Avoiding long-haul air travel during the COVID-19 pandemic

I would not recommend long-haul air travel at this time.

An epidemiological study of a 7.5 hour flight from the Middle East to Ireland concluded that 4 groups (13 people), traveling from 3 continents in four groups, who used separate airport lounges, were likely infected in flight. The flight had 17% occupancy (49 passengers/283 seats; 12 crew) and took place in summer 2020.
(Note: I am not an epidemiologist.)

The study (published open access):
Murphy Nicola, Boland Máirín, Bambury Niamh, Fitzgerald Margaret, Comerford Liz, Dever Niamh, O’Sullivan Margaret B, Petty-Saphon Naomi, Kiernan Regina, Jensen Mette, O’Connor Lois. A large national outbreak of COVID-19 linked to air travel, Ireland, summer 2020. Euro Surveill. 2020;25(42):pii=2001624. https://doi.org/10.2807/1560-7917.ES.2020.25.42.2001624

Irish news sites including RTE and the Irish Times also covered the paper.

The age of the 13 flight cases ranged from 1 to 65 years with a median age of 23 years. Twelve of 13 flight cases and almost three quarters (34/46) of the non-flight cases were symptomatic. After the flight, the earliest onset of symptoms occurred 2 days after arrival, and the latest case in the entire outbreak occurred 17 days after the flight. Of 12 symptomatic flight cases, symptoms reported included cough (n = 7), coryza (n = 7), fever (n = 6) and sore throat (n = 5), and six reported loss of taste or smell. No symptoms were reported for one flight case. A mask was worn during the flight by nine flight cases, not worn by one (a child), and unknown for three.

Murphy Nicola, Boland Máirín, Bambury Niamh, Fitzgerald Margaret, Comerford Liz, Dever Niamh, O’Sullivan Margaret B, Petty-Saphon Naomi, Kiernan Regina, Jensen Mette, O’Connor Lois. A large national outbreak of COVID-19 linked to air travel, Ireland, summer 2020. Euro Surveill. 2020;25(42):pii=2001624. https://doi.org/10.2807/1560-7917.ES.2020.25.42.2001624 (Notes to Figure 1 Caption)

“The source case is not known. The first two cases in Group 1 became symptomatic within 48 h of the flight, and COVID-19 was confirmed in three, including an asymptomatic case from this Group in Region A within 5 days of the flight. Thirteen secondary cases and one tertiary case were later linked to these cases. Two cases from Flight Group 2 were notified separately in Region A with one subsequent secondary family case, followed by three further flight cases notified from Region B in two separate family units (Flight Groups 1 and 2). These eight cases had commenced their journey from the same continent and had some social contact before the flight. The close family member of a Group 2 case seated next to the case had tested positive abroad 3 weeks before, and negative after the flight. Flight Group 3 was a household group of which three cases were notified in Region C and one case in Region D. These cases had no social or airport lounge link with Groups 1 or 2 pre-flight and were not seated within two rows of them. Their journey origin was from a different continent. A further case (Flight Group 4) had started the journey from a third continent, had no social or lounge association with other cases and was eated in the same row as passengers from Group 1. Three household contacts and a visitor of Flight Group 4 became confirmed cases. One affected contact travelled to Region E, staying in shared accommodation with 34 others; 25 of these 34 became cases (attack rate 73%) notified in regions A, B, C, D, E and F, with two cases of quaternary spread.”

Murphy Nicola, Boland Máirín, Bambury Niamh, Fitzgerald Margaret, Comerford Liz, Dever Niamh, O’Sullivan Margaret B, Petty-Saphon Naomi, Kiernan Regina, Jensen Mette, O’Connor Lois. A large national outbreak of COVID-19 linked to air travel, Ireland, summer 2020. Euro Surveill. 2020;25(42):pii=2001624. https://doi.org/10.2807/1560-7917.ES.2020.25.42.2001624

“In-flight transmission is a plausible exposure for cases in Group 1 and Group 2 given seating arrangements and onset dates. One case could hypothetically have acquired the virus as a close household contact of a previous positive case, with confirmed case onset date less than two incubation periods before the flight, and symptom onset in the flight case was 48 h after the flight. In-flight transmission was the only common exposure for four other cases (Flight Groups 3 and 4) with date of onset within four days of the flight in all but the possible tertiary case. This case from Group 3 developed symptoms nine days after the flight and so may have acquired the infection in-flight or possibly after the flight through transmission within the household.”

Murphy Nicola, Boland Máirín, Bambury Niamh, Fitzgerald Margaret, Comerford Liz, Dever Niamh, O’Sullivan Margaret B, Petty-Saphon Naomi, Kiernan Regina, Jensen Mette, O’Connor Lois. A large national outbreak of COVID-19 linked to air travel, Ireland, summer 2020. Euro Surveill. 2020;25(42):pii=2001624. https://doi.org/10.2807/1560-7917.ES.2020.25.42.2001624

“Genomic sequencing for cases travelling from three different continents strongly supports the epidemiological transmission hypothesis of a point source for this outbreak. The ability of genomics to resolve transmission events may increase as the virus evolves and accumulates greater diversity [23].”

Murphy Nicola, Boland Máirín, Bambury Niamh, Fitzgerald Margaret, Comerford Liz, Dever Niamh, O’Sullivan Margaret B, Petty-Saphon Naomi, Kiernan Regina, Jensen Mette, O’Connor Lois. A large national outbreak of COVID-19 linked to air travel, Ireland, summer 2020. Euro Surveill. 2020;25(42):pii=2001624. https://doi.org/10.2807/1560-7917.ES.2020.25.42.2001624

Authors note that a large percentage of the flight passengers were infected:

“We calculated high attack rates, ranging plausibly from 9.8 % to 17.8% despite low flight occupancy and lack of passenger proximity on-board.”

Murphy Nicola, Boland Máirín, Bambury Niamh, Fitzgerald Margaret, Comerford Liz, Dever Niamh, O’Sullivan Margaret B, Petty-Saphon Naomi, Kiernan Regina, Jensen Mette, O’Connor Lois. A large national outbreak of COVID-19 linked to air travel, Ireland, summer 2020. Euro Surveill. 2020;25(42):pii=2001624. https://doi.org/10.2807/1560-7917.ES.2020.25.42.2001624

Among the reasons for the uncertainty of this range is that “11 flight passengers could not be contacted and were consequently not tested.” (A twelfth passenger “declined testing”.) There is also some inherent uncertainty due to incubation period and possibility of “transmission within the household”, especially after the flight; authors note that “Exposure possibilities for flight cases include in-flight, during overnight transfer/pre-flight or unknown acquisition before the flight.”

Beyond the 13 people on the flight, cases spread to several social groups, across “six of the eight different health regions (Regions A–H) throughout the Republic of Ireland”. Flight groups 1 and 2 started their travel from one continent; Flight group 3 from another; Flight group 4 from a third continent.