STRENGTHS AND LIMITATIONS
Our study has some strengths. First, our study is somewhat unique in that we investigated recombinant protein vaccine against Omicron BA.2 variant infection in the real world. Previous studies conducted in China concentrated on the VE of inactivated vaccines and the Ad5-nCoV vaccine, and most of the studies were clinical studies. Second, all age groups of participants were included in our study compared to previous studies in which most considered participants were over 18 years old. Third, we performed a sensitivity analysis, potential confounding factors (age, gender, and time since the last dose of vaccination) were adjusted, and the final results indicated that the adjusted VEs were robust.
Our findings are also subject to the following limitations. First, the sample size was small, as indicated by the very wide confidence intervals. However, this outbreak in Lu’an had a limited number of positive individuals. Second, we did not estimate the VE of the heterologous booster vaccination with the Ad5-CoV vaccine and recombinant protein vaccine separately though there may be a possibility for differences in the VE, which was due to the small number of individuals in the heterologous booster vaccination group in the minority. Third, although all subjects in the control group were core close contacts, the frequency of exposure was ultimately different for each individual and may have had a slight effect on the final results. We reviewed the epidemiological survey reports of each positive individual and searched for testing results of negative controls with a high frequency of exposure, making every effort to ensure that the frequency of exposure was consistent across controls.