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The absence of universal COVID-19 testing in the U.S. makes it impossible to establish prevalence and mortality rates of SARS-CoV-2 infection in various geographic locations and in the nation as a whole. Studies of prevalence have used select subpopulations or nonrandom sampling, both of which have limitations regarding generalizability.
Now, epidemiologists in Indiana have conducted statewide random-sample testing to assess the prevalence of SARS-CoV-2 active infection (AI) and antibodies to the virus in 3658 persons (92% white) selected from a group of 15,495 aged 12 years and older. Random sampling was conducted from April 25 through 29 and was supplemented by nonrandom sampling of 898 persons (44% Hispanic and 33% Black) to describe prevalence of AI in racial and ethnic minorities.
Results were as follows:
Overall prevalence of AI, confirmed by reverse transcription–polymerase chain reaction testing, was 1.74%. Among this group, 44.2% were entirely asymptomatic; in the nonrandomly selected group of racially and ethnically diverse persons, a smaller percentage of those with AI were asymptomatic (20.2%).
Antibodies to SARS-CoV-2 were present in 2.79%.
AI prevalence was higher among household contacts than among noncontacts (33.6% vs. 2.2%).
AI prevalence was higher in selected Hispanic persons than in non-Hispanic persons (8.3% vs. 2.3%), as was antibody prevalence.
Estimates of prevalence based on this study were that 187,802 Indiana residents (2.8% of the Indiana population) were currently or previously infected with SARS-CoV-2, a number 9.6 times higher than that of confirmed cases.
The infection-mortality ratio was 0.58%.
Menachemi N et al. Population point prevalence of SARS-CoV-2 infection based on a statewide random sample — Indiana, April 25–29, 2020. MMWR Morb Mortal Wkly Rep 2020 Jul 24; 69:960. (https://doi.org/10.15585/mmwr.mm6929e1)
Comment
This study clearly suggests that actual testing reveals a higher prevalence of infection than does estimated prevalence from reported cases and allows for accurate case-fatality measurement. It also confirms that some, if not all, minorities are more adversely affected than is the overall population. Although this higher overall infection rate may well be multifactorial and difficult to sort out precisely, it suggests that difficulties of achieving social distancing may play a major part in the spread of infection. Random and nonrandom sampling both provide important information on the prevalence of the disease.