Associations between sunshine and influenza or influenza-like illness, a systematic review and meta-analysis.

Influenza and influenza-like illness (ILI) impose an immense burden to public health. Exposure to ultraviolet (UV) radiation might impact the spread of influenza. We examined associations between UV radiation and sunshine duration and influenza or ILI. We performed a systematic review and meta-analysis including studies with UV radiation, UV index or sunshine duration as exposure and influenza/ILI as outcome. A random-effects meta-analysis was conducted using the Knapp-Hartung method to calculate the pooled relative risks (RR) of the associations between UV radiation and sunshine duration and influenza. Several sensitivity analyses were performed. 45 studies which met the eligibility criteria were included in the systematic review, of which 16 were included for meta-analysis, covering more than 1,000,000 influenza cases. The pooled results showed every 1 W/m2 increase in solar radiation was significantly associated with a decrease in influenza incidence RR 0.996 95%-confidence interval [0.993 - 0.999]. In the meta-analysis of sunshine duration, pooled estimates showed no significant association between an increase in sunshine duration and influenza incidence (RR 1.003 [0.988 - 1.018]) per one hour increment. Results were robust for all sensitivity analyses. In this meta-analysis, an increase of UV radiation provided a protective effect, while sunshine duration showed no significant effect, perhaps due to lack of eligible studies on this relationship. Findings indicate that UV radiation could be a helpful parameter for predicting influenza incidence, specifically if other meteorological parameters such as temperature, humidity and wind speed and their interrelationship with UV radiation are considered.