The Centers For Disease Control and Prevention (CDC) released a study yesterday that’s not good news for the travel industry.

For starters, the CDC states that, “During the first 6 months of 2020, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spread to almost all countries and infected »4 million persons worldwide. Air travel is contributing to the extent and speed of the pandemic spread through the movement of infected persons.

“Spread of SARS-CoV-2 across international borders by infected travelers has been well documented; however, evidence and in-depth assessment of the risk for transmission from infected passengers to other passengers or crew members during the course of a flight (in-flight transmission) are limited. Although the international flight industry has judged the risk for in-flight transmission to be very low, long flights in particular have become a matter of increasing concern as many countries have started lifting flight restrictions despite ongoing SARS-CoV-2 transmission.”

So, the CDC is worried about long flights; it’s not clear how they define a long flight but the study they did involved a 10-hour flight from London to Vietnam.

The CDC goes on to say:
“In early March, when much of the global community was just beginning to recognize the severity of the pandemic, we detected a cluster of COVID-19 cases among passengers arriving on the same flight from London, UK, to Hanoi, Vietnam, on March 2 (Vietnam Airlines flight 54 [VN54]). At that time, importation of COVID-19 had been documented in association with 3 flights to Vietnam, including a cluster of 6.

They “traced 217 passengers and crew to their final destinations and interviewed, tested, and quarantined them. Among the 16 persons in whom SARS-CoV-2 infection was detected, 12 (75%) were passengers seated in business class along with the only symptomatic person (attack rate 62%). Seating proximity was strongly associated with increased infection risk (risk ratio 7.3, 95% CI 1.2–46.2). We found no strong evidence supporting alternative transmission scenarios. In-flight transmission that probably originated from 1 symptomatic passenger caused a large cluster of cases during a long flight.”

Their “findings call for tightened screening and infection prevention measures by public health authorities, regulators, and the airline industry, especially in countries where substantial transmission is ongoing. Making mask wearing obligatory and making hand hygiene and cough etiquette standard practice while on board and at airports seems an obvious and relatively simple measure. Blocking middle seats, currently recommended by the airline industry, may in theory prevent some in-flight transmission events but seems to be insufficient to prevent superspreading events. Also, systematic testing, quarantine policies, or both, for inbound passengers at arrival might be justified for countries with low levels of community transmission, high risk for case importation, and limited contact tracing capacity.”

So it turns out that one asymptomatic person, who was sitting in business class and flying from London to Hanoi on March 1, infected 16 people.

According to the study, “By March 10, all 16 (100%) of the flight crew and 168 (84%) of the passengers who remained in Vietnam had been traced; 33 (16%) passengers had already transited to other countries. We were able to quarantine, interview, and collect swab specimens for PCR testing from all passengers and crew members who remained in Vietnam.”

“Through these efforts, we identified an additional 15 PCR-confirmed COVID-19 cases, 14 among passengers and 1 among crew members”. The ages of affected persons ranged from 30 to 74 years (median 63.5 years); 9 (>50%) were male, and 12 (75%) were of British nationality.

Of the 15 persons with flight-associated cases, 12 (80%) had traveled in business class with case 1, and 2 travelers and 1 flight attendant had been in economy class. Among persons in business class, the attack rate was 62%. Among passengers seated within 2 meters from case 1, which we approximated in business class to be <2 seats away, 11 (92%) were SARS-CoV-2–positive compared with 1 (13%) located >2 seats away (risk ratio 7.3, 95% CI 1.2–46.2). Of the 12 additional cases in business class, symptoms subsequently developed in 8 (67%); median symptom onset was 8.8 days (interquartile range 5.8–13.5) after arrival.”

As my colleague Joe Brancatelli tweeted out in response to this study, “Or, in English, Coronavirus transmits VERY easily on a long flight, even in business class …”

However, I would argue, and the study acknowledges this, that “face masks were neither recommended nor widely used on airplanes in early March, in particular not among travelers from Europe.” So I suspect these numbers would be a lot different if everyone had worn masks. It will be interesting to see what the data of transmission rates of COVID-19 look like on a long flight where…

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