# 400 AU/mL IgG protective threshold against SARS-CoV-2 XBB reinfection in Chinese inactivated vaccine recipients: implications for booster vaccination

## TL;DR

The study finds that an IgG level of 400 AU/mL protects against SARS-CoV-2 XBB reinfection in people who received inactivated vaccines, especially older adults.

## Contribution

Identifies a specific IgG threshold (400 AU/mL) for protection against XBB reinfection in inactivated vaccine recipients.

## Key findings

- Median IgG levels declined by 28% over three months.
- Older adults (≥60 years) showed the fastest antibody decay, with 32.7% below the protective threshold.
- Each additional vaccination reduced reinfection risk by 13–15%.

## Abstract

Data on antibody dynamics and protective threshold generated by infection with Omicron subvariants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), particularly among populations that have primarily (>90%) received inactivated coronavirus disease 2019 (COVID-19) vaccines, remain limited. Using data from two large-scale paired serosurveys, we analyzed real-world changes in SARS-CoV-2 antibody levels associated with specific hybrid immunity.

The history of COVID-19 vaccination and SARS-CoV-2 infection were recorded for each participant. Serum samples collected at three-month intervals were analyzed for antibody levels against nucleocapsid (N) and spike (S) proteins using chemiluminescent microparticle immunoassay (CMIA). Linear mixed-effects model (LMM) and restricted cubic spline analysis were applied to assess antibody dynamics and determine protection thresholds, respectively.

A total of 4,065 participants were recruited in February and May 2023, with 2,894 completing both sampling rounds. Over three months, median IgG antibody levels declined by 28% (from 396.39 to 285.80 AU/mL). Prior infection and vaccination were significantly associated with higher antibody levels, while increasing age correlated with an annual decay of 0.38 AU/mL (most prominent in adults ≥60 years, 32.7% of whom fell below the protective threshold by follow-up). The IgG threshold for protection against XBB reinfection was 400 AU/mL, and each additional vaccination reduced reinfection risk by 13–15%.

The 400 AU/mL IgG threshold provides a actionable quantitative guideline for prioritizing booster doses in Chinese adults aged ≥60 years (the fastest antibody decay subgroup) and high-risk groups. Routine antibody testing combined with this threshold could optimize targeted COVID-19 vaccination strategies in inactivated vaccine-predominant populations.