4. Discussion
Since its emergence in 2019, the SARS-CoV-2 virus has continued to
evolve into different mutant strains, posing continuous challenges to
the control of COVID-19.29,30 To achieve the goal of
herd immunity, a variety of vaccines have been introduced
worldwide.31 One of the main vaccines promoted in
China is the inactivated vaccines, which has a high inoculation coverage
among the population up to date.32 Thus, features of
the immune memory triggered by natural infections of SARS-CoV-2 have
become complicated, as several COVID-19 convalescents were vaccinated.
In this study, we mainly focused on the SARS-CoV-2-specific humoral and
cellular immunity and the ability to protect against Omicron variants
after inoculated inactivated vaccines in 2-year convalescents and
general healthy controls. Data from our study showed that IgG, IgA, and
neutralizing antibodies were significantly higher in vaccinated
convalescents than in vaccinated healthy controls. Though both groups
had lower cross antibodies to Omicron variants, the vaccinated 2-year
convalescents kept higher cross-antibody responses than the vaccinated
healthy controls. Meanwhile, the convalescents after vaccination carried
robust T cell immune memory to WT and no T cell immune escape was
observed for Omicron BA.1, and with a higher T cell immunity level than
the vaccinated healthy controls.
Our previous longitudinal study of COVID-19 convalescents demonstrated
that SARS-CoV-2-specific humoral immunity was present within 95% of
convalescents at 1-year post-disease onset,3 which was
also concordant with other cohort studies.1,2,33,34Recently, it was reported that NAbs against SARS-CoV-2 could persist for
one and a half years among COVID-19 convalescents.4,35However, most of the studies indicated a decreasing trend of the
antibody levels with time among the convalescents.1,4In our study, through the antibody titers decreased from 6 months to 1
year, a sharply increasing can be observed among the 2-year
convalescents after administration with inactivated vaccines, showing
the robust antibody-inducing capacity of the inactivated
vaccines,7,36,37 especially among the population after
natural infection. The enhanced antibody level may play a positive role
in the convalescents to avoid re-infection. Thus, vaccination should
still be encouraged for the convalescents.38
Although the IgG antibody could be obviously reinforced by the
inactivated vaccine among the 2-year convalescents after symptoms onset,
our data revealed positive rates of IgM antibody continued the
decreasing trend even among the convalescents after vaccination,
indicating that IgG among the convalescents could be more easily
enhanced by the inactivated vaccines.39
Previous studies showed that IgA dominates the early neutralizing
antibody response to SARS-CoV-2 and may play an important role in the
prevention of severe illness.40,41 And a higher
concentration of IgA antibody can be detected in the convalescents who
received booster vaccination of mRNA vaccine, but there is no
significant increase in booster-vaccinated COVID-19 naïve
people.33 Our studies indicated that the decreasing of
IgA antibodies in follow-up sera from 6 months to 1 year after symptoms
onset, and observed inactivated vaccines could also induce an increase
of IgA antibody titer at 2 years post-infection.
The persistence of T-cell immunity stimulated by natural infection may
play an important role in alleviating severe disease in the
reinfection.42 Here we found that T cell immune memory
was much higher in the 2-year convalescents after vaccination than the
vaccinated even boosted healthy controls, demonstrating that T cell
immune memory by natural infection could persist up to 2 years after
symptoms onset. Several studies show that mRNA vaccines could stimulate
a certain level of virus-specific CD4+ and
CD8+ T-cell immunity,8,43,44 while
T-cell immunity induced by inactivated vaccines is far from being
well-defined. However, unlike inactivated vaccines, mRNA vaccines are
designed primarily to focus on the S protein, the cellular immunity
stimulated by this vaccine only targets the S
protein.8 Our data showed that inactivated vaccines
could stimulate T cell immunity against structural proteins S, M, and N
in COVID-19 naïve people. Nevertheless, when evaluating the inactivated
vaccine-related T cell responses, the potential baseline cross-T cell
immunity against M or N proteins of common cold coronaviruses would be
considered.3
Despite extensive mutations-related neutralization escape against
Omicron could be observed, the overall T cell responses induced by
vaccination or infection could cross-recognize the
variants.28 We verified that the neutralizing ability
of the antibodies to Delta, Omicron BA.1, BA.2, and BA.4/5 decreased
among vaccinated 2-year convalescents and healthy controls. However, the
cross-neutralizing ability against the variants among the convalescents
was significantly stronger than that of the vaccinated healthy controls.
This may indicate a special cross-reactive antibody-inducing feature by
natural infection and the necessity for vaccination among the
convalescents. Previous studies determined that cellular immunity
elicited by the mRNA vaccine could cross-recognize the
Omicron.26,28,45 We herein showed that the cellular
immunity stimulated by inactivated vaccine also has a cross-response to
Omicron. As it turns out, the vaccination of inactivated vaccine is
beneficial to the population against the WT and its variants, and it is
an effective method to realize herd immunity.
Our current study has several limitations. First, the number of
subjects’ PBMCs available for the ICS test is limited, thus the
statistical analysis has difficulties showing the differences between
groups in the ICS test. However, our main conclusions are based on the
statistical differences within the ELISpot assays and antibody tests. We
also lacked PBMCs from unvaccinated healthy controls to estimate T cell
responses and it was impossible to recruit additional volunteers since
herd immunity had been established in China.
Second, we did not perform PCR
testing on the healthy controls to exclude asymptomatic infection, but
this was supported by our earlier epidemiological
analysis.46 Third, we use the ELISA kit with the
antigen S-RBD as a surrogate for live virus neutralization assay, the
detection of the neutralizing ability may be inaccurate because other
regions could also be neutralizing. However, previous studies suggested
that using RBD as an immunogen might be a better strategy for eliciting
a robust NAb response compared with full-length S
protein,47 and the ELISA results were highly
responsive to neutralizing antibody levels.48
In summary, we provide the special characteristics of the cellular and
humoral immune responses in 2-year convalescents after the inoculation
of inactivated vaccines. The decreasing trends of the IgG, IgA, and
neutralizing antibodies, but not IgM of the convalescents were reversed
by the vaccination. And both cross-reactive cellular and humoral
immunity to Omicron variants in convalescents after vaccination were
higher than in the vaccinated healthy controls. Thus, promoting vaccine
inoculation is an essential way to achieve herd immunity during the
ongoing COVID-19 pandemic, even for the convalescents.