Although the COVID-19 pandemic has been the deadliest global viral outbreak in the last century, such extreme events aren’t as rare as many may think—according to the statistics, that is.

In a new analysis of novel disease outbreaks over the last 400 years, published in the Proceedings of the National Academy of Sciences (PNAS), a newly assembled record of past outbreaks without any known medical interventions was used to estimate the intensity of such extreme events and the yearly probability of them recurring.

It uncovered that there is a two per cent chance of a pandemic with a similar impact to COVID-19 happening in any year, meaning that someone born in the year 2000 would have about a 38 per cent chance of experiencing a pandemic by now—a frequency that even the researchers had not anticipated.

“We were really surprised when we first modelled it,” said William Pan, who is the paper’s co-author and an associate professor of global environmental health at Duke University. 

So much so that the team first questioned the completeness of their dataset, which originally extended all the way back to the High Middle Ages. However, to ensure that they had a robust dataset, the team honed in on the years between 1600, when public records became more commonly used to track births, deaths and the causes of death, and 1945, which the researchers defined as the advent of antibiotics.

“This allowed us to be more confident that we had data that are consistent over time,” he added.

Led by Marco Marani, an adjunct professor in the Earth and Climate Sciences Division at Duke, the study applied a novel method that Marani and his team had developed to describe how very rare extreme weather events return in time.

Their analysis, which covered a murderer’s row of pathogens such as plague, smallpox, cholera, typhus and novel influenza viruses, found considerable variability in the rate at which pandemics have occurred in the past.

But they also identified patterns that allowed them to describe the probabilities of similar-scale events happening again.

For example, the probability of a pandemic akin to the deadliest pandemic in modern history—the Spanish flu, which killed more than 30 million people between 1918 and 1920—occurring ranged from 0.3 per cent to 1.9 per cent per year over the period studied. In other words, this means that it is statistically likely for a pandemic of such an extreme scale to happen again within the next 400 years.

And it’s not just that. Data from the study have also shown that the risk of intense outbreaks is growing rapidly.

Based on the increasing rate at which novel pathogens, such as SARS-CoV-2, have spread through human populations in the past 50 years, the study estimates that the probability of novel disease outbreaks will likely grow three-fold in the next few decades.

Using this increased risk factor, the researchers have estimated that a pandemic on a similar scale to COVID-19 is likely to happen within 59 years—a result that, according to them, is “much lower than intuitively expected”.

The researchers also calculated something that isn’t included in their PNAS paper: the probability of a pandemic that is capable of eliminating all human life… which is statistically likely within the next 12,000 years.

But that doesn’t mean a pandemic like COVID-19 will not happen again in the next 59 years or that something similar to the Spanish flu will not strike for another 300 years.

“Such events are equally probably in any year during the span,” said Gabriel Katul, the Theodore S Coile Distinguished Professor of Hydrology and Micrometeorology at Duke and one of the paper’s authors. “When a 100-year flood occurs today, one may erroneously presume that one can afford to wait another 100 years before experiencing another such event. This impression is false. One can get another 100-year flood the next year.”

As an environmental health scientist who studies how human and environmental dynamics influence health outcomes, Pan can speculate on the reasons outbreaks are becoming more frequent. He cites population growth, changes in food systems, environmental degradation and more frequent contact between humans and disease-harbouring animals as significant factors. And outbreaks like the recent re-emergence of the bat-borne Nipah virus in India are what he is particularly concerned about.

“There is a lot more concern about such zoonotic pathogens becoming more pathogenic and more transmissible in humans because if they do, we’ll be in much more trouble,” said Pan.

He emphasises that although their PNAS study sought only to characterise the risks of a pandemic with their statistical analysis and not to explain what is driving them, he hopes that this will spark a deeper exploration of the factors that may be making devastating pandemics more likely and how to counteract them.

“To me, understanding how environmental change influences the emergence of disease is key,” Pan added. “There has been a lot of research suggesting that most of the zoonotic infections that have emerged in the last 20 years are caused by human encroachment into natural landscapes. Understanding why particular diseases become a global pandemic is an important area of study that, I think, we don’t have quite a good grasp on.”

With the probability only growing as time passes, Pan and his co-authors emphasise the need to adjust the public’s perception of pandemic risks and expectations for preparedness.

Pan said, “The most important takeaway is that large pandemics like COVID-19 and the Spanish flu are relatively likely. Understanding that pandemics aren’t so rare should raise the priority of efforts to prevent and control them in the future.”