This section below summarizes the scientific evidence on the relative risk of COVID-19 transmission in indoor versus outdoor settings, and factors associated with transmission (i.e., weather). It also includes information about transmission due to large indoor and outdoor events including political rallies, concerts, and religious events.

Scientific Evidence

• Relative Risk of Transmission in Indoor versus Outdoor Settings: Existing evidence supports the widely-held belief that risk of SARS-CoV-2 transmission is lower in outdoor settings. A 2021 systematic review identified five studies that identified a low proportion of reported global SARS-CoV-2 infections occurring outdoors (<10%), and the odds of indoor transmission was very high compared to outdoor transmission (18.7 times; 95% confidence interval, 6.0–57.9).

• Factors Associated with Outdoor Transmission: In general, outdoor reports of infection were associated with factors such as duration and frequency of personal contact, lack of personal protective equipment, and occasional indoor gathering during a largely outdoor experience. However, there are significant research gaps in the understanding of specific pathways including such factors as: 
  • The Role of Weather: A US commentary advised that weather probably influences COVID-19 transmission, but not at a scale enough to outweigh the effects of lockdowns or re-openings in populations. Policymakers should be aware that no human-settled area in the world is protected from COVID-19 transmission by virtue of weather, at any point in the year. Indoor transmission remains likely everywhere the virus is spreading, and outdoor transmission is still possible if other precautions (social distancing, mask use, etc.) are not taken.
  • While a rapid review suggests that weather can play a role in encouraging indoor or outdoor activity (i.e., temperatures that encourage outdoor activity being associated with lower COVID-19 transmission), two studies investigated aspects of weather finding that most infections occur indoors. For example, a study found that, even in crowded areas, the outdoor risk is much lower than indoor. However, in geographical areas prone to pollution – including large urban areas with collective housing and towers – monitoring this pollution, together with meteorological forecast, could be a way to alert the population of risky days and to reinforce mitigation measures for short period of time. Other factors examined include:
    • Windspeed: Outdoor transmission that may occur in the summer would be highest on days when wind is reduced.
    • Temperature/Ultraviolet Exposure: Warmer temperatures and moderate outdoor ultraviolet exposure may offer modest reductions in transmission; however, changes in weather alone will not be enough to fully contain the transmission of COVID-19.
    • Pollution/Particulate Matter (PM): Two studies discuss the role of air pollution or PM in outdoor transmission. High levels of urban air pollution, weather and specific climate conditions have a significant impact on the increased rates of confirmed COVID-19 total number, daily new and total deaths cases, possibly attributed to indoor and to outdoor airborne bioaerosols distribution.
• Large Gatherings or Events: A 2020 (September) rapid review found that due to lack of surveillance and tracing systems, and confounding factors and variables, there was no evidence of robustly tested transmission at outdoor mass gatherings (10,000+ people). An evidence synthesis (March 2020) found that the effect of restricting and cancelling mass gatherings and sporting events on infectious diseases is poorly established and requires further assessment. The best-available evidence suggests multiple-day events with crowded communal accommodations are most associated with increased risk; however, mass gatherings are not homogenous, and risk should be assessed on a case-by-case basis. Other findings from the literature discuss specific settings including:
  • Mass Motorcycle Rally: A study examined the impact of a mass 10-day motorcycle rally in South Dakota, finding that it had contributed to substantial community spread both locally and nationally. Estimates indicate that the cumulative COVID-19 caseload in at the state-level increased by between 3.6 and 3.9 cases per 1,000 population, or a total of about 3,088 cases as of September 2, 2020. This represents an increase of over 35% relative to the 9.7 cases per 1,000 population in South Dakota on July 31, 2020.
  • Religious Gatherings: More than 35% of the COVID-19 cases in Malaysia (April 13, 2020) were directly linked to a mass gathering of 19,000 participants that took place on February/March 2020, which also caused a regional spike of COVID-19 cases across Southeast Asia (10% participants were from outside Malaysia). When countries suspended religious mass gatherings early (e.g., Hajj), there were lower occurrences of COVID-19 transmission.
  • Professional Sporting Events: Mass gatherings at NBA or NHL games impact community spread of COVID-19; as of April 30, 2020, one additional game increased the cumulative number of COVID-19 deaths in affected US counties by 11 percent. Mass gatherings at English football matches were consistent with increased cases and deaths during April 2020.
  • Political Rallies: Three studies report findings related to political rallies held in US cities. A study that examined multiple events found increased subsequent confirmed cases of COVID-19 by more than 250 per 100,000 residents, which ultimately resulted in more than 30,000 incremental confirmed cases of COVID-19. The rallies likely led to more than 700 deaths (not necessarily among attendees) based on county-level post-event death rates. Another study that analyzed the outcomes of 19 outdoor rallies and one indoor rally concluded that even outdoor gatherings in areas with low COVID-19 incidence are followed by increased infections. A study that analyzed one political rally found little evidence that COVID-19 case growth grew more rapidly at the county- or state-level (Oklahoma) in the three weeks following the campaign rally.
  • Outdoor Amateur Sports: US-based youth soccer clubs in the US found the incidence of COVID-19 among youth soccer athletes is relatively low when compared to the incidence among children in the US in summer of 2020. No relationship was identified between club COVID-19 incidence and phase of return of soccer (i.e., contact or non-contact play).
  • Indoor Concerts: Densely populated venues, such as eight live indoor concerts of 50-100 people that took place in Osaka, Japan in February 2020, can ‘seed’ infections that can spread to other, distant areas. However, a German study reported that the expected additional effect of indoor mass gatherings on the burden of disease is low if hygiene concepts are applied and adequate ventilation exists.
  • Pedestrian Traffic: While the risk of infection while walking on busy streets (average density around 0.1 person/m²) is low, street cafés present the largest average rate of new infections caused by the proximity of those waiting in line, followed by busy outdoor markets, and then metro and train stations. While the benefits of enforcing one-way goo traffic in (wide) walkways are unclear, changing the spacing between people in queues substantially affects disease transmission risks.

Individual peer-reviewed articles and review articles were identified through PubMed, and Google Scholar. The search was limited to English sources and therefore may not capture the full extent of initiatives in non-English speaking countries. Full-text results extracted were limited to those available through Open Access or studies made available to the Ministry by our partners.

The COVID-19 Evidence Synthesis Network is comprised of groups specializing in evidence synthesis and knowledge translation. The group has committed to provide their expertise to provide high-quality, relevant, and timely synthesized research evidence about COVID-19 to inform decision makers as the pandemic continues. The following members of the Network provided evidence synthesis products that were used to develop this Evidence Synthesis Briefing Note:

• Evidence Synthesis Unit, Research Analysis and Evaluation Branch, Ministry of Health. March 30, 2021.