As governments worldwide examine the possibility of restarting economic and social life without allowing a resurgence of the pandemic, travel barriers have been lowered in many places. This controversial measure has been discussed in many forums.
A recent study published in the journal Travel Medicine and Infectious Disease in September 2020 reports a series of cases following travel on a commercial airliner that heralded the earliest imported COVID-19 infections in Greece.
The Role of Air Travel in Pandemic Spread
Air travel has been an important risk factor for introducing new severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections because airplanes carrying thousands upon thousands of people crisscross global airspace to and from many different countries. Either the plane itself or infected travelers may serve as routes of dissemination.
The risk factors for the in-flight spread of respiratory pathogens include the occurrence of a respiratory droplet or aerosol spread of the pathogens, and whether it can spread in the asymptomatic and presymptomatic periods, as well as the length of the incubation period.
COVID-19 in Greece began in Thessaloniki, at the end of February 2020, with an Italy-returned traveler followed by another four cases from outside in Athens. In the current report, the investigators looked at contacts of index patients on one international flight from Israel to Greece.
Tracing Contacts of In-Flight Cases
Contacts were traced on all flights to and from Greece in the period from February 26 through March 9, 2020, following the diagnosis of any index case who had traveled on any flight 1-4 days before the earliest symptom began or during the symptomatic phase. The mean incubation period was assumed to be about 5 days.
There were 21 index cases on the 18 flights, with about 86% being Greeks. Six of the index cases were symptomatic at the time of flight, while 12 developed symptoms at 1-3 days later. Two others had symptoms 5 and 7 days later.
Contact tracing was performed for 18 international flights to and from Greece. The flights mostly began from Northern Italy, Israel, and the United Kingdom. Most of these flights had detailed data on the flight passengers and aircrew. There were 891 contacts, 44% being Greeks.
All close contacts, including all those within two seats of the index case, and all aircrew who had close contact with the index case, as well as any other close companions, were traced and asked to observe a 14-day self-quarantine dating from the day they were most recently exposed to the infection. In case they developed fever or any respiratory symptoms, they could contact the public health hotline for further action.
Diagnostic reverse transcriptase-polymerase chain reaction (RT-PCR) testing was carried out when necessary using nasopharyngeal or oropharyngeal swabs.
Five Confirmed Infections Among Contacts
In the current study, the researchers found five cases of probable in-flight transmission. Four of these were passengers who had been seated within this ‘contact’ range, and one was a crew member. Three of the positive contact passengers were from the same family who had visited Israel as part of a larger pilgrim group to Jerusalem. Two of them remained asymptomatic throughout. Two passengers and the lone crew member developed symptoms at 4 and 5 days, and 4 days after the flight, respectively. None of them were hospitalized.
Of the tour group which was returning from Israel, over 90% tested positive on further testing. Pilgrims are “a high-risk group of travelers for the acquisition of respiratory diseases due to their exposure to crowded conditions and mixing with other people from different countries where local transmission of SARS-CoV-2 might have been documented.” This appears to justify the bans on large public gatherings to help contain the virus.
Implications and Recommendations
Earlier studies in China and France showed that flights were promoting transmission. In the current flight study, local transmission of the virus to the aircrew in Athens or Tel Aviv is considered unlikely because, in both places, the number of cases was extremely low, and there was no mixing between the local population and the passengers or aircrew at either location. On the other hand, the two index cases who were symptomatic and COVID-19-positive on the flight had been part of the Jerusalem pilgrim group.
Given that in-flight transmission appears to be a reasonable possibility, albeit at a low frequency under the current conditions of extremely low passenger volume, the use of face masks on the flight, and temperature screening at the point of exit, are probably good recommendations. Moreover, it is important to ensure timely case detection and isolation of symptomatic cases in such a setting. All these measures, along with robust follow-up, may ensure that the spread of the virus is contained even as flight travel expands.