PSMD1 and COVID-19: Lastly, the correlation between the anti-HBV titre (measured on March 3rd, 2021) and the anti-S1 IgG titre was not statistically significant at Ser1 (when 83.3% of the COVID-19-negative patients had received two doses; Spearman’s correlation coefficient r = 0.12, p = 0.1438) but was significant at Ser2 (when 80.0% of the COVID-19-negative patients had received three doses; Spearman’s correlation coefficient r = 0.17, p = 0.0452) (Fig. 5).