COVID-19 vaccination boosts the potency and breadth of the immune response against SARS-CoV-2 among recovered patients in Wuhan

COVID-19, caused by SARS-CoV-2, continues to spread globally with substantial morbidity and mortality. New SARS-CoV-2 variants of concern (Alpha, Beta, Gamma, Kappa, Delta, Omicron) have raised concerns about resistance to neutralizing antibodies elicited by prior infection, increasing the risk of reinfection. Reports (e.g., from South Africa) suggest Omicron can evade immunity from previous infection and is more likely to reinfect individuals. Long-term protection against SARS-CoV-2 relies on both antibody and T cell responses. While antibodies are crucial to prevent reinfection, humoral responses following primary infection wane over months. Memory T cells can contribute to protection, but recent work suggests functions of SARS-CoV-2-specific CD4+ T cells may decline by about one year after recovery. Decreases in both humoral and cellular immunity may increase breakthrough infections months to a year after recovery, underscoring vaccination as a key strategy for individuals regardless of infection history. Multiple effective vaccines exist globally, including inactivated platforms (e.g., BBIBP-CorV, CoronaVac), mRNA (mRNA-1273, BNT162b2), adenoviral-vectored (Ad26.COV2.S), and protein subunit (ZF2001). Most non-inactivated vaccines focus on spike (S) protein, especially RBD and NTD, to elicit neutralizing antibodies. Inactivated vaccines present whole virions including S and nucleocapsid (NP) and may elicit additional Fc-mediated effector functions. Prior studies show mRNA vaccination in recovered patients markedly enhances breadth and potency of humoral responses, but data on inactivated vaccines in recovered individuals are limited. This study aims to define the persistence and breadth of antibody responses and CD4+ T cell responses to SARS-CoV-2 and variants in recovered patients after inactivated vaccination.

The introduction synthesizes prior findings: antibody levels after primary SARS-CoV-2 infection decline over months; T cell memory can persist 6–12 months but functional exhaustion of CD4+ T cells may occur by one year post-recovery. Omicron has demonstrated increased ability to evade immunity from prior infection, raising reinfection risk. mRNA vaccination in convalescents is known to broaden and potentiate humoral responses. However, effects of inactivated vaccines on humoral and cellular immunity in recovered patients remain underexplored, motivating this study.

Design: Observational cohort of 448 previously infected (prototype strain) recovered COVID-19 participants in Wuhan. Groups: infection-only (n=47, unvaccinated) and hybrid-immunity (n=401, vaccinated after recovery) with 1 dose (n=105), 2 doses (n=160), or 3 doses (n=136). Disease severity categories recorded (asymptomatic to critical). Sampling schedule spanned 1–3, 4–6, 7–9, and 10–12 months after final vaccine dose for vaccinated participants. Vaccines: Seven products used overall; four inactivated vaccine series based on the prototype strain (e.g., BBIBP-CorV, CoronaVac, KCONVAC), produced via Vero cell culture, β-propiolactone inactivation, and aluminum hydroxide adjuvant. Laboratory assays: - ELISA to quantify IgG titers against SARS-CoV-2 prototype RBD (RBD-IgG) and NP (NP-IgG). - Neutralization assays using vesicular stomatitis virus-based SARS-CoV-2 pseudotyped particles and authentic virus to determine 50% inhibitory concentration (IC50) neutralizing titers against Wuhan-Hu-1, Delta, Omicron BA.1, and BA.2. - Cellular immunity: Peripheral blood mononuclear cells (PBMCs) assessed by intracellular cytokine staining (ICS) and activation-induced marker (AIM) assays to quantify SARS-CoV-2-specific CD4+ T cell responses. Analysis included time-stratified comparisons and correlations between antibody titers and clinical variables (disease severity, age, sex).

- Inactivated COVID-19 vaccination in recovered patients maintained high RBD-IgG and NP-IgG titers up to 9 months post vaccination, with no major titer differences among 1-, 2-, and 3-dose subgroups within 3 months post vaccination. - Neutralizing antibody (NAb) geometric mean titers (GMTs) against pseudotyped Wuhan-Hu-1, Delta, Omicron BA.1, and BA.2 were markedly higher in the hybrid-immunity group than infection-only and persisted up to 9 months, though declining over time. - Within 3 months post vaccination, hybrid-immunity NAb GMTs were elevated by approximately 3.7- to 4.8-fold versus infection-only across tested strains. - Omicron variants exhibited greater resistance to neutralization than prototype and Delta strains. - RBD-IgG titers positively correlated with NP-IgG titers (n=448). - No clear correlation was observed between RBD-IgG (and NP-IgG) levels and disease severity, age, or sex. - Vaccination increased the magnitude of SARS-CoV-2-specific CD4+ T cell responses in recovered individuals (by ICS/AIM assays), indicating enhanced cellular immunity breadth and potency alongside humoral responses.

The data show that inactivated vaccination of COVID-19 convalescents substantially augments both the potency and breadth of humoral and cellular immunity compared with prior infection alone. Elevated and durable (up to 9 months) antibody titers and higher neutralization breadth against Wuhan-Hu-1, Delta, and Omicron sublineages BA.1/BA.2 support the concept of hybrid immunity. Nonetheless, antibody waning over time and reduced neutralization of Omicron relative to earlier strains indicate ongoing immune escape and the potential need for booster strategies. The observed increase in SARS-CoV-2-specific CD4+ T cell responses suggests improved cellular immunity that may contribute to protection against severe disease upon re-exposure. These findings support vaccinating recovered individuals to mitigate reinfection risk from circulating and emerging variants.

Inactivated COVID-19 vaccination in recovered patients boosts and broadens humoral (RBD-IgG, NP-IgG, and neutralizing activity) and cellular (CD4+ T cell) responses, outperforming immunity from infection alone and providing protection against prototype and variant strains, including Delta and Omicron sublineages. Although responses wane over time and Omicron shows greater neutralization resistance, vaccination enhances overall immune preparedness in convalescents. Future research should define the durability beyond 9–12 months, the impact of additional boosters and variant-adapted formulations, quantitative correlates of protection for hybrid immunity, and detailed T cell epitope breadth against newer subvariants.