Early coronavirus infection rate in the US may have been 80 times more than initially thought, finds study
A surge in flu-like infections in the US in March 2020 now suggests that the likely number of coronavirus cases was far larger than official estimates, according to a new study of existing surveillance networks for influenza-like infections. Researchers from Penn State University estimate that 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.
Many scientists have suspected that the true rate of Covid-19 infections is higher than the number of confirmed cases due to the low availability of testing, infected individuals who show no symptoms or only mild flu-like symptoms, and a failure to identify early cases. Using the Centers for Disease Control and Prevention or CDC’s influenza-like illnesses (ILI) surveillance data over three weeks in March 2020, the researchers estimated the detection rate of symptomatic Covid-19 cases. They observed a huge spike in influenza-like illnesses, exceeding normal seasonal numbers in various states. The size of the observed surge of excess influenza-like illnesses corresponds to over 8.7M new cases during the last three weeks of March, compared to the roughly 100,000 cases that were officially reported during the same period, suggests analysis. The research team — which also includes experts from Cornell University, New York-Presbyterian Hospital and Montana State University — says that the excess ILI showed a nearly perfect correlation with the spread of Covid-19 around the country.
"We analyzed each state's ILI cases to estimate the number that could not be attributed to influenza and were in excess of seasonal baseline levels. 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," says Justin Silverman, assistant professor in Penn State's College of Information Sciences and Technology and Department of Medicine, in the analysis published in Science Translational Medicine.
After calculating that approximately 32% of people infected with SARS-CoV-2 sought medical care, the authors found that more than 8.7M SARS-CoV-2 (the virus that causes Covid-19) infections occurred between March 8 and March 28 in the US. They estimate that more than 80% of these cases remained unidentified as the outbreak spread rapidly. The analysis also suggests that new deaths doubled approximately every three days.
The researchers 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. The results showed rates much higher than initially reported but closer to those found once states began completing antibody testing. New York, for example, showed twice its previous record for influenza-like illnesses in the fourth week of March. The researchers' model suggests that at least 9% of New York 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% infection rate, or 2.7M New Yorkers.
According to the authors, excess influenza-like illnesses appear to 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 US states were under stay-at-home orders by March 28.
The research team says their findings indicate that there appears to be a much greater undiagnosed population than originally thought. The results also suggest that surveillance networks for influenza-like disease offer an important tool to estimate the prevalence of Covid-19, which has been hard to pin down, they add. "Together, these results suggest that SARS-CoV-2 spread rapidly throughout the US since its January 15th start date and was likely accompanied by a large undiagnosed population of potential Covid-19 outpatients with presumably milder distribution of clinical symptoms than estimated from prior studies of SARS-CoV-2 inpatients," say experts. "We emphasize the importance of testing these findings with seroprevalence data and discuss the broader potential to use syndromic surveillance for early detection and understanding of emerging infectious diseases," the team recommends.
The number of cases has been rising across the US. According to the CDC's June 19 update for week 24 that ended on June 13, the overall percentage of respiratory specimens testing positive for SARS-CoV-2 increased slightly from week 23 (6.1%) to week 24 (6.4%), driven by increases in five regions. As of May 23, more than 2,311,990 coronavirus cases have been reported from across the US, and over 120,400 have died in the Covid-19 pandemic.