Influenza vaccine effectiveness over 17 seasons in a large Japanese cohort: Analyses by age, virus type, underlying diseases and seasons before the COVID-19 pandemic

Background

As populations age and influenza reemerges post-COVID-19, evaluating vaccine effectiveness (VE) across age groups, comorbidities, virus types, and seasons is increasingly important.

Methods

A prospective multicenter cohort study was conducted annually by the Japan Physicians Association. Outpatients were registered with vaccination status before each influenza season and reported rapid antigen test results afterward. This analysis included 148,108 patients from 543 facilities across Japan during the 2002–2003 to 2018–2019 seasons. VE was estimated using unadjusted and adjusted logistic regression.

Results

Adjusted analyses showed significant protection in children ≤ 15 years and adults < 65 years, with VE of 56 % and 51 %, respectively. VE remained significant through the 40 s (20–47 %) but declined in those ≥ 50 and was nearly absent in individuals ≥ 80, especially ≥ 90. VE was lower in patients with underlying diseases (24 %) than in those without (47 %), with non-overlapping 95 % confidence intervals. An exception was children with bronchial asthma: incidence in unvaccinated children with asthma was higher (24.2 %) than in those without comorbidities (12.9 %), and VE was also high (60 %). Adjusted VE against influenza A remained significant through the 40 s (35–56 %) and against influenza B through the 20 s (27–49 %). Unadjusted VE against influenza A declined from 40 % (2002–2009) to 15 % (2010–2019), possibly due to antigenic drift or egg adaptation.

Conclusion

Seasonal influenza vaccines showed moderate effectiveness, especially in individuals < 40 years and those without comorbidities. Effectiveness declined with age, underscoring the need for improved vaccines and targeted strategies for older adults.