The novel coronavirus may have been introduced into the US as early as December 13 to 16, 2019, suggests a study which assessed archived samples from routine blood donations collected by the American Red Cross.
While the first confirmed case of COVID-19 in the US was identified on January 19, 2020, the scientists, including Sridhar V Basavaraju from the Centers for Disease Control and Prevention (CDC) in the US, noted that antibodies reactive against the coronavirus were detected in 106 of the 7,389 specimens.
According to the research, published in the journal Clinical Infectious Diseases, 84 of the samples specifically had neutralising activity against the spike protein of the SARS-CoV-2 virus, which enables it to enter host cells.
“The presence of these serum antibodies indicate that isolated SARS-CoV-2 infections may have occurred in the western portion of the US earlier than previously recognised, or that a small portion of the population may have pre-existing antibodies that bind SARS-CoV-2,” the researchers wrote in the study.
Since some parts of SARS-CoV-2, such as the S2 subunit of its spike protein, is more conserved across human coronaviruses, they said the findings raised questions on whether the detection of reactive antibodies indicate actual infections with the novel coronavirus earlier than recognised.
In order to overcome this doubt, the scientists performed more specific tests of the samples against the S1 subunit of the novel coronavirus.
“The S1 subunit has been reported to be a more specific antigen for SARS-CoV-2 serologic diagnosis than the whole S protein,” they explained in the study.
Citing recent research, the scientists also noted that the sera from patients with confirmed infection with human coronaviruses did not contain IgM or IgA antibodies.
“Therefore, the presence of IgM or IgA antibodies and S1-specific binding activity may distinguish antibodies to SARS-CoV-2 from antibodies to human common coronaviruses,” they added.
On further analysis, the researchers found that 84 of the 90 reactive sera had neutralising activity against the SARS-CoV-2 virus, and 39 had both IgG and IgM SARS-CoV-2 S-specific antibodies.
They said two sera had surrogate neutralisation activities, and one had SARS-CoV-2 S1-specific antibodies.
Based on the analysis, the scientists said “at least some of the reactive blood donor sera could be due to prior SARS-CoV-2 infection.”
“The findings of this report suggest that SARS-CoV-2 infections may have been present in the US in December 2019, earlier than previously recognised,” the researchers wrote in the study.
However, the scientists believe further studies involving retrospective analyses of human specimens with molecular or serologic methods are necessary to further corroborate the findings.