Fig. 1: A 4th dose of inactivated SARS-CoV-2 vaccine boosted NAbs against wildtype (WT) virus but not Omicron BA.1.

Thirty-eight HCWs who have already received 3 doses of inactivated SARS-CoV-2 vaccine volunteered in this clinical study and received a 4th homologous dose 6 months (26 wks) after the 3rd dose. a Neutralization assays were performed to measure NAbs titers against pseudoviruses with S protein from a WT strain (blue circle) or Omicron BA.1 variant (orange triangle). b Linear regression was performed on the fold-change of half pseudovirus neutralization titers (PVNT50) from 3rd 2 wks to 4th 2 wks and PVNT50 at 3rd 2 wks. The orange dashed line is a calculated threshold. The peak NAb levels could be further elevated by the 4th dose if the peak level after the 3rd dose is below this threshold. c The change of NAbs was shown for participants with the top 25% highest PVNT50 against WT virus. d The change of NAbs was shown for participants with the lowest (bottom 25%) PVNT50 against WT virus. NAbs at 3rd 2 wks and 4th 2 wks against WT virus (e) or Omicron variant (f) were compared respectively. The two-tailed spearman correlation coefficient was calculated for PVNT50 to WT and Omicron at 3rd 2 wks (g), 3rd 26 wks (h), or 4th 2 wks (i). For (a, b, e–i), n = 38 biologically independent samples. For (c, d), n = 9 biologically independent samples. Data were shown as Geometric mean ± 95% confidence level (Cl). The two-tailed spearman correlation coefficient was used for (b). Two-tailed Wilcoxon matched-pairs signed rank test was used for (c–f). Source data are provided as a Source Data file.