Naturally occurring COVID-19 antibodies may wane after 4 months

In coronavirus disease 2019 (COVID-19) convalescent patients, levels of naturally occurring antinucleocapsid (N) antibodies remain above the acceptable threshold for only 4 months after infection, a recent study has found.

Using blood samples from 943 patients, collected over the course of 7 months after initial virus detection, the researchers measured anti-N immunoglobulin-G (IgG) levels through a chemiluminescent microparticle immunoassay. The primary outcome was the longitudinal decay rate of these IgGs.

As expected, antibody levels waned over time. Among those who had contributed convalescent samples only once, anti-N IgG levels reached 9 signal/cutoff units (S/C) when donated within 100 days since the last day of symptoms, and only 6 S/C when donated between 100–150 days. Hereafter, most patients only had anti-N IgG levels of 3 S/C.

For within-patient longitudinal analysis, the researchers took data from 472 convalescent participants who had donated blood at least two times. Of these, 251 had their highest anti-N IgG levels during the first day of donation and the lowest levels during the final day.

Mixed linear models were used to evaluate antibody dynamics over 200 days and showed that for most participants, the rate of reduction tended to cluster around 0.12 S/C per day, though a few outliers showed faster rates, reaching 0.48 S/C per day.

Moreover, the researchers found that all blood samples donated within 120 days of the final symptom day showed anti-N IgG levels above the predefined threshold of 1.4 S/C. Beyond 120 days, antibody levels showed high variation and already fell below the cutoff by day 123, with an average value of 1.32±0.40 S/C.