Tuesday, August 24, 2021

PNAS Research: Intensity and Frequency of Extreme Novel Epidemics


125 Years of Pandemics (Not Including COVID-19) – Credit ECDC 


Although I'm not yet 70 years of age, I've lived through 4 officially declared pandemics (1957, 1968, 2009, and 2020), worked as a paramedic, and was heavily involved in the preparations for the fizzled  `Swine Flu' pandemic of 1976 (see Deja Flu, All Over Again), and worked the `Russian Flu' pseudo-pandemic of 1977 (the return of H1N1 after 20 years). 

When I began this blog in January of 2006, avian H5N1 was just moving out of Southeast Asia into Europe, and we were less than 3 years since the end of the SARS epidemic (see SARS And Remembrance). 

Based on my personal experiences, along with a strong interest in historical plagues and epidemics, I was already convinced that the growing threat of emerging infectious diseases - and the occasional pandemic - would play a large role in the history of the 21st century.  

A conclusion reinforced by reading the works of the late (May 22, 1936 - May 15, 2014) anthropologist and researcher George Armelagos of Emory University (see The Third Epidemiological Transition), who posited that humans were entering the age of newly emerging infectious diseases, re-emerging diseases carried over from the 2nd transition, and a rise in antimicrobial resistant pathogens.

In a 2010 paper, Armelagos along with Kristin Harper, updated his original paper.  Both papers are well worth reading. 

Int J Environ Res Public Health. 2010 February; 7(2): 675–697.

Published online 2010 February 24. doi: 10.3390/ijerph7020675.

The Changing Disease-Scape in the Third Epidemiological Transition

Kristin Harper and George Armelagos

Since starting this blog, we've seen two declared pandemics (H1N1 in 2009, COVID in 2020), several major outbreaks of emerging and re-emerging infectious diseases (Ebola, H7N9, H5N6, Zika, Measles, etc.), multiple epizootics (ASF, H5N8 in Europe, Asia, the Middle East, & North America), and a steady rise in antimicrobial resistant pathogens. 

All of which leads me to believe the biggest barrier that might prevent me from seeing a 5th pandemic is my lifetime of Southern cooking and an elevated cholesterol level. 

In an attempt to better quantify the risks - or odds - of seeing another pandemic in the near future, researchers at the University of Padova (Italy) - and Duke University and Marquette University here in the United States - have put together a statistical analysis on the probability in any given year of seeing a major pandemic. 

Although we have seen prolonged gaps between pandemics (1918 to 1957 is one example), in the last 50 years we've also seen two gaps of just over 10 years (1957 to 1968 and 2009 to 2020).  There were, of course, numerous other `close calls' over those 5 decades, with regional and global epidemics, including HIV, SARS-CoV, Zika, and Ebola in West Africa. 

Using math skills far above my pay grade, researchers at these three universities calculated that the risks of seeing another COVID-level pandemic in any given year is roughly 2% . . . and suggest that rate may increase 3-fold over the next few decades.

The full research article is behind a paywall, but I've reproduced the Link and Abstract below, after which you'll find a link and excerpts to a press release from Duke University. 

Intensity and frequency of extreme novel epidemics

Marco Marani, Gabriel G. Katul, William K. Pan, and Anthony J. Parolari

PNAS August 31, 2021 118 (35) e2105482118; https://doi.org/10.1073/pnas.2105482118

Edited by Simon Asher Levin, Princeton University, Princeton, NJ, and approved July 15, 2021 (received for review March 21, 2021)


Observational knowledge of the epidemic intensity, defined as the number of deaths divided by global population and epidemic duration, and of the rate of emergence of infectious disease outbreaks is necessary to test theory and models and to inform public health risk assessment by quantifying the probability of extreme pandemics such as COVID-19. Despite its significance, assembling and analyzing a comprehensive global historical record spanning a variety of diseases remains an unexplored task. 

