Morning or afternoon flu shot? It likely doesn’t matter

Clinical takeaway: Do not delay or reschedule influenza vaccination to target a particular hour. Available evidence suggests that receiving the vaccine—rather than receiving it at a specific time of day—is what matters clinically.

Influenza diagnosis rates varied slightly depending on the hour of vaccination, but there was no consistent advantage to receiving a flu shot in the morning rather than the afternoon. The differences were also too small to be clinically meaningful. Vaccination time was not associated with the risk of influenza hospitalization.

Across 3 influenza seasons, approximately 0.9% to 1.5% of vaccinated patients had a subsequent influenza encounter, while 0.11% to 0.19% were hospitalized with influenza. Among those later diagnosed, the median interval between vaccination and influenza diagnosis was 98 days.

The findings come from an analysis of 23.1 million influenza vaccinations administered to 15.9 million US adults during the 2023-2024, 2024-2025, and 2025-2026 seasons. Median patient age was 58 years, and 59% were women. Vaccinations were most commonly administered between 8 and 10 am, with a second peak from noon to 2 pm.

To account for potential differences between people who prefer morning and afternoon appointments, investigators also conducted a within-person analysis of 5.46 million adults vaccinated during at least 2 seasons, representing 12.6 million vaccinations. This allowed researchers to compare the same individuals when they received vaccines at different hours in different years.

The authors concluded that vaccine effectiveness “does not meaningfully differ by time of day of vaccine administration.” Although residual confounding remains possible, the study’s large sample and within-person design support a practical message: the best appointment time is the one the patient can keep.

Source: Wong KK, et al. (2026 Jul 24). JAMA Netw Open. Time of Day of Influenza Vaccination and Subsequent Influenza Risk