Covid-19 Antibodies Wane Rapidly

A new UK study found that the percentage of UK residents testing seropositive for covid-19 IgG antibodies has decreased, from 6 percent in June to 4.4 percent in September, even though the country likely experienced an additional 200K infections during that 3-month interval. Other recent studies also found a rapid decline in IgG antibodies. A BMJ editorial published two months ago proposed that IgA antibodies, not detected with the usual finger-stick blood test, might last longer after infection. However, a study conducted at Mass General found that covid-19 IgA antibodies spiked shortly after infection and then declined even more rapidly than did IgC antibodies. Still, it’s likely that, after initial infection, the immune system can mount an effective antibody response to a recurrent infection more rapidly than can the immune system in a previously uninfected individual.

On this blog I’ve focused largely on trying to estimate population-wide covid prevalence and current rate of contagion. It’s become clear that antibody tests can’t answer the prevalence question, while contagion rates can be more directly measured through diagnostic tests administered at frequent intervals to stratified random samples.

What does the short half-life of antibodies imply for vaccine effectiveness? If covid-19 proves to be a seasonal infection, like influenza, then I suppose that an annual vaccine administered in the fall, near the beginning of covid season, would be the best bet. If a large enough proportion of the population gets immunized, then acquired herd immunity can be achieved during each season, effectively extinguishing the virus until next fall rolls around and the virus returns…

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