SEASONAL influenza is a contagious viral respiratory illness that can cause severe disease, particularly in older adults and people with chronic conditions. Annual influenza vaccination remains the main public health strategy to reduce hospitalisation and death, yet questions persist about whether repeated yearly doses blunt immune responses over time. A new community-based study from Hong Kong offers timely insight into how repeat vaccination shapes immunity in the absence of natural infection.
Why Repeated Influenza Vaccination Matters
Concerns about repeat influenza vaccination stem from observations that antibody responses shortly after vaccination may be lower in people vaccinated year after year. Most previous research has focused on immune responses around 30 days after vaccination, when antibody levels typically peak.
The Hong Kong study took advantage of a rare situation: influenza circulation was virtually absent for more than 2 years during the COVID-19 pandemic. This allowed researchers to assess vaccine-induced immune responses without the confounding effects of recent influenza infection.
Measuring Antibody Responses Over Time
Researchers followed a community cohort across two influenza seasons (2021/22 and 2022/23), measuring antibody responses to eight contemporary vaccine strains. Using standard haemagglutination inhibition assays, they examined how much antibody levels rose after influenza vaccination and how quickly those antibodies waned over subsequent months.
They found that people who received influenza vaccination more frequently showed smaller increases in antibody titres at around 2 weeks after vaccination, particularly in the later season. This supports earlier suggestions that repeated vaccination can dampen short-term immune boosting.
Six-Month Protection Tells a Different Story
However, the landscape shifted when looking beyond the early response. Although repeat vaccinees sometimes had slower antibody waning for certain influenza A and B strains, antibody levels at 6 months after vaccination were similar regardless of vaccination history.
For clinicians, this distinction is important. Early post-vaccination antibody rises are often used as markers of vaccine immunogenicity, but longer-term antibody persistence may be more relevant for protection across an influenza season.
Implications for Clinical Practice
The findings suggest that while repeated influenza vaccination may reduce the initial antibody surge, it does not substantially lower antibody levels later in the season. This helps reassure clinicians and patients that annual influenza vaccination remains beneficial, even with repeated uptake.
Importantly, the study does not assess clinical outcomes such as infection or hospitalisation, so it cannot determine vaccine effectiveness directly. Nonetheless, it supports current recommendations for annual influenza vaccination, especially in populations at higher risk of complications.
Reference
Zhong JC et al. Repeat influenza vaccination effects in 2021/22 and 2022/23 in a community-based cohort in Hong Kong. J Infect Dis. 2026; DOI:10.1093/infdis/jiag051.
