According to the study, more than 100 million people in the United States may have COVID-19 by the end of 2020.

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An analysis published Thursday in the journal Nature indicated that more than 100 million people in the United States – or 31% of the population – would be infected with COVID-19 by the end of 2020.

According to researchers from Columbia University’s Mailman School of Public Health, who modelled the virus’s transmission, fewer than one-fourth of all cases were counted due to insufficient testing and a substantial percentage of people being symptom-free.

“The vast majority of infectious were not accounted for by the number of confirmed cases,” study co-author Jeffrey Shaman said in a press release.

“It is these undocumented cases, which are often mild or asymptomatic infectious, that allow the virus to spread quickly through the broader population,” said Shaman, a professor of environmental health sciences at Columbia University.

Official estimates indicated that more than 25 million people in the United States were infected with COVID-19 in 2020. It was acknowledged, however, that this could be an undercount due to lack of testing early in the pandemic, as well as asymptomatic cases throughout the year, according to the Centers for Disease Control and Prevention.

For this study, the researchers simulated the transmission of the coronavirus within and between all 3,142 U.S. counties using population, mobility and confirmed case data.

Only about 22% of all infections that occurred in 2020 were counted, meaning more than 103 million people may have actually had the virus by the end of the year, the data showed.

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Infections were more widespread in some areas of the country, with parts of the upper Midwest and Mississippi valley, including the Dakotas, Minnesota, Wisconsin and Iowa, having more than 60% of the populations there infected by the end of last year, the researchers said.

In addition, as many as 48% of Chicago residents , 52% of Los Angeles residents, 42% of Miami residents, 44% of New York City residents and 27% of Phoenix residents most likely were infected over the course of the year, they said.

These cities peaked at different times, with New York and Chicago having heavy spring and fall-winter waves, but little activity during summer, while Los Angeles and Phoenix experienced waves in the summer and fall-winter periods.

Miami experienced waves during all three periods.

At the end of 2020, one in 130 people nationally had COVID-19 and was contagious at year’s end, though in some metropolitan areas the percentage of individuals who were contagious was much higher, the researchers said.

Fatality rates fell due in large part to improved treatments for severe illness, including the use of steroids, and public health measures designed to limit disease spread such as masking and social distancing, according to the researchers.

The percentage of infected people who died from COVID-19 fell from 0.8% in the spring of last year to 0.3% by Dec. 31.

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Urban areas such as New York City that saw a peak in infections in the early stages of the pandemic saw the worst numbers in terms of deaths because of delays in testing availability and masking mandates, overwhelmed hospitals and lack of effective treatments, the researchers said.

Based on their findings for last year, in 2021, the virus will continue to spread to those who haven’t yet been infected, they said.

Although vaccines protect against severe and fatal disease, breakthrough infections, including those that are mild or asymptomatic, will likely contribute to the spread of the virus.

In addition, new, more contagious variants such as the Delta strain could make reinfection and breakthrough infections more likely.

“While the landscape has changed with the availability of vaccines and the spread of new variants,” study co-author Sen Pei said in a press release.

“It is important to recognize just how dangerous the pandemic was in its first year,” said Pei, an assistant professor of environmental health sciences at Columbia University.

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