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SARS-CoV-2 immunity within the U.S. stays low

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

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

It is likely that the actual prevalence of COVID-19 in a population based only on the number of cases is leading to an underestimation of the spread of the disease. Previous US seroprevalence studies aimed at measuring the proportion of the population infected by blood serum with SARS-CoV-2 focused on high-risk populations.

A team of researchers from the United States Centers for Disease Control (CDC), ICF Inc., Quest Diagnostics, and BioReference Laboratories in the United States therefore believed that using residual sera from commercial testing laboratories offered a practical approach to estimating SARS could. CoV-2 prevalence in a general population by testing for the antibodies present in the sera. The research results are published in the journal JAMA Internal Medicine.

Seroprevalence from samples taken for routine testing

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

Approximately 15% of the samples tested included people living in non-urban areas, which is close to the actual population distribution in the United States and therefore has a more significant geographic distribution than previous studies.

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

Although there was no overall difference in seroprevalence between men and women, in some states, such as Iowa and Louisiana, they found that women had higher seroprevalence. In contrast, in states like Maryland and Pennsylvania, they found that men had higher seroprevalence. Antibodies were detected more often in people aged 18 to 49 years than in people over 65 years of age.

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

Low prevalence of SARS-CoV-2 infection in the US

“We found that most people in the US had no evidence of previous SARS-CoV-2 infection,” the authors write, with the seroprevalence as of September 2020 being less than 10%. This is similar to previous studies in the US and other countries. The study also showed mixed seroprevalence in metropolitan areas and non-metropolitan areas, suggesting that SARS-CoV-2 spread is not homogeneous. The changes in seroprevalence from July 2020 to September 2020 were modest.

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

According to the authors, there are some limitations to the study. Because the samples tested were individuals taken for routine examination or clinical treatment, they may not represent the general US population. Even if the samples included people with COVID-19, the blood may have been collected before any detectable levels of antibodies were present.

The authors write that more study is 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 sera collected from commercial laboratories can estimate how the SARS-CoV-2 prevalence will change over the next few months.

The authors write, “Our results underscore the need for continued public health prevention efforts, including the use of face masks and social distancing, to limit the spread of SARS-CoV-2 in the US.”

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