Initial Covid-19 infection rate in US may be far more than reported: Study

The number of early Covid-19 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 which 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," Silverman explained.

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-19 cases, and there appears to be a much greater undiagnosed population than originally thought," Silverman said.

 

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.

"We realised that deaths across the US had been doubling every three days and that our estimate of the infection rate was consistent with three-day doubling since the first observed case was reported in Washington state on January 15," Silverman said.

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 that 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.

"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 explained.

"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," he added.