Saturation time of exposure interval for cross-neutralization response to SARS-CoV-2: Implications for vaccine dose interval

Serum anti-nucleocapsid antibody correlates of protection from SARS-CoV-2 re-infection regardless of symptoms or immune history

BACKGROUND

High spike-based vaccine coverage led to a high seroprevalence of anti-spike (S) antibodies against SARS-CoV-2 in Japanese adults in 2024. Nevertheless, the COVID-19 epidemic continues, and individuals with hybrid immunity are becoming more common in these populations.

METHODS

We conducted a prospective cohort study to measure serum anti-SARS-CoV-2 antibody levels in 4496 Japanese adults as part of the national seroepidemiological survey. This study evaluated the correlation between first-visit anti-SARS-CoV-2 antibody levels and their effectiveness in providing protection until the second visit during the Omicron BA.5 epidemic.

RESULTS

Reduced symptomatic infection risk was found to be associated with anti-S antibody, anti-nucleocapsid (N) antibody, and BA.5 neutralizing antibody levels. However, the reduced asymptomatic infection risk associated with anti-S antibody or BA.5 neutralizing antibody levels was limited. In contrast, higher anti-N antibody levels were strongly linked to a reduced asymptomatic infection risk. Furthermore, higher anti-N antibody levels were also associated with a reduced risk of re-infection in individuals with hybrid immunity.

CONCLUSION

These observations highlight the potential of anti-N antibody level as a correlate of protection against SARS-CoV-2 asymptomatic infection and re-infection. The findings indicate that individuals with hybrid immunity have a distinct protective immunity against both symptomatic and asymptomatic infection beyond serum anti-S and neutralizing antibodies against circulating viral strains, which correlate with serum anti-N antibodies.

Infection-mediated immune response in SARS-CoV-2 breakthrough infection and implications for next-generation COVID-19 vaccine development

Post-vaccination infections, termed breakthrough infections, occur after the virus infection overcomes the vaccine-induced immune barrier. During the early stages of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron wave, high serum-neutralizing antibody titers against the Omicron variant were detected in individuals with breakthrough infections as well as those who received a third vaccine dose (i.e., booster recipients). Additionally, these cases indicated that Omicron antigens triggered an immune response that differed from that triggered by the vaccine strain before analysis of the effectiveness of new vaccines updated for the Omicron variants. Moreover, the magnitude and breadth of neutralizing antibody titers induced by breakthrough infections are correlated with the upper respiratory viral load at diagnosis and the duration between vaccination and infection, respectively. Unlike booster vaccine recipients, patients with breakthrough infections have varying durations between vaccination and infection. Accordingly, optimal booster vaccination intervals may be estimated based on the cross-neutralizing antibody response induced over time. Examination of breakthrough infection cases has provided valuable insights that could not be yielded by only examining vaccinated individuals alone. These insights include estimates of vaccine-induced immunity against SARS-CoV-2 variants and the various factors related to the clinical status. This review describes the immune response elicited by breakthrough infections; specifically, it discusses factors that affect the magnitude and breadth of serum antibody titers as well as the appropriate booster vaccination strategy. This review provides key aspects that could contribute to developing next-generation COVID-19 vaccines through breakthrough infection cases.