In DepthCOVID-19

NAS letter suggests ‘normal breathing’ can expel coronavirus

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Science  10 Apr 2020:
Vol. 368, Issue 6487, pp. 119
DOI: 10.1126/science.368.6487.119

Science's COVID-19 coverage is supported by the Pulitzer Center.

On 31 March, University of California, San Diego, atmospheric chemist Kimberly Prather set off an online debate by saying people should stop surfing during the coronavirus outbreak. Her reasoning: Viruses can travel long distances if carried by wind. “I ignited a bomb,” Prather says, as commenters pushed back, pointing to official assurances that the novel coronavirus is transmitted only short distances in respiratory droplets from a sneeze or cough.

The surfing controversy just adds to the fog surrounding how the novel coronavirus is transmitted. When people cough and sneeze, the droplets they expel fall to the ground within 1 or 2 meters. The fallen droplets deposit virus on surfaces, where people can pick it up and infect themselves by touching their faces. But if the coronavirus can be suspended in an ultrafine mist of particles smaller than 5 micrometers—known as aerosols—people could potentially spread infection when they exhale. And aerosol particles are so light that they can float like dust for hours in air.

8 meters

The distance a single sneeze can propel aerosol particles

The National Academy of Sciences (NAS) weighed in last week, suggesting it's likely the novel coronavirus can spread this way. A 1 April letter to Kelvin Droegemeier, head of the White House Office of Science and Technology Policy, noted that current studies are inconclusive. But, it added, “The results of available studies are consistent with aerosolization of virus from normal breathing.”

The debate was kicked off by a finding published 17 March in The New England Journal of Medicine that the new coronavirus, SARS-CoV-2, can float in mechanically generated aerosols for up to 3 hours and remain infectious. People readily shed such particles, and an analysis published 26 March in JAMA reported that a single sneeze can propel them up to 8 meters. “From the physics it's very clear the emissions go beyond [2 meters],” says the paper's author, Lydia Bourouiba, a physicist at the Massachusetts Institute of Technology.

The NAS letter also pointed to a preprint posted on medRxiv on 26 March by Joshua Santarpia and colleagues at the University of Nebraska Medical Center that found widespread evidence of viral RNA in rooms of patients being treated for COVID-19. Viral RNA turned up on hard-to-reach surfaces and in air samples more than 2 meters from the patients. Another preprint posted 10 March on bioRxiv found the coronavirus can be resuspended in the air when health care workers remove their personal protective equipment, clean the floors, and move through infected areas. Considering all the data, “The presence of viral RNA in air droplets and aerosols indicates the possibility of viral transmissions via these routes,” the NAS letter concluded.

“This added airborne pathway helps explain why [the virus] is spreading so fast,” Prather says.

To others, the question is far from settled. A 27 March scientific brief from the World Health Organization states that aerosol transmission “may be possible in specific circumstances and settings that generate aerosols,” such as when severely ill patients are intubated. However, the brief adds, an analysis of more than 75,000 coronavirus cases in China revealed no cases of airborne transmission. As for studies such as Santarpia's, it notes that detection of viral RNA doesn't necessarily mean infectious virus persists.

Meanwhile, some researchers are considering another possible transmission route: feces. According to the Centers for Disease Control and Prevention, no cases of fecal transmission have been documented even though the novel coronavirus has been detected in the feces of some patients. CDC says the risk “is expected to be low based on data from previous outbreaks of related coronaviruses, such as severe acute respiratory syndrome and Middle East respiratory syndrome.”

  • With reporting by Jocelyn Kaiser.

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