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.