NEW YORK – Researchers from the Laboratory Corporation for America have found that around 90 percent of people infected with SARS-CoV-2 were still positive for antibodies against the virus 300 days after infection.
Detailed in a study published this week in EClinicalMedicine, the findings provide some of the longest-term longitudinal data collected to date on the persistence of anti-SARS-CoV-2 antibodies in infected individuals, though the researchers note that additional work will be required to determine what, if any, level of protection this persistence provides against reinfection.
In the study, the researchers looked at antibody levels against either the SARS-CoV-2 nucleocapsid (N) or spike (S) proteins in 39,086 individuals across all 50 US states who tested positive for the virus by RT-PCR between March 2020 and January 2021.
For assessing antibody levels they used four qualitative assays (two to the N protein and two to the S protein) from two unnamed vendors. According to the US Food and Drug Administration Emergency Use Authorizations for the assays, both N-protein assays (designated in the paper as assay C and D) have 100 percent sensitivity 14 days after a positive PCR result, while both S-protein assays (assays A and B) have 97 percent sensitivity or greater. All four assays have 99 percent specificity or higher.
Among the roughly 39,000 people tested, seropositivity was roughly 90 percent for both S-protein and N-protein IgG antibodies 21 days after the initial positive PCR result. After 293 days, N-protein seropositivity dropped to 68 percent. S-protein seropositivity was at 88 percent after 300 days, which the authors noted was the furthest time point for which they had data.
Antibody persistence varied with gender and age, with men showing a larger decrease in N-protein positivity after 180 days and individuals younger than 65 showing higher seropositivity rates for both proteins after roughly 180 days.
The study also observed variability among the assays, with assay A showing substantial variability in positivity rate over time and a sustained downward trend beginning around 250 days, while assay B exhibited little variability and no downward trend in positivity at 300 days. Assay C and D showed an even more pronounced difference, with the former sharply declining from a peak of around 90 percent positivity at 50 days to 57 percent positivity at 184 days, while the latter remained around 97 percent positive through 260 days with little variation.
The researchers noted that the difference in the assay results reflected differences in sensitivity and suggested that, in this instance, the lower sensitivity assays were arguably more informative as they gave earlier evidence of antibody titers falling in the population over time due to their higher threshold for positivity.
"At the individual level the best limit of detection would be the one most closely matched to the clinically relevant immunity threshold," they wrote. However, they noted that "in the case of SARS-CoV-2, it is not possible to say at this time what is the minimal antibody titer necessary for immunity."