The US may never know the true tally of its coronavirus infections - but scientists are getting closer to an accurate count. New research suggests that many millions of infections went undiagnosed in March, even as testing ramped up.
The nation's current case count hovers around 2.4 million. But a study published Monday in the journal Scienceestimated that the US saw 8.7 million coronavirus cases from March 8 to 28.
The country's official tally indicates there were about 100,000 infections during that time period, suggesting that the real-life outbreak was more than 80 times larger than the one reported.
To arrive at this estimate, the researchers looked at reports of flu-like illnesses among outpatients using the Centres for Disease Control and Prevention's influenza surveillance system.
Many states, including Colorado, Louisiana, Maryland, New Jersey, New York, Oregon, Pennsylvania, and Washington, reported a surge of illnesses in March - about 2.8 million - that weren't attributable to the flu or seasonal respiratory diseases.
The researchers think most of those were coronavirus cases that went undiagnosed because of limited testing capacity or false-negative test results. They also estimated that only one-third of coronavirus patients in the US sought medical care in the first place, since many coronavirus cases are asymptomatic or mild.
So the research suggests the actual surge of coronavirus cases was likely three times larger than those extra cases of flu-like illness: 8.7 million cases.
The findings support what epidemiologists have long suspected: The coronavirus is far more prevalent and less deadly than current figures suggest.
About 5 percent of US coronavirus cases reported so far have been fatal. A surge of undiagnosed cases in March could lower that fatality rate to between 0.07 and 1.4 percent, the researchers found. Recent estimates suggest that the global fatality rate is about 1 percent.
Previous models predicted that the actual number of US coronavirus cases could be anywhere from five to 20 times the current number. The new study suggests that estimate isn't far off.
By the end of March, the researchers estimated, the US had confirmed less than 13 percent of that month's "flu-like illnesses" were coronavirus infections. Many of these undiagnosed cases were in New York, the original centre of the US outbreak.
More than 8 percent of New York's population - or 1.6 million people - had been infected by March 28, according to the researchers' estimates. (The state's antibody testing later indicated that nearly 14 percent of New Yorkers, or 2.7 million people, had been infected by March 29.) But data at the time suggested only 0.3 percent of the state, or 58,000 people, had been infected.
That means New York may have failed to diagnose up to 2.6 million cases in March.
"At first I couldn't believe our estimates were correct," Justin Silverman, an assistant professor at Penn State University who coauthored the study, said in a statement.
But the researchers cross-referenced their model with the rise of coronavirus deaths in the US. Using data from The New York Times, they found that deaths were doubling every three days in March.
That same doubling rate accounted for the rise of coronavirus cases from a single infection in January to 8.7 million infections in March (assuming that the majority of cases didn't seek medical care or get official diagnoses).
By the third week of March, according to the study, coronavirus infections identified as "flu-like illnesses" reached their peak nationally. Many states then saw a decline in these infections over the following week.
That could mean that increased testing or social-distancing measures worked - or that fewer people presented themselves to healthcare providers.
But infections continued to rise in New York and New Jersey.
"Our results suggest that the overwhelming effects of COVID-19 may have less to do with the virus' lethality and more to do with how quickly it was able to spread through communities initially," Silverman said.
"A lower fatality rate, coupled with a higher prevalence of disease and rapid growth of regional epidemics, provides an alternative explanation of the large number of deaths and overcrowding of hospitals we have seen in certain areas of the world."
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