Initial coronavirus infection rate in US may be 80 times greater than reported, says study

The number of early coronavirus cases in the US may have been more than 80 times greater, and likely doubled nearly twice as fast as originally believed, according to a study.

The research further says the undercounting may have been due to testing issues, and the prevalence of asymptomatic individuals.

In the study, published in the journal Science Translational Medicine, the scientists estimated the detection rate of symptomatic COVID-19 cases using the Centers for Disease Control and Prevention's (CDC's) influenza-like illnesses (ILI) surveillance data over a three week period in March 2020.

"We analysed each state's ILI cases to estimate the number that could not be attributed to influenza, and were in excess of seasonal baseline levels," explained Justin Silverman, study co-author from the Pennsylvania State (Penn State) University in the US.

"When you subtract these out, you're left with what we're calling excess ILI - cases that can't be explained by either influenza, or the typical seasonal variation of respiratory pathogens."

The study noted that the excess ILI showed a nearly perfect correlation with the spread of COVID-19 around the country.

This suggests that ILI data is capturing COVID cases, and there appears to be a much greater undiagnosed population than originally thought.

According to the scientists, the size of the observed surge of excess ILI corresponds to more than 8.7 million new cases during the last three weeks of March, compared to the roughly 1,00,000 cases that were officially reported during the same time period.

The researchers realised that deaths across the US had been doubling every three days and that their estimate of the infection rate was consistent with three-day doubling since the first observed case was reported in Washington state on January 15.

Using this process, the scientists estimated infection rates for each state, noting that states showing higher per capita rates of infection also had higher per capita rates of a surge in excess ILI.

Their calculation showed rates much higher than initially reported but closer to those found once states began completing antibody testing.

Citing an example, the researchers said at least 9 per cent of New York's entire population was infected by the end of March.

After the state conducted antibody testing on 3,000 residents, they found a 13.9 per cent infection rate, or 2.7 million New Yorkers, the scientists added.

They said excess ILI may have peaked in mid-March as fewer patients with mild symptoms sought care, and states implemented interventions which led to lower transmission rates.
Nearly half of the US states were under stay-at-home orders by March 28, the study noted.

The findings, according to the researchers, suggest an alternative way of thinking about the COVID-19 pandemic.

(with inputs from PTI)