A systematic review finds evidence for airborne transmission of SARS-CoV-2 over longer distances than previously believed.

Since the beginning of the COVID-19 pandemic, scientific dispute about routes of SARS-CoV-2 transmission has continued. It has been generally accepted that transmission occurs through direct contact and respiratory droplets traveling ‘short-range’ (one to two meters). However, anecdotal evidence indicates that airborne transmission over longer distances may also occur via virus-containing aerosols with much smaller particle size than droplets. British researchers conducted a systematic review of 18 observational studies through January 2022 that reported on SARS-CoV-2 transmission events (primarily outbreak investigations) in nonhealthcare indoor community settings in which long-distance (more than two meters) airborne transmission was the most likely route.

Transmission settings included singing events, apartment blocks, quarantine hotels, restaurants, buses, a food processing facility, a courtroom, a department store and a fitness facility. Exposure lasted between five minutes and three hours, and involved distances up to 15 m. At the time of transmission, most of the index cases were asymptomatic or presymptomatic. Due to the unstructured evaluation of the contact persons in 14 of the studies (e.g. nonstandardised contact workup, lack of genomic sequencing of isolates), transmission modes other than long-distance airborne could not be excluded for all cases. Four studies provided sound evidence for long-distance airborne transmission through detailed epidemiological workup.

Comment: Although these observational studies provide only low-certainty formal evidence for airborne SARS-CoV-2 transmission, a human trial controlled for several exposure variables will never be performed for ethical reasons. Thus, such retrospective investigations provide the best evidence we can get. It also seems logical that infectious droplets (created, for example, by talking loudly or singing) will dry out and transform into aerosols that can travel longer distances while still infectious. An editorialist notes parallels to tuberculosis (for which aerosol transmission is generally accepted) and calls for better indoor ventilation to limit the spread of SARS-CoV-2.
THOMAS GLÜCK, MD
Professor of Medicine, University of Regensburg, Chief, Department of Medicine, District Hospital Trostberg, and Chief, Infectious Diseases Consulting Service, Traunstein and Trostberg, Bavaria, Germany.

Duval D, et al. Long distance airborne transmission of SARS-CoV-2: rapid systematic review. BMJ 2022; 377: e068743.
Dancer SJ. Airborne SARS-CoV-2. BMJ 2022; 377: o1408.

This summary is taken from the following Journal Watch title: General Medicine.