Participants with advanced COVID-19 had higher neutralizing titers (moderate/severe/critical GMT 247.36 IU/ml) compared to the 23 participants with mild COVID-19 (GMT 178.48 IU/ml), but the results of the two-tailed unpaired t test did not reach statistical significance (p value = 0.2443), as illustrated in Table 2 .
The SARS-CoV-2 neutralizing responses in convalescent COVID-19 individuals are known to show a decreasing trend with time after diagnosis. 3 , 7 , 21 , 23 , 24 , 25 , 26 The titers of neutralizing antibodies decreased modestly, with the titers at 3–4 months after diagnosis generally being approximately 4-fold lower than those at 1 month. 13 , 15 Our study suggests that convalescent COVID-19 individuals, after more than a year and four months, account for up to only 25% of positive NAbs on the 512nd day.