Examination of commercially available serological assays is important to comprehend the immune response and the duration of post-vaccination protection. The antibody titres of SARS-CoV-2 spike protein were significantly higher in women at months 1 and 7 after vaccination among younger participants. A significant negative correlation was observed between age and changes in the antibody titres after vaccination only in men. Moreover, a significant positive correlation was observed between the antibody titres at months 1 and 7 after vaccination in participants with negative nucleocapsid protein, although a significant negative correlation was found in those with positive nucleocapsid protein antibody titres at month 7 after vaccination.
Immunogenicity based on age and sex
Antibody responses after vaccination showed high interindividual variation in levels. Recent studies provide conflicting evidence on the persistence of SARS-CoV-2 immunity induced by mRNA vaccines [15,16,17]. Antibody kinetics showed differences in immunogenicity according to sex, age group, and coexisting conditions [1, 3, 15, 18]. In this study, we observed that between-sex differences in antibody titres for spike protein after vaccination differed according to age groups; only among participants younger than 30 years, the antibody titres of SARS-CoV-2 spike protein were significantly higher in women at months 1 and 7 after vaccination. This result could differ from the previous study [1] showing between-sex differences in all age groups due to the timetable for the measurement of antibody titres because we did not examine the peak level in each individual. The difference in the age groups could be another reason.
In our study, the correlation between age and antibody titres for spike protein was negatively significant at month 1 after vaccination in both sexes, as previously reported [19]. This finding is consistent with the results showing that larger differences are observed in antibody titres between younger and older age groups [1]. When the correlation was examined between age and the reduction levels of the antibody titres from month 1 to month 7 after vaccination, there was a significant correlation only in men. This finding was according to the between-sex difference in the antibody titres at month 7 after vaccination among participants aged 50 years or older. Immune maintenance capacity may be lower in men in the aged population.
Infection affects the changes in antibody titres of spike protein after vaccination
Our findings using paired samples in each participant showed a positive correlation between the antibody titres for spike protein at months 1 and 7 after vaccination. However, in participants with positive nucleocapsid protein antibody at month 7 after vaccination, their antibody titres for spike protein at the two time points were high, and the correlation was significantly negative. Serologic testing of antibodies specific to SARS-CoV-2 nucleocapsid protein is used as a marker of a previous infection and may aid in surveying asymptomatic infection and assessing past SARS-CoV-2 infection prevalence. In the subset of the study population previously exposed to SARS-CoV-2 based on seropositivity for nucleocapsid antibodies, higher anti-spike IgG levels are already measured before the vaccine but no significant difference in anti-spike IgG levels from unexposed individuals is observed after vaccination [16]. We could not determine the time when the participants were positive for nucleocapsid protein antibodies at month 7 after vaccination infected SARS-CoV-2. Among the 19 participants with positive nucleocapsid protein antibody at month 7, only three participants had positive antibody titres of spike protein before vaccination and their antibody titres of spike protein were higher at month 1 than those at month 7 after vaccination or the other participants at month 1. The rest participants with positive nucleocapsid protein at month 7 (n = 16) may be infected after dose 1 and before blood collection at month 7. Although immunity against SARS-CoV-2 reduced after administration of the second dose of vaccine, [1, 2] the infection’s timing may affect antibody titres of spike protein after vaccination. At least five out of 19 participants with positive nucleocapsid protein at month 7 (Fig. 2) showed high antibody titres of spike protein at both time points, and their titres were inversely higher at month 7 after vaccination. Previous studies have reported that the antibody titres for spike protein are high [1, 19, 20] and their decay is slow [21, 22] in populations with a previous infection of SARS-CoV-2. These findings could be related to the fact that SARS-CoV-2 infection rates in those with nucleocapsid protein antibody positivity are low, compared to those with antibody-negativity [23, 24].
Limitations
We did not examine the factors related to antibody titres, such as comorbidities, except for age and sex. [25] We did not evaluate neutralising antibody kinetics; the correlation between antibody titres and neutralising function would be dependent on the time after vaccination. [1] We did not examine the precise timing for the decrease in antibody titres after vaccination, although it may be brisk until 70–80 days after vaccination. [1] Data on previous positivity are available only on a limited number of subjects (anti-S titer before vaccination) and there is no information on swabs before and after vaccination (gold standard for the diagnosis of infection). The determination of positivity at month 7 after vaccination was made with antibodies to the nucleocapsid protein. This method has several critical issues, in particular a low sensitivity in subjects with asymptomatic or mild infection. The participants were mostly healthy, and therefore, may not represent the general population. Although the sample size was small, we presented new knowledge regarding the divergence in antibody titres for spike protein after vaccination, between the sexes and according to age group, and emphasized the optimisation of these assays for evaluating post-vaccination antibody status.