Zhong Shuyi, Thompson Mark G, Cowling Benjamin J
School of Public Health, University of Hong Kong, Hong Kong, China.
Westat, Rockville, Maryland, USA.
Influenza Other Respir Viruses. 2025 Mar;19(3):e70052. doi: 10.1111/irv.70052.
Influenza vaccination is recommended annually based on the evolving nature of influenza viruses and the waning of vaccine-induced immunity. The timing of vaccination is usually before the winter influenza season in most temperate locations, where the seasonality is clear and influenza activities on average last no longer than 6 months. However, many tropical and subtropical areas have year-round influenza activity and multiple epidemics within 1 year, against which annual influenza vaccination may not offer sufficient protection at the individual level.
A twice-annual vaccination program could utilize standard inactivated influenza vaccines or enhanced influenza vaccines. Here, we discuss three reasons to consider twice-annual vaccination as a strategy to improve protection.
The first, mentioned above, is that some locations experience prolonged or year-round influenza activity. The second reason is based on the observation that vaccine effectiveness significantly declines about 6 months after vaccination particularly for A(H3N2) strains, and therefore, vaccination twice a year might be beneficial to maintain a higher level of immunity in the second half of each year. The third reason is to allow for receipt of the most updated vaccine strains, given that these are updated twice each year by the World Health Organization. We also discuss three potential barriers or challenges. The first potential challenge is knowledge gaps, because there are very few existing studies that used twice-annual vaccination. The second potential barrier is a concern over whether more frequent vaccination would lead to reduced immunogenicity or reduced clinical protection in the longer term. The third relates to concerns about cost or feasibility.
We discuss these issues and recommend comparative assessment of the incremental benefits and cost of twice-annual vaccination versus annual vaccination, as well as other vaccination strategies aiming to reduce influenza disease burden particularly in tropical and subtropical locations where there can be year-round influenza activity.
鉴于流感病毒不断变化的特性以及疫苗诱导免疫力的减弱，建议每年接种流感疫苗。在大多数季节性明显且流感活动平均持续不超过6个月的温带地区，接种疫苗的时间通常在冬季流感季节之前。然而，许多热带和亚热带地区全年都有流感活动，且1年内会出现多次疫情，针对这些地区，每年接种流感疫苗在个体层面可能无法提供足够的保护。
每年接种两次的疫苗接种计划可以使用标准的灭活流感疫苗或增强型流感疫苗。在此，我们讨论将每年接种两次作为提高保护效果的策略的三个理由。
第一个理由如上文所述，即一些地区经历长期或全年的流感活动。第二个理由基于这样的观察结果，即疫苗效力在接种后约6个月会显著下降，尤其是对于A(H3N2)毒株，因此，每年接种两次疫苗可能有利于在每年下半年维持较高水平的免疫力。第三个理由是考虑到世界卫生组织每年对流感疫苗株更新两次，每年接种两次能够接种到最新的疫苗株。我们还讨论了三个潜在的障碍或挑战。第一个潜在挑战是知识差距，因为现有的关于每年接种两次疫苗的研究非常少。第二个潜在障碍是担心更频繁地接种疫苗从长期来看是否会导致免疫原性降低或临床保护作用减弱。第三个问题涉及对成本或可行性的担忧。
我们讨论了这些问题，并建议对每年接种两次疫苗与每年接种一次疫苗的增量效益和成本，以及其他旨在减轻流感疾病负担的疫苗接种策略进行比较评估，特别是在全年都可能有流感活动的热带和亚热带地区。
