As the United States enters the fourth month of the pandemic, social distancing and hygiene practices continue to be encouraged to decrease transmission of the coronavirus disease 2019 (COVID-19). While vaccine development is underway, the path forward to reopening businesses and relaxing social distancing policies relies on the development of antibody testing and contact tracing. However, a recent epidemiology study conducted in Boston, Massachusetts reported that 10% of an unselected population had developed antibodies against severe acute respiratory syndrome (SARS)-CoV-2, which is far from the 70%–90% immunity needed to achieve herd immunity. Targeted antibody screening would risk excluding asymptomatic carriers, which contributes to the difficulty in containing the virus1. Immunity screening may offer an opportunity to select which individuals in the population can safely resume life. However, studies are needed to determine if the presence of antibodies provide immunity and limit transmission of the virus and, if immunity develops, studies must be undertaken to determine the length of time immunity will persist.
Around the globe, studies are underway to determine which antibody test can perform with the highest accuracy and identify those who have been infected (sensitivity) and those who have not (specificity). An initial study reported that the Centers for Disease (CDC) developed ELISA (enzyme-linked immunoabsorbent assay) serology screen detects antibodies against the SARS-COV-2 spike protein with a sensitivity of 96% and specificity of 99%. They reported minimal cross reactivity with common coronaviruses but increased cross reactivity with rare coronaviruses, Middle East Respiratory Syndrome (MERS-CoV) and severe acute respiratory syndrome (SARS-CoV)2. A large-scale population study at the National Institutes of Health (NIH) is underway, with the goal to analyze blood samples from 10,000 volunteers for the detection of anti-SARS-CoV-2 protein IgG and IgM using an ELISA. This study will provide more information about community exposure and may potentially be used to identify geographic regions that are vulnerable to disease spread and identify emerging hot spots for disease transmission. We are also learning that even those at high risk of disease exposure may not have immune protection, as a study conducted in Germany reported that only 1.6% of health care workers tested positive for SARS-CoV-2-IgG antibodies3. For patients with confirmed covid-19 disease. antibody levels (IgG and IgM) against the SARS-CoV internal nucleoprotein and surface spike protein receptor binding domain were noted to increase at 10 days or more after the onset of symptoms. A high viral load did not correlate with antibody response and a higher proportion of patients had an earlier IgG seroconversion than IgM4. A second study of individuals with confirmed coronavirus infections reported that 99% of patients with confirmed coronavirus infections by PCR developed IgG antibodies over a period of 7–50 days from symptom onset, indicating that timing of antibody testing should be 2–3 weeks after symptoms. Antibody titers increased over time, although the participants were followed for only 3 weeks5. While there is evidence that the immune system is activated with exposure to the disease, no study to date has determined if the presence of antibodies confers immunity to a subsequent infection.
Conflict of interest disclosures
The author declares that there is no financial conflict of interest with regard to the content of this report.
1. Pan X, Chen D, Xia Y, et al. Asymptomatic cases in a family cluster with SARS-CoV-2 infection. Lancet Infect Dis 2020;20:410–1.
2. Freeman B, Lester S, Mills L, et al. Validation of a SARS-CoV-2 spike protein ELISA for use in contact investigations and sero-surveillance. bioRxiv 2020:2020. Doi: https://dx.doi.org/10.1101%2F2020.04.24.057323.
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