Through large-scale population testing, our study overs valuable insights into the dynamic changes in the Nabs against specific strains of SARS-CoV-2. Monthly monitoring of Nab titers enabled us to thoroughly understand the changes following the BA.5 breakthrough infection period and the subsequent establishment of immunity.¹² In 2021 and 2022, vaccines maintained sufficient neutralizing antibody levels against the Beta and Delta variants, although additional containment measures, such as behavioral guidelines, also contributed to reducing viral spread. According to a report by the Chinese CDC,¹³ JN.1 became the dominant strain nationwide in the third week of January 2024, accounting for nearly 50% of all cases, and reached 90% by the end of February 2024. During this period, the nationwide COVID-19 positivity rate peaked at 14.3%, slightly lower than the 18% observed in our study. Additionally, we noted a significant decline in the number of patient tests compared to earlier periods (Fig. 1a). These findings suggest that the current pattern of case increases every four to seven months is unlikely to result in peaks exceeding those of previous waves, unless a new strain with significant mutations emerges. JN.1, which emerged in late 2023, is phylogenetically distinct from BA.5.¹⁴ However, we observed that Nabs against XBB.1.5 and EG.5 increased during the JN.1 wave rather than during the BA.5 wave. This indicates that JN.1 can evade the immune barrier established by prior BA.5 infection.^15,16 Conversely, vaccines based on XBB.1.5 significantly elevated the titers of Omicron-specific Nabs against JN.1.¹⁷

Instead of focusing on small cohorts or cross-sectional analyses, we collected hundreds of serum samples each month for over a year to observe the dynamic changes in the titers of Nabs against circulating strains. This approach allowed us to demonstrate antibody responses have evolved in real world scenarios. Although nucleic acid testing and clinical diagnoses indicated no large-scale outbreak after February 2023, infection rates increased periodically every few months. These subsequent cases were predominantly asymptomatic or mildly symptomatic. This underscores the importance of dynamically monitoring viral mutations and changes in Nab levels. RNA viruses, including SARS‑CoV‑2, exhibit high mutation rates due to the limited proofreading capacity of their RNA‑dependent RNA polymerases. In our study, we observed that emerging variants—such as XBB.1.5, EG.5, and JN.1—displayed significantly altered neutralizing antibody titers compared to BA.5. These changes in GMTs closely correlate with specific spike protein mutations that reduce antibody binding and facilitate immune escape.^18,19 This concern is heightened during periods of immune stress screenings, as evolving strains may render existing antibodies less effective. The continuous evolution of RNA viruses under immune pressure highlights the need for ongoing surveillance. While widespread vaccination and prior SARS‑CoV‑2 infections have undoubtedly raised overall population immunity, the classical concept of herd immunity—which posits that infection spread is contained once a critical immunity threshold is reached—may not fully apply to COVID‑19.²⁰ This limitation is due to several factors. First, immunity (whether from vaccination or infection) wanes over time, reducing the duration of protection. Second, the emergence of variants with enhanced immune escape properties further undermines long‑term group protection.^21,22 For instance, our data demonstrate that even in populations with substantial prior exposure, a decline in cross‑neutralizing antibody titers can coincide with the emergence of new variants, which suggests that the immune barrier is compromised. Highlighting the importance of continuous immunological surveillance and adaptive vaccine strategies. The recurring development of new variants to enhance transmissibility can be seen as a natural process of maximizing their evolutionary fitness. Additionally, the covert spread of SARS-CoV-2 in regions with limited genetic monitoring²³ and persistent SARS-CoV-2 infections in individuals with weakened immune²⁴ systems further facilitate the emergence of new variants. Our findings indicate that monitoring dynamic changes in neutralizing antibody titers can provide early warnings of waning immunity and emerging immune evasion. Although Nab measurements alone are not fully predictive of whether a new variant will become dominant, a significant decline in titers—for example, a 2- to 3-fold reduction compared to levels associated with effective protection—especially when coupled with the emergence of variants that are less efficiently neutralized, suggests a compromised immune barrier. Incorporating such immunological signals with genomic and epidemiological data can inform the timely update of vaccine immunogens. This integrated approach is essential for optimizing vaccination strategies and effectively counteracting evolving viral threats on a global scale.

Numerous studies indicate that advanced age is associated with reduced Nab titers against SARS‑CoV‑2, which may contribute to increased susceptibility to Omicron subvariants among elderly individuals.^25,26 However, several researchers have also observed that Nab responses exhibit considerable age-related variability. In some cohorts, the decline in antibody titers with age appears minimal—an effect that may be masked by confounding factors such as the vaccine platform used and the time elapsed since vaccination.²⁷ Our analysis revealed that, apart from BA.5, there were no statistically significant correlations between age and neutralizing antibody titers for XBB.1.5, EG.5, and JN.1. Even for BA.5, the observed correlation was extremely weak, indicating that age was not a major determinant of Nab titers in our cohort. We also analyzed the impact of gender on Nab generation (Supplementary Fig. 2). The results showed that females had higher Nab titers against BA.5, consistent with previous research suggesting that women develop more robust immune responses to infections and vaccinations and are more prone to autoimmune diseases compared to men.^28,29 While our study provides a comprehensive analysis of humoral immunity in the general population, it does not specifically include high-risk populations, such as heavy smokers and immunocompromised individuals. These groups are known to exhibit distinct immune responses and may be more vulnerable to emerging SARS-CoV-2 variants.

Overall, our data illustrates the dynamic changes in Nabs following the December 2022 breakthrough infection across a large population over a period of more than a year. Continuous tracking of Nabs against emerging SARS-CoV-2 variants has proven critical for maintaining effective pandemic control strategies, including vaccination. Since the immunogens in earlier vaccines were based on the original strains, they did not provide effective pre-existing immunity and were only moderately effective against Omicron lineages,³⁰ As new variants continue to emerge and evolve, the choice of vaccine becomes increasingly vital. Our study demonstrates the critical role of viral evolution in immune escape. Molecular and structural analyses into future research will provide a comprehensive understanding of SARS-CoV-2’s mechanisms of adaptation, guiding vaccine development and therapeutic strategies against emerging variants. Moreover, ongoing monitoring of antibodies within the population is essential for understanding the establishment of immunity and the development of broad-spectrum Nabs.