Introduction
…Seasonal influenza has been with humans throughout history (Viboud and Epstein 2016). It imposes extreme costs on contemporary societies, with 2017-18 being a notable high outlier (CDC 2018). Beyond the significant discomfort to those it strikes, it saps productivity when individuals cannot work (Duarte et al. 2017) and absorbs health care resources (Schanzer and Schwartz 2013). Influenza also has less known long-range consequences. Notably, individuals exposed to influenza in utero have lower earnings as adults and are more likely to depend on government assistance (Almond 2006; Schwandt 2017), and are more likely to suffer from serious health problems later in life (Lin and Liu 2014). They are also more likely to have a heart attack (Kwong et al. 2018). Influenza is a type of viral respiratory infection. Traditional public health measures to combat it include vaccination (Maurer 2009; and White 2016) and paid sick leave to keep contagious workers at home (Barmby and Larguem 2009; and Pichler and Ziebarth 2016). Coincidental reductions in interpersonal contact (such as from holiday school closings and public transportation strikes) can also reduce prevalence (Adda 2016). Finally, a recent meta-analysis shows that ingested vitamin D pills help to protect against these types of infections (Martineau et al. 2017)].
Conclusion
…Sunlight, likely operating through the well-established vitamin D channel, plays a significant role in flu incidence. A recent meta-analysis of 25 randomized controlled trials of vitamin D supplementation (Martineau et al. 2017) demonstrated significant benefits of vitamin D supplements for reducing the likelihood that an individual will contract an acute upper respiratory infection. The current study considers sunlight as an alternate, natural path through which humans can and do secure vitamin D. This study's findings reinforce the Martineau et al. evidence that vitamin D protects against such infections. The Martineau meta-analysis imposed stringent criteria for including a trial, thereby ruling out a variety of confounding factors, such as selection effects. This analysis adds three findings to this literature. First, the relationship between relative sunlight and influenza is likely driven by late summer and early fall sunlight, and not by sunlight the rest of the year. Second, externalities of immunity contribute strongly to the protective effects of sunlight. Third, these externalities, the underpinnings of herd immunity, operate most forcefully at a middle level of population density. Apart from its methodological contributions, this study reinforces the long-held assertion that vitamin D protects against acute upper respiratory infections. One can secure vitamin D through supplements, or through a walk outdoors, particularly on a sunny day.
