# Age-Specific Differences in the Dynamics of Neutralizing Antibody to Emerging SARS-CoV-2 Variants Following Breakthrough Infections: A Longitudinal Cohort Study

## TL;DR

Children initially have higher neutralizing antibodies against SARS-CoV-2 variants but lose protection faster than adults and the elderly, suggesting they are more vulnerable to new variants.

## Contribution

This study reveals age-specific differences in neutralizing antibody dynamics and effectiveness against emerging SARS-CoV-2 variants following breakthrough infections.

## Key findings

- Children had higher initial neutralizing antibody levels against SARS-CoV-2 variants but experienced rapid decline.
- Children's broad-spectrum neutralizing antibody effectiveness became significantly lower than adults and the elderly by late 2023.
- Children showed higher proportions of stable antibody levels, while adults and the elderly showed more early increases.

## Abstract

Background: The continuous evolution of SARS-CoV-2 necessitates the development of targeted strategies based on the immunological profiles of distinct age groups. Despite this imperative, comprehensive insights into the dynamics and broad-spectrum efficacy of neutralizing antibodies (NAbs) against emerging variants across different age groups, particularly in children, remain inadequate. Methods: Following the termination of China’s dynamic ‘zero-COVID-19’ policy in January 2023, which coincided with a widespread Omicron outbreak and numerous breakthrough infections, a longitudinal cohort study was established encompassing all age groups in Hubei, China. Follow-up assessments were conducted in March (Visit 1), June (Visit 2), and October (Visit 3) 2023. A total of 320 individuals were randomly selected and stratified into three age categories: children (<18 years, n = 80), adults (18–59 years, n = 167), and the elderly (≥60 years, n = 73). The NAbs against emerging SARS-CoV-2 variants BA.5, XBB.1.5, EG.5, and JN.1 were evaluated for each group. Trajectory modeling was employed to classify antibody trends into five categories: low-level stability, median-level stability, high-level stability, early increase, and late increase. Results: In March 2023, children exhibited significantly higher NAb levels against BA.5, XBB.1.5, EG.5, and JN.1 compared to adults and the elderly. However, these levels rapidly declined. From June to October 2023, no significant difference in NAb levels was observed between children and the other age groups. Regarding the broad-spectrum effectiveness of NAbs, the effectiveness in children was comparable to that of adults and the elderly in March 2023. However, from June to October 2023, children’s effectiveness became significantly lower than that of the other age groups. Trajectory analysis revealed that the highest proportions of high-level stability (31.3%) and median-level stability (42.5%) were observed among children. In contrast, adults and the elderly were most commonly categorized into the early increase (adult 46.7%, elderly 49.3%) and median-level stability (adult 22.1%, elderly 20.5%) categories. Conclusions: Although children initially demonstrate higher levels of NAbs, these levels decrease more rapidly than in adults and the elderly, eventually equalizing in later stages of recovery. Furthermore, the broad-spectrum effectiveness of NAbs in children is narrower than in other age groups. These findings suggest that children are at an elevated risk of infection with newly emerging variants, underscoring the urgent need to intensify focus on reinfections among children and develop tailored strategies to protect this vulnerable population.