Background: Influenza poses infection threats to healthcare workers (HCW) and the risk of nosocomial transmission. Sustained adherence to seasonal influenza vaccination uptake annually is important in epidemic control.

Aim: To assess the adherence pattern of nurses to seasonal influenza vaccination over five years and its associated factors.

Methods: A cross-sectional study was conducted among nurses after the winter influenza season in Hong Kong in March 2019. Based on the influenza vaccine uptakes in the 2014/15-2018/19 seasons, respondents were stratified into three groups: "full adherence"(5 uptakes), "partial adherence"(1-4) and "non-adherence"(0). Stepwise multivariable logistic regression was performed to determine the associations between adherence to annual influenza vaccination, respondents´ characteristics and considerations for vaccination.

Findings: Of 1,306 nurses recruited, a majority were female (88%), with a median age of 36 years (IQR 30-46). Influenza vaccination uptake rate increased from 36% in the 2014/15 to 47% in the 2018/19 season. After stratification, 39%, 40% and 21% were non-, partial and full adherers respectively. Full adherence was significantly associated with female (adjusted odds ratio [aOR] 0.60), aged 40 or over (aOR 2.92), long-term care facility nurse (aOR 0.56) and uptake during studentship (aOR 3.83), local prevalence of seasonal influenza (aOR 0.51), and expert opinion (aOR 4.04).

Conclusions: This study identified a limited proportion of nurses being fully adherent to seasonal influenza vaccination. Monitoring adherence, improving access to vaccine and intervention tailored to the less adherent HCWs are crucial.