A recent article suggested the rapid decay of anti–SARS-CoV-2 IgG in early infection, but the rate was not described in detail. In this correspondence just published on The New England Journal a total of 31 of the 34 participants had two serial measurements of IgG levels and the remaining 3 participants had three serial measurements.

The first measurement was obtained at a mean of 37 days after the onset of symptoms (range, 18 to 65), and the last measurement was obtained at a mean of 86 days after the onset of symptoms (range, 44 to 119).

The initial mean IgG level was 3.48 log10 ng per milliliter (range, 2.52 to 4.41). On the basis of a linear regression model that included the participants’ age and sex, the days from symptom onset to the first measurement, and the first log10 antibody level, the estimated mean change (slope) was −0.0083 log10 ng per milliliter per day (range, −0.0352 to 0.0062), which corresponds to a half-life of approximately 73 days over the
observation period (Fig. 1A). The 95% confidence interval for the slope was −0.0115 to −0.0050 log10 ng per milliliter per day (half-life, 52 to 120 days).

These findings raise concern that humoral immunity against SARS-CoV-2 may not be long lasting in persons with mild illness, who compose the majority of persons with Covid-19.