The number of early Covid-19 cases in the US may have been more than 80 times greater, and doubled nearly twice as fast as originally believed, said a new study.
“At first I couldn’t believe our estimates were correct,” said Justin Silverman, Assistant Professor at the Pennsylvania State University.
“But 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 added.
In a paper published in the journal ‘Science Translational Medicine’, the researchers estimated the detection rate of symptomatic Covid-19 cases using the Centers for Disease Control and Prevention’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 numbers that could not be attributed to influenza and were in excess of seasonal baseline levels,” said Silverman.
“When you subtract these out, you’re left with what we’re calling excess influenza-like illnesses — cases that can’t be explained by either influenza or the typical seasonal variation of respiratory pathogens,” he said.
The researchers found that the excess influenza-like illnesses showed a nearly perfect correlation with the spread of Covid-19 around the country.
Remarkably, the size of the observed surge of excess influenza-like illnesses corresponds to more than 8.7 million new cases during the last three weeks of March, compared to the roughly 100,000 cases that were officially reported during the same time period.
The researchers also used this process to estimate 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 influenza-like illnesses.
Their estimates showed rates much higher than initially reported, but closer to those found once states began completing antibody testing.
In New York, for example, the researchers’ model suggested that at least 9 per cent of the state’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.
So far, over 2.3 million people have tested positive for Covid-19 in the US, while 120,000 have died, according to data compiled by the Johns Hopkins University.