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How did COVID-19 spread so extensively in the United States?

The United States has now reported well over five million cases of COVID-19, which is more than any other country in the world. A new study from the University of Notre Dame is helping to explain how the coronavirus managed to spread so extensively. 

According to the researchers, more than 100,000 Americans were already infected with COVID-19 by early March, yet only 1,514 cases had been officially reported and a national emergency had not been declared. 

Study lead author Alex Perkins is an associate professor in the Department of Biological Sciences and an expert in Infectious Disease Epidemiology and Population Biology.

“We weren’t testing enough,” said Professor Perkins. “The number of unobserved infections appears to be due to very low rates of case detection during a critical time, when the epidemic was really starting to take hold in this country. Part of it was the availability of testing, but another huge part was case definitions and the fact that they were overly restrictive early on.”

The Centers for Disease Control and Prevention reports that the first confirmed case of COVID-19 in the United States was in January. The early focus was on banning travel to and from Wuhan, China.

“While we were focusing on people coming back from China, we were ignoring people coming from Europe, and actually quite a bit of the initial seeding of the virus into various parts of the United States came from Italy,” said Professor Perkins.

By mid-March, several states had begun closing schools and restaurants and imposing lockdowns, but a lack of a coordinated national response created a number of variables that proved to be detrimental. 

The Notre Dame team focused the study on the January to March timeframe, when little to no action had been taken to address COVID-19 on a wide scale. 

“It was such a crucial period in terms of how this situation started,” said Professor Perkins. “We look at the United States now and compare it to other countries like South Korea or Germany, New Zealand or Vietnam, any number of countries who have done a much, much better job controlling transmission.” 

“The key differences really come down to the time period we examine in this study. Those countries had adequate surveillance up and running at that time, whereas we show that throughout most of February the United States missed the vast majority of infections that were already out there. This particular timeframe that we focus on is really important for figuring out how we got here in the first place.”

The researchers used a computer model to simulate data reported by Johns Hopkins University beginning on January 1. The model accounted for confirmed cases and deaths, asymptomatic infections, case fatality rates, and local transmission. 

First, the experts generated an estimate of the total infections across the United States through March 12. They factored in how the detection of symptomatic infections evolved over time, and predicted the number of undocumented infections during this time.

The researchers said the number of unobserved and unreported infections, as well as the number of uncertainties early on in the pandemic, demonstrate just how critical containment strategies are when battling infectious diseases.

“I think the fact that there were so many infections by the second half of February speaks to the importance of and what we could have done in terms of containment,” said Professor Perkins. 

“If you think about Ebola or SARS (severe acute respiratory syndrome) or other emerging infectious diseases, there have been cases that show up in the U.S. or other countries where officials move quickly to get ahead of them.” 

“They isolate those people, they do contact tracing – and transmission is extinguished. I think a lot of us were hoping that would be the situation with this disease. By the time we got to February, the problem had grown so big, containing the virus wasn’t possible.”

The study is published in Proceedings of the National Academy of Sciences.

By Chrissy Sexton, Staff Writer


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