics are not that rare should increase the priority of efforts to prevent and control them in the future, he said.

The study, led by Marco Marani, Ph.D., of the University of Padua in Italy, used new statistical methods to measure the scale and frequency of epidemics for which there was no medical intervention. immediate over the past four centuries. Their analysis, which covered a row of pathogens from a murderer including plague, smallpox, cholera, typhus and new influenza viruses, found considerable variability in the rate at which pandemics have spread. produced in the past. But they also identified patterns that allowed them to describe the probabilities of events of similar scale recurring.

In the case of the deadliest pandemic in modern history – the Spanish flu, which killed over 30 million people between 1918 and 1920 – the probability of a pandemic of a similar magnitude occurring ranged from 0, 3% to 1.9% per year over the period studied. . Taken another way, these numbers mean that it is statistically likely that a pandemic of such an extreme magnitude would occur within the next 400 years.

But the data also shows that the risk of intense outbreaks is increasing rapidly. Based on the increasing rate at which new pathogens such as SARS-CoV-2 have released into human populations over the past 50 years, the study estimates that the likelihood of new disease outbreaks is likely to triple over the course of decades to come.

Using this increased risk factor, the researchers estimate that a pandemic of a similar magnitude to COVID-19 is likely within 59 years, a result they write is “much lower than what is expected. intuitively waits “. While not included in the PNAS article, they also calculated the likelihood of a pandemic capable of killing all human life, finding it statistically likely over the next 12,000 years.

This does not mean that we can count on a 59-year reprieve from a COVID-type pandemic, nor that we are immune to a calamity of the magnitude of the Spanish flu for another 300 years. Such events are also likely in any year during the period, said Gabriel Katul, Ph.D., Theodore S. Coile, distinguished professor of hydrology and micrometeorology at Duke and another of the authors. of the article.

“When a 100-year flood occurs today, it’s safe to assume that you can afford to wait another 100 years before experiencing another such event,” Katul explains. “That impression is wrong. We may have another 100-year flood the following year.”

As an environmental health scientist, Pan can speculate as to why epidemics are becoming more frequent, noting that population growth, changes in food systems, environmental degradation, and more frequent contact between humans and disease-carrying animals can all be important factors. He insists on the statistical analysis sought only to characterize the risks, not to explain what motivates them.

But at the same time, he hopes the study will spark further exploration of the factors that may make devastating pandemics more likely and how to counter them.

“This underscores the importance of an early response to epidemics and of building capacity for pandemic surveillance at local and global scales, as well as establishing a research agenda to understand why large epidemics are becoming more common,” said Pan. .

Marani, the lead author of the article, holds an adjunct position at Duke, where he was previously a professor of civil and environmental engineering. Another co-author, Anthony Parolari, Ph.D., of Marquette University, is a former postdoctoral researcher at Duke.