Positivity Rates for Home COVID Tests Tracked Public Statistics But Now Diverge
Since the use of home COVID-19 testing has become widespread, pandemic trends have become more difficult to track for public health officials. New research suggests positivity rates had been in line with publicly reported results but that situation now is changing. Here is more information.
SAN FRANCISCO – Positivity rates from SARS-CoV-2 home testing appear to be in line with publicly reported tests, although the results are beginning to diverge, according to a new research letter.
“Throughout the COVID-19 pandemic, COVID-19 test sites have been required to report SARS-CoV-2 test results to local or state public health departments, and these data are used for detecting new surges of transmission,” according to the University of California San Francisco-led study. “ With increasing availability of home antigen tests, however, it is unclear how to interpret time trends in officially reported case counts and test positivity.”
The letter in JAMA Network Open recounts how the COVID-19 Citizen Science Study was launched in March 2020 to gather patient-reported data about the COVID-19 pandemic. The study team invited participants by word of mouth or social media or from recruitment partners via email, telephone, or patient portal message. Those who agreed provided informed consent and baseline demographic information.
In addition, race and ethnicity were self-reported by the participants and were analyzed to better understand differences in unreported test frequency and test positivity.
Researchers asked participants each week about recent COVID-19 testing and test results. In March 2022, they advise, a question was added to distinguish tests conducted with a “Fully at-home test kit, with my own sample collection and reading of my own results” vs tests where “a healthcare provider collected my sample” or that were “sent to a clinical lab.”
In order to compare their data with SARS-CoV-2 testing reported nationally, the study team downloaded data from the Johns Hopkins Coronavirus Resource Center on Aug. 15, 2022, and plotted smoothed 7-day moving averages of total daily tests and test positivity.
The study enrolled more than 100,000 participants, of whom 18,642 (18%) reported completing at least 1 SARS-CoV-2 test from March 16, 2022, to Aug.15, 2022. Most, 67.8%, of the participants were female (and non-Hispanic white, 82.1% and had a mean age of 55.
“During this time period, the proportion of SARS-CoV-2 testing conducted at home increased from approximately 60% to more than 80%,” the researchers point out. “The percentage test positivity on home tests was similar to officially reported tests through June, but then started to diverge with lower positivity in-home tests (P < .001 for interaction of official test positivity and time).”
Overall, the study determined that female, non-Hispanic white, younger, and higher social status participants were more likely to test at home, while males and young adults were more likely to test positive on home tests.
“In this cohort study, we found home testing to be increasingly common through spring and into summer 2022, most recently comprising more than 80% of all SARS-CoV-2 testing reported,” the authors explain. “Home test positivity appears to track closely with national data from reported tests, but these trends are starting to diverge. Home testing patterns differ by demographic subgroup, as previously shown, perhaps because of differential COVID-19 worry or availability and cost of test kits.”
Among the limitations of the study, according to the researchers, is that home testing might be repeated during illness episodes and might be less common in the United States than the engaged participants would suggest. The results also could have been, officially reported despite being conducted at home, such as by employers.
“Our findings confirm common wisdom that official COVID-19 case counts increasingly underestimate the number of people who test positive and vastly underestimate the number of true infections,” the study concludes. “The percentage test positivity in officially reported tests appears to reflect home test positivity, though these trends may be diverging.”