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Even experts with deep knowledge of SARS-Cov-2 transmission are split on the question of whether it is safe to get on an airplane these days as the number of COVID-19 cases in the United States.
Bob Wachter, M.D., chair of the Department of Medicine at University of California, San Francisco (UCSF), devoted his increasingly indispensable grand rounds on all-things COVID-19-related this week to aerosols, droplets and facemasks. Wachter asked the three speakers to weigh in on safety of airplane travel, which is on a lot of people's minds in a time normally devoted to summer vacations. verdict.
The verdict was split. Donald Milton, M.D., Dr.Ph., an expert on infectious bioaerosols at the University of Maryland School of Public Health, said he was waiting for general rate of COVID-19 cases in the population to go down before he flew. Milton acknowledged that he was personally trying to be “extremely safe” because he wanted to continue to visit his 102-year-old father. But Milton, who said he served on a FAA committee about airline cabin environmental health 20 years, also said the airflow in airplane cabins was problematic. According to Milton, the ventilation in the airplane cabins blows down from the ceiling while the upward “thermal flume” coming off the passengers’ bodies rises. As a result of the upward and downward movement of air pushing against each other, air travels sideways through airline cabins, said Milton, which, he suggested, might mean greater exposure to SARS-CoV-2 if it is present.
Michael Edmond, M.D.,M.P.H.,M.P.A., MBA, chief quality officer at University of Iowa Hospitals and Clinics and a proponent of face shields to protect against COVID-19, said he would fly — and did so fairly recently, albeit wearing an N95 respirator.
Monica Gandhi, M.D., M.P.H., an infectious disease expert at USCF, was in agreement with Edmond and said she would fly but would be certain to wear a mask.
Airborne transmission: Droplet vs. aerosols?
Aside from ventilation and airlflow, one of the main issues with the safety of airplane travel — indeed, the safety of any activity that brings people together — is characteristics of airborne transmission of SARS-CoV-2.
Milton, as an aerosol expert, used a different classification system and nomenclature. But in many contexts, airborne transmission is divided into respiratory droplets and aerosols. Respiratory droplets are fairly large (greater than 5 micrometers in diameter) and don’t travel very far because of their size. They fall to the ground within three to six feet of the person producing them and don’t stay airborne very long. Aerosols are, by comparison, tiny (less than 5 micrometers) and can travel great distances and stay in the air for long periods. They have been compared to pollen or smoke particles.
Much of the current strategy against stopping the spread of SARS-CoV-2 — especially masks and staying six feet apart from each other — is predicated on SARS-Cov-2 being transmitted largely droplets. Aerosols could get through and around many masks, and keeping six feet apart wouldn’t make much difference if SARS-CoV-2 bobs around in the air in aerosols for long periods and over relatively long distances.
In an opinion piece in this week’s JAMA, Michael Klompas, M.D., M.P.H., Meghan A. Baker, M.D., Sc.D., and Chanu Rhee, M.D., M.P.H., of Brighman and Women’s Hospital in Boston go through the evidence of whether SARS-CoV-2 is spread by droplets or aerosols. Anyone interested in the topic should read their piece for themselves; the full text is available here. The three authors mention some experimental evidence of aerosol spread, including a study that showed coughing and speaking can produce secretions that travel up to 27 feet and a meta-analysis that showed N95 respirators are more protective against infection. But based on COVID-19's epidemiology, reproduction number (the number of people an infected person infects), and other findings, Klompas, Baker, and Rhee conclude that “the balance of evidence is inconsistent with aerosol-based transmission of SARS-CoV-2, particularly in well-ventilated spaces.” They hedge their bet some and also say that people produce droplets and aerosol, so “transmission may take place along a spectrum.”
In his UCSF grand rounds presentation, Milton compared measles, which is spread through aerosols and can infect people who are not in close proximity to the infected person, with tuberculosis, which tends to transmitted only when there is proximity to the infected person for relatively long periods of time. “This is a lot more like TB than measles in terms of the source strength of aerosols,” he said of SARS-CoV-2.
What does the CDC say about flying?
CDC doesn’t take a strong stand on whether it is safe to fly or not in its public messages about travel. The federal agency’s website says that “most viruses and other germs do not spread easily on flights because of how air circulates and is filtered on airplanes.” But it also notes that social distance may be difficult on crowded flights and that security lines and airport terminals could bring a person in close contact with others.
What has been the actual experience on airplanes? Have people contracted COVID-19 as airline passengers?
A July 7 report from the medical advisory group of the International Air Transport Association (IATA), a trade association for the airline industry, says the group only knows of one instance of on-board transmission to multiple people. The report says it occurred on a March 2 flight from the United Kingdom to Vietnam when a single passenger infected 14 others.
But the report also says that an “informal survey” of 18 airlines by the association identified four instances of in-flight transmission from passengers to crew members and four other instances of pilot-to-pilot transmission that didn’t necessarily occur during a flight.
The IATA report also mentions an investigation that involved public health authorities and four airlines that identified 1,100 passengers who had confirmed cases of COVID-19 (preumably this was in the early days of the pandemic). The flights they were on had 125,000 fellow passengers. One passenger and two crew members were possibly infected through in-flight transmission, according to the IATA.