A global dataset of historical epidemics from 1600 to present is here compiled and examined using novel statistical methods to estimate the yearly probability of occurrence of extreme epidemics. Historical observations covering four orders of magnitude of epidemic intensity follow a common probability distribution with a slowly decaying power-law tail (generalized Pareto distribution, asymptotic exponent = −0.71). The yearly number of epidemics varies ninefold and shows systematic trends. Yearly occurrence probabilities of extreme epidemics, Py, vary widely: Py of an event with the intensity of the “Spanish influenza” (1918 to 1920) varies between 0.27 and 1.9% from 1600 to present, while its mean recurrence time today is 400 y (95% CI: 332 to 489 y).

The slow decay of probability with epidemic intensity implies that extreme epidemics are relatively likely, a property previously undetected due to short observational records and stationary analysis methods. Using recent estimates of the rate of increase in disease emergence from zoonotic reservoirs associated with environmental change, we estimate that the yearly probability of occurrence of extreme epidemics can increase up to threefold in the coming decades.

          (Continue . . . )

 The press release, summary, follows:

Statistics say large pandemics are more likely than we thought

Most people are likely to experience an extreme pandemic like COVID-19 in their lifetime 

DURHAM, N.C. -- The COVID-19 pandemic may be the deadliest viral outbreak the world has seen in more than a century. But statistically, such extreme events aren’t as rare as we may think, asserts a new analysis of novel disease outbreaks over the past 400 years.

The study, appearing the week of Aug. 23 in the Proceedings of the National Academy of Sciences, used a newly assembled record of past outbreaks to estimate the intensity of those events and the yearly probability of them recurring.

It found the probability of a pandemic with similar impact to COVID-19 is about 2% in any year, meaning that someone born in the year 2000 would have about a 38% chance of experiencing one by now. And that probability is only growing, which the authors say highlights the need to adjust perceptions of pandemic risks and expectations for preparedness.

“The most important takeaway is that large 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 paper’s co-authors. Understanding that pandemics aren’t so rare should raise 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 disease outbreaks for which there was no immediate medical intervention over the past four centuries. Their analysis, which covered a murderer’s row of pathogens including 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.

In the case of the deadliest pandemic in modern history – the Spanish flu, which killed more than 30 million people between 1918 and 1920 -- the probability of a pandemic of similar magnitude occurring ranged from 0.3% to 1.9% per year over the time period studied. Taken another way, those figures mean it is statistically likely that a pandemic of such extreme scale would occur within the next 400 years.

But the data also show the risk of intense outbreaks is growing rapidly. Based on the increasing rate at which novel pathogens such as SARS-CoV-2 have broken loose in 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 estimate that a pandemic similar in scale to COVID-19 is likely within a span of 59 years, a result they write is “much lower than intuitively expected.” Although not included in the PNAS paper, they also calculated the probability of a pandemic capable of eliminating all human life, finding it statistically likely within the next 12,000 years.

That is not to say we can count on a 59-year reprieve from a COVID-like pandemic, nor that we’re off the hook for a calamity on the scale of the Spanish flu for another 300 years. Such events are equally probable in any year during the span, said Gabriel Katul, Ph.D., the Theodore S. Coile Distinguished Professor of Hydrology and Micrometeorology at Duke and another 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,” Katul says. “This impression is false. One can get another 100-year flood the next year.”

(Continue . . . )

In the 1950s and 1960s, the biggest threat to mankind appeared to be nuclear annihilation, and nations spent trillions of dollars beefing up their Departments of Defense. 

Today that risk has greatly receded, but it has been replaced by a myriad of new threats, including abrupt climate change, emerging infectious diseases, cyber-terrorism, bioterrorism, and potentially catastrophic space weather events. 

If we hope to get through this 21st century reasonably intact, we need to make similar commitments to funding national, regional, and global Departments of Resilience.  If we continue to rely on the token `preparedness' efforts of the past, we are doomed to failure.