Scientific American:

On March 11 the World Health Organization officially designated the novel coronavirus outbreak a pandemic. Defined as the worldwide spread of a new disease, such a declaration is the first to be made since the 2009 H1N1 swine flu. As of this writing, there have been approximately 336,000 confirmed cases of the new disease, called COVID-19, resulting in more than 14,600 deaths worldwide.

Although a coronavirus—a family of viruses that cause illnesses ranging from the common cold to severe acute respiratory syndrome (SARS)—had not previously triggered a pandemic, this is not the first time we have seen the global transmission of a serious disease. Studying past outbreaks can help scientists better estimate the trajectory of COVID-19 and identify the best measures to slow its spread.

“Historically, we could look at everything back to the 1918 influenza pandemic. But in more contemporary times, we’d be looking at the 2015–2016 Zika outbreak in Central and South America, the global SARS outbreak from 2002 to 2003 and the Ebola outbreak in West Africa from 2014 to 2016,” says Jeremy Youde, dean of the College of Liberal Arts at the University of Minnesota Duluth and an expert on global health politics.

Whereas COVID-19 is caused by a coronavirus and not an influenza virus, the 1918 flu pandemic—which caused at least 50 million deaths worldwide, according to the Centers for Disease Control and Prevention—might be the best model to understand this novel pathogen’s behavior. It is also an outbreak for which massive social interventions were undertaken.

“Past influenza pandemics give some sense of what the overall [trajectory] of a virus like this would be because the reproductive number of this virus”—defined as how many people each infectious person transmits the disease to in a completely susceptible population—“is pretty similar to that of a pandemic flu,” says Marc Lipsitch, a professor of epidemiology and director of the Center for Communicable Disease Dynamics at Harvard University. Although it is difficult to determine exact figures for an emerging disease, reports put the reproductive number of COVID-19 between 2 and 2.5. The median reproductive number for the 1918 flu pandemic was around 1.8. Lipsitch estimates that between about 20 and 60 percent of the global population will ultimately become infected with the novel coronavirus, or SARS-CoV-2.

Although every virus and resulting disease is different, a look at epidemic dynamics of both COVID-19 and the 1918 flu points to similar successful containment procedures. In a 2007 study published in JAMA, Howard Markel of the Center for the History of Medicine at the University of Michigan Medical School and his co-authors analyzed the excess deaths from pneumonia and influenza (meaning how many more there were than usual during nonpandemic years) in 43 U.S. cities from September 8, 1918 through February 22, 1919. Despite the fact that all of the cities implemented nonpharmaceutical interventions, it was the timing of activation, the duration and the combination of measures that determined their success. The researchers found “a strong association between early, sustained, and layered application of [such] interventions and mitigating the consequences of the 1918–1919 influenza pandemic in the United States.”

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