Short-term safety and reactogenicity of same-day COVID-19 and influenza vaccination in very old, community-dwelling adults.

purposeTo assess short-term safety after routine same-day seasonal influenza and COVID-19 vaccination in community-dwelling adults aged 65 years and older, predominantly very old, and to identify factors associated with adverse events within 48 h.

methodsWe conducted a prospective observational cohort study in a hospital-based infectious diseases department and a collaborating outpatient clinic in the Czech Republic, enrolling adults aged 65 years and older vaccinated between 1 September and 30 November 2025. Participants completed 48 h symptom diaries and measured body temperature at 24 and 48 h. We compared local reactions between vaccine injection sites within individuals and used regression models to examine demographic and clinical correlates of adverse events and the predictive value of 24 h temperature.

resultsThe cohort included 169 participants, 75.1% of whom were women, with a mean age of 84.3 years and substantial multimorbidity. Within 48 h, 44.4% reported at least one adverse event, most reactions were mild, and no participant had fever at 48 h. Local reactions were more frequent at the COVID-19 than the influenza injection site. Higher comorbidity count predicted any adverse event (odds ratio 1.3, 95% CI 1.0-1.6), and prior COVID-19 infection predicted systemic adverse events (odds ratio 2.2, 95% CI 1.1-4.4). Higher body temperature at 24 h was associated with higher counts of total, systemic, and local adverse events at 48 h, with the strongest association observed for systemic symptoms (β = 0.4, p < 0.001).

conclusionSame-day influenza and COVID-19 vaccination was generally well tolerated in very old, medically complex adults, supporting coadministration with anticipatory counselling for those with higher comorbidity burden or prior COVID-19 infection.