What Is a Second Wave of a Pandemic, and Could It Happen for COVID-19?

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Colorized scanning electron micrograph of an apoptotic cell (purple) heavily infected with SARS-CoV-2 virus particles (yellow), isolated from a patient sample. (coronavirus) Image captured at the NIAID Integrated Research Facility (IRF)...
Courtesy, NIAID

This article was originally published May 15, 2020, and updated October 15, 2020.

When a global outbreak of disease begins to slow, feelings of relief and a sense of moving on ensue. But historically, based on past unusually deadly for 20- to 40-year-olds. The influenza (H1N1) pandemic of 2009 had two waves, the second of which caused significantly more illness than the first.

There was much speculation about whether a second wave of disease would sweep the world during the COVID-19 pandemic. To investigate this possibility, scientists developed models of different intervention scenarios and found that the likelihood of an additional round of illness and death by COVID-19 increased significantly when interventions to limit the spread of disease were relaxed. In particular, experts warned, the premature reopening of the economy in areas where new cases of disease continued to be reported all but ensured that a second wave would happen.

China, where the COVID-19 outbreak was first detected, was the first country to move past the initial wave of disease, a milestone of relief that came in the first half of February 2020. But, just weeks after newly reported cases dropped to zero in many places in China, researchers became concerned about re-emergence of the disease particularly in areas where restrictions on social distancing and travel had been eased. These concerns were realized in mid-May, when a cluster of new COVID-19 cases was detected in western Wuhan, after a stretch of just 35 days without new infections.

In late September 2020, while countries such as the United States and India were still in their first wave of the COVID-19 pandemic, health officials declared the emergence of a second wave of the disease in Europe. In most European countries, daily cases were higher during the second wave, compared with the first.

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