Effectiveness of COVID-19 and seasonal influenza vaccines in older adults in Hong Kong

Abstract

The COVID-19 pandemic caused enormous public health impact. The development and introduction of COVID-19 vaccines allowed the safer relaxation of stringent public health and social measures that had been implemented in many parts of the world to slow transmission. However, the effectiveness and potential safety risks of these vaccines were subjects of debate throughout the pandemic. In contrast, the influenza vaccination pro-gram has been in place for many years, with the World Health Organization and various international governments recommending annual administration of seasonal influenza vaccines. In subtropical and tropical locations twice-annual influenza vaccination has been discussed but without much evidence on the potential effectiveness.

To evaluate the effectiveness of twice-annual influenza vaccination, the effectiveness of COVID-19 vaccines, and the relative risk conferred by hybrid immunity in older adults in Hong Kong, I analyzed data from two cohorts of older adults recruited from the community in Hong Kong. A total of 504 participants were followed up in the RETAIN study, which is an ongoing randomized trial of once-annual versus twice-annual influenza vaccination. A total of 1,221 participants were followed up in the PIVOT study, which is an ongoing randomized-controlled trial of enhanced influenza vaccines. In the RETAIN study I assessed the relative effectiveness of twice-annual influenza vaccination over 8 years and evaluated hybrid immunity to COVID-19, and in the PIVOT cohort I esti-mated the effectiveness of COVID-19 vaccines. The COVID-19 analyses were con-ducted across four SARS-CoV-2 epidemic waves in Hong Kong during 2022-24, uti-lizing Cox regression models to estimate vaccine effectiveness.

The relative vaccine effectiveness (rVE) for the twice-annual vaccination in the RE-TAIN study was estimated in 3 time periods: Pre-COVID, Post-COVID and across the whole study period. The rVE estimates were 17% (95% CI: -83%, 63%), 43% (95% CI: -146%, 89%), 30% (95% CI: -44%, 69%) respectively. The VE for COVID-19 vac-cines in the PIVOT study when comparing 3 doses of BNT162b2 (BioN-Tech/Pfizer/Fosun) vaccines to 2 doses of BNT162b2 vaccines in wave 5 was 39% (95% CI: -4%, 64%). The estimated rVE for 3 doses of CoronaVac (Sinovac) versus 2 doses of CoronaVac in wave 5 was 21% (95% CI: -32%, 53%). In the smaller RETAIN co-hort I examined hybrid immunity in the latter waves of SARS-CoV-2, finding that prior vaccination and infection reduced the risk of COVID-19 in the 7th wave by 92% (95% CI: 41%, 99%), and by 42% (95% CI: -26%, 74%) in wave 8.

The outcomes of my analyses suggested the potential benefit of booster doses of COVID-19 vaccines in older adults. Twice-annual influenza vaccination provided a similar level of protection against clinical influenza as once-annual vaccination.