Dear Editor,
My letter is concerned with a topic of utmost importance: The effect of the flu vaccine on flu-related death. That’s the ultimate endpoint of interest for vaccine effectiveness (VE) research, yet it is not captured in most studies that employ the test-negative design.
Faksova et al. (January 2026) reported in your journal results from a large cohort study of the flu vaccine in 2024–2025 in elderly residents (age≥65) of three Nordic countries. The estimated VE against flu-related death was 63%, which corresponds to a risk ratio (RR) of 0.37. How robust is the result? What does sensitivity analysis show?
The authors relied on a matched design to avoid confounding bias, a major threat in observational research. Unvaccinated members of the cohort were matched to the vaccinated (one-to-one matching) on several variables, including key comorbidities.
Sweden was not included in the mortality analysis (small numbers), and the mortality graphs were displayed by country (Denmark, Finland). I will examine the data from Denmark, a matched cohort of over one million people.
Graphs of cumulative mortality by vaccination status were displayed as of 14 days after a matched index date. For a vaccinated person, the index date was the vaccination date; for their unvaccinated counterpart, that matched date was just a random date on the calendar. I will return to this point later when we discuss their health status.
Hellbound and Down 