- Prevalence of Covid-19 ranged from 2.49% to 4.16%
- In Santa Clara County, that means an estimated 48,000 to 81,000 people were infected
- That is 50-85-fold more than the number of confirmed cases
The first early results are on from coronavirus antibody testing in a general population: Covid-19 (SARS-CoV-2) is much more widespread than positive tests indicate.
The goal of the study is to find out how prevalent Covid-19 has been beyond those who have tested positive. Many Americans have had the disease in the recent past but had few or no symptoms. They are believed to be largely immune from a repeat infection, or from infecting anybody else.
Knowing how many truly had coronavirus will allow scientists to calculate the first accurate death rate. Until now, the publicized mortality rates have been skewed higher than they really are because officials have only counted deaths among those who tested positive. A more accurate number will come from calculating deaths among everybody who had coronavirus, including those who were never diagnosed with it.
According to a preprint scientific paper on a part of the study conducted in Santa Clara, California, the population prevalence of COVID-19 in Santa Clara ranged from 2.49% to 4.16%.
That works out to a range of an estimated 48,000 to 81,000 people infected in Santa Clara County by early April. Those numbers are 50-85-fold more than the number of confirmed cases.
The scientists of the study say the population prevalence of Covid-19 (SARS-CoV-2) antibodies in Santa Clara County implies that the infection is "much more widespread than indicated by the number of confirmed cases."
Coronavirus in Santa Clara County, California is "much more widespread than indicated by the number of confirmed cases."Study scientists
That make the true death rate, at least in Santa Clara, significantly lower than earlier publicized figures.
The scientists are continuing their research in other locations.
The study paper abstract follows, and the link to it is below that:
Background Addressing COVID-19 is a pressing health and social concern. To date, many epidemic projections and policies addressing COVID-19 have been designed without seroprevalence data to inform epidemic parameters. We measured the seroprevalence of antibodies to SARS-CoV-2 in Santa Clara County. Methods On 4/3-4/4, 2020, we tested county residents for antibodies to SARS-CoV-2 using a lateral flow immunoassay. Participants were recruited using Facebook ads targeting a representative sample of the county by demographic and geographic characteristics. We report the prevalence of antibodies to SARS-CoV-2 in a sample of 3,330 people, adjusting for zip code, sex, and race/ethnicity. We also adjust for test performance characteristics using 3 different estimates: (i) the test manufacturer's data, (ii) a sample of 37 positive and 30 negative controls tested at Stanford, and (iii) a combination of both. Results The unadjusted prevalence of antibodies to SARS-CoV-2 in Santa Clara County was 1.5% (exact binomial 95CI 1.11-1.97%), and the population-weighted prevalence was 2.81% (95CI 2.24-3.37%). Under the three scenarios for test performance characteristics, the population prevalence of COVID-19 in Santa Clara ranged from 2.49% (95CI 1.80-3.17%) to 4.16% (2.58-5.70%). These prevalence estimates represent a range between 48,000 and 81,000 people infected in Santa Clara County by early April, 50-85-fold more than the number of confirmed cases. Conclusions The population prevalence of SARS-CoV-2 antibodies in Santa Clara County implies that the infection is much more widespread than indicated by the number of confirmed cases. Population prevalence estimates can now be used to calibrate epidemic and mortality projections.