Efficacy and immune responses of COVID-19 vaccines
The RBD of the SARS-CoV-2 spike protein is the primary target for
neutralizing antibodies induced by COVID-19 vaccines. Specific IgG
antibodies against conformational but not sequential RBD epitopes have
the potential to block the binding of SARS-CoV-2 with ACE2 and confer
protective immunity179. Mutations in RBD, as
identified in several VOC, display increased binding affinity to
ACE2180 and/or reduced neutralization ability of sera
from convalescent and BNT162B2 vaccinated
individuals181-184. Noticeably, BNT162b2-induced IgG
antibodies had higher avidity to mutated RBD than those induced by
natural infection66. The variant Omicron exhibited
approximately fourfold greater immune escape relative to the Beta
variant183. Prior COVID-19 was associated with higher
neutralization capacity for the ancestral virus after BNT162b2
vaccination183. More importantly, two doses of mRNA
COVID-19 vaccines provided almost non-existent, whereas a booster dose
yielded almost 75% protection against symptomatic infection of
Omicron184. It is predicted that BNT162b2 boosted, or
vaccination (two doses) combined with previous infection can prevent
73% symptomatic infection by Omicron, which is significantly higher
than in individuals with BNT162b2 vaccination only183.
Other types of vaccines have also been administered around the world.
CVnCoV is a vaccine based on unmodified RNA and induced only a 48%
reduction in the incidence of symptomatic disease185.
CV2CoV is the second-generation unmodified mRNA vaccine but with
optimized non-coding regions and enhanced antigen expression. Compared
to CVnCoV, CV2CoV induced higher titers of binding and neutralizing
antibodies against SARS-COV-2 variants, and memory B and T cell
responses non-human primates186. The inactivated
Sinopharm/BBIBP COVID-19 vaccine is widely used in developing countries
including China due to its low storage requirements. Seroconversion
rates in unexposed individuals after first and second dose reached 40%
and 100% respectively, and younger individuals and women had the
highest antibody concentrations. Previous SARS-CoV-2 infection was
associated with a strong antibody response after a single dose of the
BBIBP vaccine. A sharp increase in antibody concentrations was observed
following SARS-CoV-2 infection after the first and second
doses187. Virus-like particle-based COVID-19 vaccines
induced high levels of neutralizing antibodies and protection against
infection with SARS-CoV-2 and its variants in mice and rhesus
macaques188,189. PreS-RBD vaccine was developed based
on a recombinant fusion protein consisting of the human hepatitis B
virus-derived PreS antigen and two SARS-CoV-2 RBD domains. It induced a
robust anti-RBD IgG response in rabbits consisting of an early IgG1 and
sustained IgG4 which can be detected in serum and mucosa secretions.
Moreover, the vaccine-induced antibodies potently inhibited the
interaction of RBD with ACE2 and possessed the neutralizing ability of
the omicron VOC190. The efficacy and immune responses
elicited by different types of COVID-19 vaccines are listed in Table 3.