Synergistic influence of air temperature and vaccination on COVID-19 transmission and mortality in 146 countries or regions

OBJECTIVE: We aimed to determine the influence of vaccination and air temperature on COVID-19 transmission and severity. METHODS: The study data in 146 countries from January 6, 2020 to July 28, 2022 were aggregated into 19,856 weeks. Country-level weekly incidence, time-varying reproduction number (Rt), mortality, and infection–fatality ratio (IFR) were compared among groups of these weeks with different vaccination rates and air temperatures. RESULTS: Weeks with <15 °C air temperature and 60% vaccination showed the highest incidence (mean, 604; SD, 855; 95% CI, 553–656, unit, /100,000 persons; N = 1073) and the highest rate of weeks with >1 Rt (mean, 41.6%; SD, 1.49%; 95% CI, 39.2–45.2%; N = 1090), while weeks with >25 °C and <20% showed the lowest incidence (mean, 24; SD, 75; 95% CI, 22–26; N = 5805) and the lowest rate of weeks with >1 Rt (mean, 15.3%; SD, 0.461%; 95% CI, 14.2–16.2%; N = 6122). Mortality in weeks with <15 °C (mean, 2.1; SD, 2.8; 95% CI, 2.0–2.2, unit, /100,000 persons; N = 4365) was five times of the mortality in weeks with >25 °C (mean, 0.44; SD, 1; 95% CI, 0.41–0.46; N = 7741). IFR ranged between 2% and 2.6% (SD, 1.9%–2.4%; 95% CI, 2.0–2.7%) at < 20% vaccination level, 1.8% (SD, 2%–2.2%; 95% CI, 1.7–2.0%) at 20–60% vaccination level, and 0.7%–1% (SD, 1%–1.8%; 95% CI, 0.7–1.1%) at > 60% vaccination level and at all air temperatures (all P < 0.001). CONCLUSIONS: Vaccination was insufficient to mitigate the transmission since the significantly elevated weekly incidence and >1 Rt rate in weeks with high vaccination, while IFR was reduced by high vaccination. Countries with long-term low air temperature were affected by high transmission and high mortality.