Spotlight influenza: Estimation of influenza vaccine effectiveness in elderly people with assessment of residual confounding by negative control outcomes, Finland, 2012/13 to 2019/20

Abstract

Background Cohort studies on vaccine effectiveness are prone to confounding bias if the distribution of risk factors is unbalanced between vaccinated and unvaccinated study subjects. Aim We aimed to estimate influenza vaccine effectiveness in the elderly population in Finland by controlling for a sufficient set of confounders based on routinely available register data. Methods For each of the eight consecutive influenza seasons from 2012/13 through 2019/20, we conducted a cohort study comparing the hazards of laboratory-confirmed influenza in vaccinated and unvaccinated people aged 65–100 years using individual-level medical and demographic data. Vaccine effectiveness was estimated as 1 minus the hazard ratio adjusted for the confounders age, sex, vaccination history, nights hospitalised in the past and presence of underlying chronic conditions. To assess the adequacy of the selected set of confounders, we estimated hazard ratios of off-season hospitalisation for acute respiratory infection as a negative control outcome. Results Each analysed cohort comprised around 1 million subjects, of whom 37% to 49% were vaccinated. Vaccine effectiveness against laboratory-confirmed influenza ranged from 16% (95% confidence interval (CI): 12–19) to 48% (95% CI: 41–54). More than 80% of the laboratory-confirmed cases were hospitalised. The adjusted off-season hazard ratio estimates varied between 1.00 (95% CI: 0.94–1.05) and 1.08 (95% CI: 1.01–1.15), indicating that residual confounding was absent or negligible. Conclusion Seasonal influenza vaccination reduces the hazard of severe influenza disease in vaccinated elderly people. Data about age, sex, vaccination history and utilisation of hospital care proved sufficient to control confounding.

High vaccine effectiveness against severe Covid-19 in the elderly in Finland before and after the emergence of Omicron

“…We found no meaningful difference in the effectiveness of the 2 mRNA vaccines. The effectiveness of Vaxzevria was, however, slightly lower, although it was still better than the average protection observed among the elderly after seasonal influenza vaccination [27,28]. Nevertheless, individuals who were first vaccinated with 2 doses of Vaxzevria and later boosted with mRNA vaccine were as well protected as those who received 3 doses of mRNA vaccine.…”

Preparing for an influenza season 2021/22 with a likely co-circulation of influenza virus and SARS-CoV-2

“…Many efforts have been undertaken in the last decade to improve the design of these studies. Baum et al obtained confounder-adjusted estimates of influenza VE in the elderly population in Finland using a register-based cohort study design based on medical and demographic register data [ 19 ]. One of the covariates that they used as confounders is the presence of underlying conditions based on the subject-specific 1-year history of inpatient and outpatient hospital visits before season onset.…”

The evolutionary footprint of influenza A subtype H3N2 strains in Bangladesh: implication of vaccine strain selection

“…For instance, the vaccine of the 2019–2020 season made a close cluster with north American 2018 strains while globally circulated 2019–2020 strains were in a distinctive cluster, which might result in low (39%) VE in the USA for season 2019–2020 30 . Such disparity between circulating viruses and the vaccine strain could have resulted in low immunity of populations, leading to reduced VE previously reported globally 31 – 34 . On the other hand, the overall VE was estimated below 40%, even after genetic proximity with circulating strains 34 – 36 , due to the co-circulation of different strains and/or sudden decline of a dominant strain 37 .…”