Number of coronavirus-infected Americans may be 6 to 24 times higher than officially reported, says CDC
The findings may reflect persons who had mild or asymptomatic illness or who did not seek medical care or undergo testing but who still may have contributed to ongoing virus transmission in the population
The true extent of the Covid-19 pandemic in the US is currently unknown. Experts have argued that the over 3.8 million confirmed cases reported (as of July 21) likely represent only a fraction of all the infections that have occurred in the US so far. Researchers now estimate that the number of individuals infected with Covid-19 could be six to 24-fold higher than the number of confirmed cases reported in the US. This finding may reflect persons who had mild or asymptomatic illness or who did not seek medical care or undergo testing but who still may have contributed to ongoing virus transmission in the population, say researchers from the US Centers for Disease Control and Prevention (CDC).
The analysis of 10 diverse geographical sites across the US shows that the proportion of Americans with SARS-CoV-2 (the virus that causes Covid-19) antibodies ranges from 1% in the San Francisco Bay area (collected April 23-27) to 6.9% of persons in New York City (collected March 23-April 1). Detection of antibodies to SARS-CoV-2 in a person’s blood likely indicates that they were infected at some point since the start of the pandemic. Thus, serologic assays can be used to provide population-based estimates of infection that include people who had mild or asymptomatic infection or who were never tested despite having symptoms, say experts.
Serum samples were tested from 16,025 persons, 8,853 (55.2%) of whom were women; 1,205 (7.5%) were 18 years or younger and 5,845 (36.2%) were 65 years or older. “We used convenience samples of residual clinical specimens obtained from 2 commercial diagnostic laboratories to conduct a serologic survey. We tested 16,025 residual sera specimens from 10 sites collected from March 23 through May 12. Our goal was to estimate the seroprevalence (of antibodies to SARS-CoV-2) in the population — that is, the proportion of the population with evidence of the previous infection with SARS-CoV-2 in 10 geographically diverse US sites with known community transmission,” write authors.
The research team compared the number of estimated cases in the population based on their seroprevalence estimates with the reported cases as of the last day of specimen collection. From this comparison, they estimated that the number of infections ranged from 6 to 24 times the number of reported cases. “During March to early May 2020, most persons in 10 diverse geographic sites in the US had not been infected with SARS-CoV-2 virus. The estimated number of infections, however, was much greater than the number of reported cases in all sites. Six to 24 times more infections were estimated per site with seroprevalence than with coronavirus disease 2019 case report data. For most sites, it is likely that greater than 10 times more SARS-CoV-2 infections occurred than the number of reported Covid-19 cases; most persons in each site, however, likely had no detectable SARS-CoV-2 antibodies,” say authors in the analysis published in JAMA Internal Medicine.
The research team also includes experts from Florida Department of Health, New York State Department of Health, Utah Department of Health, Louisiana Department of Health, California Department of Health, Connecticut Department of Public Health, Washington State Department of Health, Pennsylvania Department of Health, Missouri Department of Health and Senior Services, and the Minnesota Department of Health.
The sites and dates of the collection are western Washington State (March 23-April 1); the New York City metro area (predominantly Manhattan, Bronx, Brooklyn, Queens, and Nassau counties; March 23-April 1); south Florida (restricted to Miami-Dade, Broward, Palm Beach, and Martin counties; April 6-10); Philadelphia metro statistical area counties and Lancaster and Cumberland counties (April 13-25); San Francisco Bay area, including San Jose (April 23-27); Minneapolis-St Paul-St Cloud combined statistical areas (April 30-May 12); and all of Missouri (April 20-26), Utah (April 20-May 3), Connecticut (CT) (April 26-May 3), and Louisiana (LA) (April 1-8). Overall, the estimated numbers of infections for seven sites — Connecticut, Florida, Louisiana, Missouri, New York City metro area, Utah, and western Washington State —were at least 10 times greater than the number of reported cases. “These infections likely include asymptomatic and mild infections for which healthcare was not sought, as well as symptomatic infections in persons who either did not seek care or in whom SARSCoV-2 viral testing was not performed,” says the team.
Based on their findings, the researchers recommend that because persons often do not know if they are infected with SARS-CoV-2, the public should continue to take steps to help prevent the spread of Covid-19, such as wearing cloth face coverings when outside the home, remaining 6 feet apart from other people, washing hands frequently, and staying home when sick.
In June, the CDC had said over 20 million Americans may have been infected with Covid-19, a number that was 10 times higher than the official number of cases reported back then. Dr Robert Redfield, CDC Director, said only about one in every 10 Covid-19 cases in the US has been identified, implying that for every person who tests positive, another 10 cases have likely gone undiagnosed. The estimates were based on serology testing used to determine the presence of antibodies that show whether an individual has ever had Covid-19.
Discussing the implications of the latest findings, an editorial in JAMA says that the study rebukes the idea that current population-wide levels of acquired immunity (so-called herd immunity) will pose any substantial impediment to the continued transmission of SARS-CoV-2 in the US, at least for now. “The size of the epidemic projected through early May 2020 in this study falls far short of the estimated herd immunity threshold of approximately 60% to 70%. 7 of the 10 study locations are currently experiencing substantial, as-yet uncontrolled increases in new Covid-19 cases. These data should also quickly dispel myths that dangerous practices like “Covid parties” are either a sound or safe way to promote herd immunity,” write experts from the Massachusetts General Hospital, Boston, and Harvard Medical School, Boston.
According to the experts, increasing evidence suggests that acquired immunity may be short-lived in some persons with SARS-CoV-2 infection, particularly those with mild or asymptomatic infections. “Importantly, whatever protection population-level seropositivity might confer may be less durable than initially anticipated. The results also challenge the idea that there is a trade-off between implementing a prompt, effective public health response to the epidemic and acquiring higher levels of population-level immunity that might be protective in the future. As the authors underscore, the vast majority of individuals in all 10 study locations had no serologic evidence of prior SARS-CoV-2 infection, both in locations with relatively contained epidemics (San Francisco) and in those that were affected most heavily (New York),” says the editorial.