Vaccine AdvisorInfluenza Vaccine Effectiveness Decreases With Repeat Vaccination Among Children

Annual flu vaccination still protects children from hospitalization, but repeat vaccination erodes that protection by 13.6% on average. A 10-year test-negative study of 34,237 hospitalized children found current-season-only recipients achieved the highest VE (66.1%), while repeat vaccinators saw modestly lower protection (61.5%) and prior-season-only recipients fared worst (40.2%).

Clinical Considerations

• Overall VE of 57.2% against influenza hospitalization confirms the program works, but strain-specific protection varies sharply: H1N1 (67.7%), influenza B (60.6%), H3N2 (37.2%)
• Repeat vaccination reduced VE by up to 29.1% for certain subtypes, with the steepest single-season decline occurring in 2022–2023 (VE change, -104.2%), though the effect did not persist the following season
• Influenza B was the exception: repeat vaccinators showed higher protection (70%) than current-season-only recipients (65%), suggesting subtype-specific immune dynamics
• Hospitalized children skew toward higher underlying disease burden, limiting generalizability to the broader pediatric outpatient population

Practice Applications

• Continue recommending annual influenza vaccination; overall hospitalization protection remains substantial despite repeat-vaccination attenuation
• Counsel families that current-season vaccination alone outperforms skipping a year; prior-season-only receipt drops VE to 40.2%
• Monitor emerging strategies to mitigate repeat-vaccination effects, including adjuvanted or high-dose pediatric formulations under investigation
• Document vaccination history to contextualize breakthrough illness in repeat-vaccinated children presenting with influenza symptoms

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