Is social distancing enough to prevent coronavirus spread on long flights?

"Long flights not only can lead to importation of COVID-19 cases but also can provide conditions for superspreader events," significant findings show as countries resume international travel.

Interior of a passenger airplane (photo credit: INGIMAGE)
Interior of a passenger airplane
(photo credit: INGIMAGE)
It's possible that simply spacing out people on airplanes isn't enough to prevent the spread of coronavirus on long flights, according to two studies published this week by the CDC after multiple passenger-to-passenger or crew member transmission stories were released. 
Both studies were published on September 18, each of which identified and investigated a scenario in which multiple people were diagnosed with the novel coronavirus days after traveling on the same flight while spaced out in business class.
In one scenario, a businesswoman and her sister were traveling across Europe after visiting Italy, where the virus had gained a stronghold at the time. They visited other European countries before the business woman flew to Hanoi on a 15-hour flight from London. On that flight, 16 people were later diagnosed with the virus. 
In a second scenario, four people, including two flight attendants, were found to have contracted the coronavirus seemingly from a man on a 10-hour flight from Boston to Hong Kong. While it remains possible that the travelers could have picked up the virus from different locations, all four of those positively diagnosed were found to have the same strain.  
"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," the study wrote. 
"At that time, importation of COVID-19 had been documented in association with three flights to Vietnam. No in-depth investigations among passengers on those flights were conducted, and no evidence indicated that transmission had occurred during the flights themselves," the study added. 
This led to the investigation
 
A 27-YEAR-OLD businesswoman from Vietnam, has been based in London since the beginning of February. On February 18, she traveled to Italy with her sister, who was later confirmed to have been infected with the virus after the pair had returned to London together on February 20. On February 22, the businesswoman and her sister returned to Milan, Italy, before traveling to Paris, France, for the yearly Fashion Week. On February 25, they returned to London, and four days later the businesswoman herself began to develop a sore throat and cough while "attending meetings and visiting entertainment hubs with friends."
On March 1, the woman boarded a 15-hour flight to Hanoi, and was seated in business class, and on March 5 she was diagnosed with the virus while in Vietnam.
"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," the study wrote. "Passengers and crew had traveled on to 15 provinces in Vietnam, ranging from Lao Cai and Cao Bang in the north to Kien Giang in the south."
Of the people having been on that particular flight, 15 were infected, 14 who were passengers and one who was a crew member. The businesswoman was the only symptomatic person on board, and was the only person who had been known to have been in close contact with another case. In addition, the incubation periods for all persons with confirmed flight-associated cases overlapped with the timing of the flight.
The study noted that the most likely route of transmission during the flight is aerosol or droplet transmission from the woman, assumed to the probable index case, the study noted. 
"Our findings have several implications for international air travel, especially because several countries have resumed air travel despite ongoing SARS-CoV-2 transmission," the study concluded. "First, thermal imaging and self-declaration of symptoms have clear limitations, as demonstrated by [the businesswoman], who boarded the flight with symptoms and did not declare them before or after the flight. 
"Second, long flights not only can lead to importation of COVID-19 cases but also can provide conditions for superspreader events. It has been hypothesized that a combination of environmental factors on airplanes (humidity, temperature, air flow) can prolong the presence of SARS-CoV-2 in flight cabins. 
 
The number of probable secondary cases detected in our study is on the upper end of hypothesized estimations for SARS-CoV-2 transmission on airplanes in the absence of face mask use, although the movement of aerosols and droplets in the specific conditions of a flight cabin remains poorly understood. 

IN THE second scenario, the CDC investigated a situation where four people who arrived in Hong Kong from Boston, Massachusetts, after a 15-hour flight were later found to have contracted the coronavirus somewhere along the way. While many may presume that the travelers could have been infected from different places anywhere along their travels, researchers note that what's interesting about this particular scenario is that the virus sequence of each of the four passengers was identical, unique, and belonged to a clade not previously indented in Hong Kong. 
In this scenario, a couple flew on March 9 from Boston to Hong Kong in business class. On March 10, the husband, 58, developed a fever and a cough, and on March 13 developed abdominal discomfort followed by more severe abdominal symptoms two days later. His wife, 61, who sat directly in front of him on March 10 also began developing symptoms a day after the flight, experiencing a sore throat, and on March 11 developed a fever and a cough.
On March 14 they were hospitalized and the next day they tested positive for coronavirus.
On that same flight, two flight attendants also tested positive for the coronavirus days after the flight. A 25-year-old specific-to-business-class flight attendant was notified that he had been in close contact with coronavirus patients on the flight, and on March 16 attended an outpatient clinic. A day later he was subsequently quarantined and hospitalized. 
Similarly, another flight attendant from the same flight, 51, was also diagnosed positively for the coronavirus. She had stayed in Boston March 5-9, and on the 18th developed coronavirus symptoms. Three days later she was hospitalized. 
"Given the case histories and sequencing results, the most likely sequence of events is that one or both of passengers A and B contracted SARS-CoV-2 in North America and transmitted the virus to flight attendants C and D during the flight. The only location where all four persons were in close proximity for an extended period was inside the airplane," the study noted. 
"Our results strongly suggest in-flight transmission of SARS-CoV-2," it added.