Serum anti-nucleocapsid antibody level induced after primary infection is an immunological surrogate of protection against SARS-CoV-2 re-infection in hybrid immunity holders
Authors: Miyamoto Sho , Numakura Koki , Kinoshita Ryo , Arashiro Takeshi , Takahashi Hiromizu , Hibino Hiromi , Hayakawa Minako , Kanno Takayuki , Sataka Akiko , Ainai Akira , Arai Satoru , Suzuki Motoi , Yoneoka Daisuke , Wakita Takaji , Suzuki Tadaki 
Published: June 2024

Background

In 2024, there was quite high seroprevalence of anti-spike (S) protein antibodies against SARS-CoV-2 in Japanese adults, owing to the high vaccination coverage by spike-based vaccines. Nevertheless, the COVID-19 epidemic continues, albeit with low rates of severe illness, and hybrid immunity holders are becoming more common in these populations. It is necessary to determine the immunological protection correlates against SARS-CoV-2 re-infection in individuals with hybrid immunity because the currently available immune correlates were established by analyzing individuals possessing vaccine-induced immunity only.

Methods

We conducted an ad hoc prospective cohort study to measure serum anti-SARS-CoV-2 antibody levels in 4,496 Japanese adults as part of the national COVID-19 seroepidemiological survey. This ad hoc study evaluated the correlation between anti-S and anti-nucleocapsid (N) antibody levels at the first visit and their effectiveness in infection prevention until the second visit, including undiagnosed re-infections during the Omicron BA.5 epidemic period from December 2022 to March 2023.

Findings

We assessed the combined effect of anti-N and anti-S antibody levels and found that the reduced infection risk associated with anti-S antibody levels was limited. Contrastingly, higher levels of anti-N antibodies were strongly linked to a reduced infection risk in the entire cohort and in individuals with hybrid immunity.

Interpretation

We demonstrate a high correlation between reduced re-infection risk in hybrid immunity holders and high serum anti-N antibody levels, highlighting its potential as an immunological surrogate of protection against SARS-CoV-2 re-infection. The findings indicate that individuals with hybrid immunity are protected by a distinct form of immunity, beyond the presence of serum anti-S antibodies, which correlates with serum anti-N antibody levels.

Funding

The national COVID-19 seroepidemiological survey as a public health investigation was funded by the Ministry of Health, Labour and Welfare of Japan (MHLW). The ad hoc study based on the survey data as a research activity was funded by the Japan Agency for Medical Research and Development (AMED).

Research in context

Evidence before this study

We searched PubMed for studies published between January 1, 2022, and April 18, 2024, using the search terms “SARS-CoV-2” in combination with the search terms “antibody,” “Omicron,” AND “Correlate(s) of Protection,” with no language restrictions. Studies on the correlates of protection (CoP) using antibody titers to prevent Omicron infection have primarily been performed during Omicron BA.1/2 waves. One report indicated serum correlates of protection involving anti-spike (S) antibodies against Omicron BA.5, but the anti-S antibody titer thresholds varied according to previous infection histories. The investigation of quantitative immunological markers that serve as correlates of protection against infection among populations with various immune histories through vaccination and infection should include asymptomatic or undiagnosed re-infected cases, which would be useful for the development of next-generation COVID-19 vaccines that would control future COVID-19 epidemics. However, the immune correlates of protection against re-infection, especially among hybrid immunity holders with a history of infections and vaccination, remains unclear.

Added value of this study

Our study evaluated immunological markers for infection prevention in adults with both vaccination and infection histories during the Omicron sublineage epidemic period. The reduction in re-infection risk during the Omicron BA.5 epidemic period correlated with higher anti-nucleocapsid (N) antibody levels. Conversely, anti-S antibody titers induced by both vaccines and infections were less strongly correlated with protection. These results may account for the variation in anti-S antibody titers’ effectiveness in protecting against Omicron sublineages, highlighting the usefulness of anti-N antibody levels for estimating the antiviral immunity level in hybrid immunity holders, the majority of the population with high vaccination coverage.

Implications of all the available evidence

Previously established immunological correlates for the prevention of SARS-CoV-2 infection are serum anti-S antibody levels and neutralization titers induced by vaccination or infection. In contrast, serum anti-N antibody responses are considered to be immune responses induced by infection. Our findings suggest that infection-induced anti-N antibody levels represent a non-mechanical immunological surrogate for protection against re-infection. According to the study’s results, people with hybrid immunity have an unique immunity that correlates with serum anti-N antibody levels above and beyond the presence of serum anti-S antibodies, suggesting the potential for the development of a next-generation COVID-19 vaccine that can induce more effective immunity by mimicking hybrid immunity.