Antibody Testing and Seroprevalence Studies 

This week, I wanted to discuss the relevance of antibody testing and seroprevalence studies in the fight against COVID-19. Serology tests can identify SARS-CoV-2 IgG or IgM antibodies in the bloodstream to determine whether patients, who may have had mild or asymptomatic illness and did not seek medical care or undergo testing, have previously been infected. Current serological tests are not designed for diagnosis of an acute infection, but can provide useful information for surveillance efforts to determine the true spread of the COVID-19 pandemic, measure the impact of public health efforts, and guide responses moving forward. 

The NIH and CDC are currently conducting nationwide seroprevalence surveys to better understand how much of the US population has been infected and how the virus is spreading over time. The NIH is focusing on collecting samples through at-home finger-prick blood collection from as many as 10,000 adult volunteers. Blood samples that return positive for antibodies against SARS-CoV-2 may also undergo further testing to evaluate immune response to provide insight as to why these cases were less severe than those that lead to hospitalization.

The CDC is conducting large-scale geographic surveys, community level surveys, and smaller-scale surveys which focus on specific populations, such as health care workers and pregnant women. Results published from samples collected from March 13 – May 12, 2020 from all age groups in 10 diverse geographic sites in the US found that the estimated number of infections was much greater than the number of reported cases (Havers et al. 2020). However, the overall prevalence was under 10%, suggesting that a large majority of people remain susceptible to the virus and that the threshold prevalence of 60%* which is needed for natural herd immunity to be effective is elusive (Altmann et al. 2020). 

Antibody tests can also provide information to indicate potential donors for convalescent plasma therapy. While the efficacy of this therapy for COVID-19 is still uncertain, a preprint from a non-randomized trial run by the Mayo Clinic with 35,322 patients receiving transfusions, found reduced mortality rate in patients who received a higher antibody level at an earlier time (Joyner et al. 2020). Randomized, placebo-control studies of convalescent plasma are ongoing.

*A mathematical model published on August 14, 2020 in Science estimates the threshold for herd immunity at 43%, accounting for the effects of a heterogeneous population on transmission and immunity (Britton et al. 2020).