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.