SARS-CoV-2 immunity in the U.S. remains low

Seroprevalence studies indicate that less than 10% of people in the U. S. have detectable antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from July to September 2020.

The United States has been severely affected by the COVID-19 pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

It is likely that the actual prevalence of COVID-19 in a population-based only on the number of cases leads to an underestimation of the disease spread. Previous seroprevalence studies in the U.S., intended to capture what proportion of the population has been infected with SARS-CoV-2 using blood serum, have focused on high-risk populations.

Hence, a team of researchers from the U.S. Centers for Disease Control (CDC), ICF Inc., Quest Diagnostics, and BioReference Laboratories, all in the U.S., thought using residual sera from commercial testing labs could provide a practical approach to estimating SARS-CoV-2 prevalence in a general population by testing for the antibodies present in the sera.  The research findings are published in the journal JAMA Internal Medicine.

Study: Estimated SARS-CoV-2 Seroprevalence in the US as of September 2020. Image Credit: CI Photos / Shutterstock
Study: Estimated SARS-CoV-2 Seroprevalence in the US as of September 2020. Image Credit: CI Photos / Shutterstock

Seroprevalence from samples collected for routine tests

The researchers selected sera from samples collected for non-COVID-19 routine testing between July and September 2020 by two private commercial laboratories over four periods of about two weeks each. The samples were tested for antibodies to SARS-CoV-2 using commercial tests for IgG. They calculated the overall estimated seroprevalence by jurisdiction, age, and sex. Using this, the team predicted the total number of infections in each jurisdiction.

About 15% of the samples tested included people living in non-metropolitan areas, which is close to the true U.S. population distribution and thus has a more significant geographic distribution than studies done before.

Over the collection period, the team tested 177,919 samples in total from all 50 U.S. states, and Washington D. C., and Puerto Rico. The seroprevalence range was between 0% in South Dakota in period 2 to 23% in New York in period 1. The seroprevalence was 13% in the South and estimated to be less than 10% in the West and Midwest. They found that less than 10% of the samples had detectable antibodies in the jurisdictions with enough samples to calculate an estimate.

Although overall, there was no difference in seroprevalence between men and women, in some states like Iowa and Louisiana, they found women had a higher seroprevalence. In contrast, in states like Maryland and Pennsylvania, they found men had a higher seroprevalence. Antibodies were detected more in people in the age group 18 to 49 than in people over 65.

Among areas with enough samples across all four periods to estimate total seroprevalence, New York showed the largest decrease of 6.3%, and Georgia showed the largest increase of 6.2%.

Low prevalence of SARS-CoV-2 infection in U. S.

“We found that most people in the U.S. did not have evidence of previous SARS-CoV-2 infection,” write the authors, with the seroprevalence being less than 10% as of September 2020. This is similar to previous studies in the U. S. and other countries. The study also showed a mixed seroprevalence in metropolitan and non-metropolitan areas, suggesting SARS-CoV-2 spread is not homogeneous. The changes in seroprevalence from July 2020 to September 2020 were modest.

Seroprevalence studies may show a higher number of COVID-19 cases than reported cases. However, the estimated numbers could still be lower than the actual number of cases. This could be because of the decrease in antibodies over time, lesser antibodies in asymptomatic cases, or other reasons for decreasing antibodies that are not clearly understood yet.

According to the authors, there are some limitations to the study. Since the samples tested were those of people taken for routine screening or clinical care, they may not represent the general U. S. population. Furthermore, even if the samples included people with COVID-19, the blood may have been collected before they had detectable levels of antibodies.

The authors write more studies are needed to understand how the presence or absence of antibodies affects continued immunity to the disease and future reinfection. Other possible components of the innate immune response such as B-cell and T-cell memory could affect immunity to SARS-CoV-2, which is not captured by seroprevalence studies using immunoassays for antibodies.

Continued testing of the sera collected by commercial laboratories will allow estimating how the SARS-CoV-2 prevalence is changing over the next few months.

The authors write, “Our results reinforce the need for continued public health preventive measures, including the use of face masks and social distancing, to limit the spread of SARS-CoV-2 in the U.S.”

Journal reference:
Lakshmi Supriya

Written by

Lakshmi Supriya

Lakshmi Supriya got her BSc in Industrial Chemistry from IIT Kharagpur (India) and a Ph.D. in Polymer Science and Engineering from Virginia Tech (USA).

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