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.