INTRODUCTION
Since the first coronavirus disease 2019 (COVID-19) case was reported in Wuhan in December 2019 and resulted in many deaths and serious cases worldwide, several new variants of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus have been identified[1,2]. Subsequently, SARS-CoV-2-related mutant variants have emerged. The SARS-CoV-2 B.1.617.2 (Delta) variant was first identified in October 2020 in Maharashtra, India, which was approximately 50–60% more transmissible than the original variant[3]. Thereafter, the B.1.1.529 (Omicron) variant was detected in South Africa in November 2021, and this variant has fueled more daily cases across many countries from the time it was discovered until today, compared to any previous variants[1,4,5]. Moreover, an increasing number of studies have consistently reported that the Omicron variant has increased transmissibility, severity, and immune escape abilities, which resulted in lower vaccine effectiveness (VE) against SARS-CoV-2 infection compared with previous variants[1,2,5-8]. The two-dose (mRNA, and inactivated vaccines) VEs were reported to be 50.5–74.6% against the Delta variant infection and 0–44% against the Omicron variant infection[9-15]. Limited data showed that the three-dose (mRNA vaccines) VE was 86.0–93.7% against the Delta variant infection and 7–47.4% against the Omicron variant infection[15-17]. These findings confirmed the substantially decreased protection from the booster dose against the Omicron variant infection. However, the effectiveness against severe illness and death related to the Omicron variant remains much better[18].
A total of three types of COVID-19 vaccines were authorized in China, including inactivated, Ad5-nCoV, and recombinant protein vaccines, among which inactivated vaccines were used most frequently[19,20]. There are few real-world studies on the VE of COVID-19 vaccines, especially recombinant protein vaccines against SARS-COV-2 in China, and most were limited to the Alpha or Delta variant[11,16,19,21]. The adjusted VEs of the inactivated vaccines and Ad5-nCoV vaccine were 51.8% (95% CI: 20.3–83.2%) and 61.5% (95% CI: 9.5–83.6%) against the Delta variant, respectively. In contrast, the adjusted VEs of the inactivated vaccines and Ad5-nCoV were 16.3% (15.4–17.2%) and 13.2% (10.9–15.5%) against the Omicron variant, respectively[22]. As China has undertaken a strategy entitled the “dynamic zero-COVID” beginning in August 2021, there were much far fewer large-scale local outbreaks after the first epidemic wave in 2020 than in other countries[16,23]. Therefore, this posed a challenge evaluating the effectiveness of COVID-19 vaccines in China. Recently, Huang, et al.[22] reported that high and durable two- and three-dose inactivated VE against Omicron that was associated with preventing severe or critical illness and death across all age groups, but provided lower effectiveness against Omicron infection. However, further data are needed to support the VE of recombinant vaccines to date.
A SARS-CoV-2 Omicron BA.2 variant outbreak was reported in Lu’an City in April 2022, providing an opportunity to estimate the VE of three types of COVID-19 vaccines used in China and globally (inactivated vaccines, recombinant protein vaccine, and Ad5-nCoV vaccine) in actual use so that policies could be adjusted to keep up-to-date with mutated variants, duration of vaccine-induced protection, and implementation of booster doses, including fourth doses.