Seroepidemiology of respiratory syncytial virus and influenza in infants: A prospective cohort study in China, 2023-2024

Abstract

Objectives

Respiratory syncytial virus (RSV) and influenza are two major pathogens responsible for acute respiratory infections in infants and children. We investigated the dynamic changes of serum antibodies against RSV and influenza in early childhood, aiming to assess exposure rates and factors associated with later exposure, to inform the development of targeted intervention strategies.

Methods

The study performed a community-based prospective cohort seroepidemiological study in children in Jiangsu Province, Eastern China, from February 2023 to August 2024, comprising 16 months of follow-up. Healthy infants aged between 2 and 3 months were recruited. Blood samples were collected from the cohort longitudinally at three study visits: visit 1 (enrollment, 2-3 months of age), visit 2 (5-7 months of age), and visit 3 (18-19 months of age). We detected immunoglobulin (Ig)G antibody concentrations against the RSV prefusion F (pre-F) protein, influenza A/H1N1, influenza A/H3N2, and influenza B using enzyme-linked immunosorbent assays. A four-fold or greater increase of antibody levels compared with baseline or the previous visit was considered as an exposure to RSV or influenza A/B viruses. Confirmed and hospitalization cases were collected through active monitoring of acute respiratory infections during the follow-up. Binary logistic regression models were employed to identify factors associated with exposure.

Results

A total of 357 participants aged 2-3 months were involved in the study. The pre-F IgG geometric mean concentration (GMC) against RSV was 5131.7 ng/ml (95% confidence interval [CI] 4834.0-5447.7) in infants at 2-3 months of age. This decreased to 612.5 ng/ml (95% CI 544.3-689.3) by 5-7 months of age. After that, the GMC of RSV pre-F IgG antibody gradually increased to 1608.1 ng/ml (95% CI 1239.7-2086.0) by 18-19 months. For influenza A/H1N1, A/H3N2, and B viruses, the GMCs of IgG antibodies were 912.4 ng/ml (95% CI 837.1-994.4), 613.9 ng/ml (95% CI 575.5-654.9), and 960.8 ng/ml (95% CI 896.9-1029.3) at 2-3 months of age, respectively. By 5-7 months of age, they declined to 637.5 ng/ml (95% CI 548.1-741.6), 483.9 ng/ml (95% CI 431.3-543.0), and 727.4 ng/ml (95% CI 646.8-818.1). At 18-19 months, GMC against A/H1N1 increased to 955.9 ng/ml (95% CI 833.2-1096.6) again, whereas the response to influenza B showed a marked increase, reaching 1531.9 ng/ml (95% CI 1344.3-1746.0). In contrast, antibody level against A/H3N2 remained relatively stable, with a GMC of 469.5 ng/ml (95% CI 413.6-533.0). The exposure rates were 52.9% for RSV, 28.0% for A/H1N1, 19.6% for A/H3N2, and 37.1% for influenza B. Exploratory analysis further revealed that maternal antibodies serve as protective factors against exposure to both RSV and influenza.

Conclusions

The data revealed the vulnerability of infants to RSV and influenza. These findings provide crucial evidence supporting the necessity of maternal vaccination during pregnancy and infant vaccination in early life.