As the COVID-19 pandemic continues to spread across the world, air travel restrictions are slowly being eased in some regions. Now, a new report published in the JAMA Network Open journal in August 2020 shows that the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can spread among passengers on international flights unless proper precautions are taken.
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Tourist Group with Contact History
The paper reports a viral transmission event on a commercial Tel Aviv-Frankfurt flight on March 9, 2020, with 102 passengers on board.
This included a 24-member tourist group, who had had contact a week before the flight with a hotel manager subsequently diagnosed with COVID-19. None of the tourists had been diagnosed with the infection before the flight, and none wore masks or took other precautionary measures to limit transmission. The flight lasted over 4.5 hours.
The tourist group was medically tested at the airport destination, with a throat swab being taken for SARS-CoV-2 testing as well. All the passengers were later interviewed over the phone after 4-5 weeks. Contacts with COVID-19 positives were enquired about, as well as any symptoms or a history of being tested for the virus prior to the interview.
All passengers who had occupied seats within 2 rows of the index cases in the flight were offered antibody testing for SARS-CoV-2 IgG, as well as to any passenger who reported symptoms. All positive and borderline results were confirmed by plaque reduction neutralization test (PNRT).
Positives Detected Among Contacts
The researchers found that 7 of the 24 tourists were positive for the virus on the throat swab test on arrival. Four of them had symptoms on the flight, while 2 had not yet developed symptoms, and the seventh remained asymptomatic.
Of the remaining 78 passengers, 71 were interviewed, and 13 of them supplied samples for serum testing, at 6-9 weeks after the flight. One of the patients reported a positive viral RNA test by polymerase chain reaction at 4 days after the flight, despite the absence of symptoms. The investigators carried out the antibody test 7 weeks from the flight date, at which point it was positive, as well as a positive PNRT. The patient in question had no contact with any COVID-positive patients either before or after the flight.
Two Secondary Cases
Seven other passengers had symptoms potentially indicating COVID-19, within two weeks of taking the flight, including headache and myalgia, and hoarseness, from 5 days of flying. This patient had been in quarantine for two weeks from the day after landing and had no contact with known COVID-positives. He also had not been tested before. The antibody test and the PNRT were positive and borderline, respectively.
The other six samples from symptomatic patients, and an additional five samples from asymptomatic passengers, were uniformly negative except for one borderline positive by antibody testing but with a negative PNRT.
The researchers were unable to rule out the in-flight transmission of the virus in one symptomatic patient who had already been in contact with a known case, and 46 asymptomatic passengers with no testing. They concluded that two transmission events might have occurred from the 7 index cases who tested positive on arrival at the destination airport. They were unable to narrow down the time of transmission to before or after the flight.
Respiratory Droplet Transmission
Transmission by respiratory droplets can occur if passengers are close to an index case, as well as depending on how passengers and crew members move about, how they deposit fomites, and how they move towards and through the departure gate. In the current study, both the secondary cases were seated within two rows of one of the index cases.
Airflow from the cabin took place from ceiling to floor and from the front to the back of the plane may have contributed to this low rate of transmission. This could have been further reduced by the wearing of masks – possibly. Thus, the study shows that airborne transmission may occur in a flight, and from data on SARS and influenza, such transmission may occur beyond a distance of two rows from an index case.
Earlier Incidents
This case supports the findings of an earlier study published on the preprint server bioRxiv*, which showed that in-flight transmission of SARS-CoV-2 is a real risk. After a flight, laboratory-confirmed COVID-19 were reported in 12 patients, of whom 10 were admitted to hospital, between January 25 to February 28, 2020. All the infected individuals were passengers rather than flight crew, who wore masks and may, therefore, have been protected. The median age of the infected group was 33 years, and 70% were female.
Of the 12, 9 patients tested positive on the initial test, but only half of them had positive findings on chest CT. All had a mild infection and were hospitalized within 2 days of symptom onset. Two of the positives remained asymptomatic and did not develop any chest CT scan signs throughout the course of the infection.
In this older study, the researchers drew attention to the role of asymptomatic infection in promoting transmission, when individuals are on a single flight for several hours. They said, “The risk of in-flight transmission can vary widely, depending on whether people wear masks, on the physical proximity to the index patient(s), the stage of illness and number of index patient(s), the type of air-ventilation system, the size of the aircraft cabin, and the duration of the flight.” They also pointed out the role of air travel in spreading the virus globally. They pointed to the possibility of mass transmission of the virus on such flights, particularly when precautions are not taken.
Stringent Combined Interventions Most Effective
And in another paper published in the journal The Lancet, research highlighted the importance of interventions such as quarantine of infected patients and their contacts, and preventing contacts between infected and susceptible adults at their workplace are the most effective interventions for restricting transmission of a viral respiratory illness. This study modeled viral spread under various COVID-19 control regimes in Singapore at 80 days, after reaching the 100-case mark of community transmission, assuming various levels of transmission and symptoms.
With an R0 of 1.5, 2, and 2.5, the median number of infections by day 80 was about 280,000, 727,000 and over 1.2 million, respectively, in the absence of any intervention. With combined control measures, the reduction in the median number of infections was over 99% to only 1,800 cases at R0 of 1.5, and by 93% and 78% when the R0 is 2 and 2.5, respectively. If only quarantine was implemented, this would yield 15,000 cases, the case numbers being reduced by 95% for R0 =1.5.
These simulations assume that about 7.5% of infections are asymptomatic. They assumed that asymptomatic cases infect only half the number that symptomatic cases do.
When the proportion of asymptomatic cases rises, to about 50%, as many as 277,000 infections may arise with combined interventions, rather than 1,800 cases at baseline, with an R0 of 1.5, thus, asymptomatic case proportion plays a crucial role in determining the optimal control strategy.
Implications
When most infections are symptomatic, quarantine and social distancing should be promoted at workplaces and schools. Adults should be targeted in the following program, since they are less likely to absent themselves when sick, from the workplace, compared to sick children attending school. This is confirmed by the observation that symptomatic adults continued on their tour using a commercial flight, exposing others to the risk of infection.
The earlier flight-related viral transmission study promoted travel bans, strict screening to detect symptomatic cases before the flight, and universal mask use during the flight, as well as two weeks of quarantine for all individuals who become symptomatic on the flight. These papers make it clear that more study is urgently needed before air travel restrictions are released, to put guidelines in place that will limit the risk of such viral spread around the world.
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