Transmission of respiratory illnesses outdoors ‘definitely happens’ but less than indoors
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Outdoor transmission of SARS-CoV-2 and other respiratory viruses is possible, but the odds are much lower than indoor transmission, according to a recent study.
“We wanted to better understand the risk of getting or transmitting SARS-CoV-2 when people congregate outdoors,” Nooshin Razani, MD, MPH, director of the Center for Nature and Health at the University of California, San Francisco Benioff Children’s Hospital Oakland, told Healio. “We sought to inform policies that reconcile the many health benefits of being outdoors with the very important need to reduce COVID transmission.”
Razani and colleagues conducted a systematic review of peer-reviewed papers indexed in PubMed, EMBASE and Web of Science as well as preprints in Europe PMC through Aug. 12, 2020, that described cases of human-to-human transmission of SARS-CoV-2. The researchers included reports of other respiratory virus transmission for reference.
Five studies in their analysis determined that less than 10% of reported global SARS-CoV-2 infections have occurred outdoors and that the odds of indoor transmission were 18.7 times higher in comparison to outdoor transmission (95% CI, 6-57.9).
Additionally, the researchers found five studies that described influenza transmission outdoors and two that described adenovirus transmission outdoors, but varying study quality and definitions of “outdoor transmission” limited their ability to draw conclusions. However, they did determine that factors such as duration and frequency of personal contact, lack of personal protective equipment and occasional indoor gathering during a largely outdoor experience were all associated with outdoor reports of infection.
According to Razani, “outdoor transmission definitely happens.”
“Bottom line: existing evidence supports the wide-held belief that the risk of SARS-CoV-2 transmission is lower outdoors, but there are significant gaps in our understanding of specific pathways. Access to outdoor spaces is important because the odds of infection are lower,” Razani said. “But being outdoors doesn't make you immune to infection and doesn't stop you from spreading infection. You still need masks and distance outdoors.”
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