Long-term immunogenicity in previously vaccinated healthcare workers with inactivated virus vaccine after SARS-CoV-2 infection or booster vaccination

Immunity against SARS-CoV-2 infection in vaccinated individuals varies based on the vaccine type, duration after vaccination or infection, and SARS-CoV-2 variant type. We conducted a prospective observational study to evaluate the immunogenicity of a booster vaccination with AZD1222 after two doses of CoronaVac (booster group) compared to individuals who had SARS-CoV-2 infection after receiving two doses of CoronaVac (infection group). We used a surrogate virus neutralization test (sVNT) to evaluate immunity against wild-type and Omicron variant (BA.1) at 3 and 6 months after infection or booster dose. Of the 89 participants, 41 were in the infection group, and 48 were in the booster group. At 3 months post-infection or booster vaccination, the median (IQR) sVNT against wild-type was 97.87 % (97.57–97.93 %) and 97.65 % (95.38–98.00 %), p = 0.66, respectively, while the sVNT against Omicron was 18.8 % (0–47.10 %) and 24.46 (11.69–35.47 %), p = 0.72 respectively. At 6 months, the median (IQR) sVNT against wild-type was 97.68 % (95.86–97.92 %) in the infection group, higher than 94.7 % (95.38–98.00 %) in the booster group (p = 0.03). Results showed no significant difference in immunity against wild-type and Omicron at 3 months between the two groups. However, the infection group exhibited better immunity than the booster group at 6 months.