It is estimated that by November 2020, 14.3% of the U.S. population had antibodies to COVID-19, indicating that the virus infected many more people than reported – but still not enough to get close to the ratio needed. is not for herd immunity, according to a study published today in JAMA Network open.
In the cross-sectional study, researchers from study sponsors Pfizer and Merck analyzed data from randomized community surveys of community studies and five such regional and national Centers for Disease Control and Prevention (CDC) surveys to estimate infections underreporting multipliers. Seroprevalence surveys reveal the part of a population that has antibodies against a certain disease, such as COVID-19.
After adjusting for underreporting using validated multipliers, the analysis revealed an estimated median of 46,910,006 infections with SARS-CoV-2, the virus that causes COVID-19; 28,122,752 symptomatic infections; 956 174 hospitalizations; and 304,915 deaths from April to mid-November.
According to these numbers, 14.3% of Americans were infected with the new coronavirus by November 15, 8.6% had symptomatic infections, with an infection-hospitalization ratio of 2.0% and a case of fatal consequences for people with symptoms of 1.1%.
In contrast, the CDC reported 10,846,373 COVID-19 cases and 244,810 deaths in the same period, with 1,037,962 cases recorded in the last 7 days of that period (an average of 148,280 new daily reported cases).
An estimated 35% of COVID deaths were not reported
The authors noted that seroprevalence surveys are essential to monitor the progress towards herd immunity, which occurs when about 60% of the population has antibodies that may help slow the transmission of the community. By comparison, the number of cases reported is underestimated because many symptomatic people do not seek testing or medical care, and the estimated 40% who have no symptoms.
“The U.S. population remains a long way from herd immunity, even with millions of new infections each week,” the researchers wrote. “The estimated number of deaths from COVID-19 is also significantly higher than the reported deaths in the U.S. through November 15, 2020, which supports the conclusion that approximately 35% of COVID-19 deaths are not reported.”
They added that several methodological issues need to be considered when using seroprevalence surveys to derive under-reporting multipliers for adjusting disease monitoring data to estimate the burden of disease. These issues include time from infection to antibody development, decrease in antibodies and reporting confirmed infections. The multipliers also shift over time with changes in the portion of the population that have been tested, diagnosed, and reported.
A sensitivity analysis of the CDC seroprevalence surveys suggested that using the number of reported cases at the end of the survey period provides a useful estimate of the underreport multipliers, especially early in the pandemic, ‘the authors said. “Therefore, the number of reported cases on the last day of the seroprevalence survey was used in the COVID-19 estimate of disease burden in this study.”
The researchers concluded that supervision should continue to measure progress toward herd immunity. “Additional surveys of seroprevalence are needed to monitor the pandemic, including after the development of safe and effective vaccines,” they said.