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Asymptomatic Transmission of SARS-CoV-2 on Evacuation Flight

Our take —

This study sought to assess the extent of SARS-CoV-2 transmission among asymptomatic persons on a long-haul flight from Milan, Italy to South Korea. It found an estimated attack rate of 0.3% from infections likely obtained during the flight, and an overall prevalence of 2.3% for the disease from people who boarded the flight without symptoms. While it is impossible for them to determine for sure whether these infections were from the flight or prior, it highlights that with high adherence to passengers using high-quality masks whenever possible and social distancing by at least 6 feet, transmission can be very low during air travel.

Study design

Prospective Cohort

Study population and setting

The study objective was to assess the extent of transmission of SARS-CoV-2 during an airplane flight among both asymptomatic and symptomatic cases. On March 31, 2020, an evacuation flight from Milan, Italy, to South Korea included 310 passengers who enrolled in the study. Passengers were given N95 masks, socially distanced during preboarding only, and medical staff performed physical exams, medical interviews, and took temperature for all passengers. They identified 11 symptomatic passengers who were removed from the flight. Of the 299 passengers who arrived in South Korea, they all immediately entered medical isolation at a government facility for two weeks and were examined twice per day by medical staff via temperature checks and symptom screening. They also all received testing on their 1st and 14th day of quarantine. All crew members (n=10) and medical staff (n=8) on the flight were similarly quarantined. They also replicated a similar level of transmission in another evacuation flight of 205 passengers.

Summary of Main Findings

Of the 310 passengers total, 11 had symptoms and were barred from boarding, and another 7 cases who did board later tested positive during the quarantine, for a total prevalence of 5.8% who attempted to board or boarded the flight. Of those who did board, 6 had a confirmed positive result on day 1 and transferred to a hospital. None of them had underlying conditions or comorbidities. On day 14, another person with no underlying disease also tested positive with mild symptoms (coughing, runny nose, muscle aches) and transferred to a hospital, for a likely attack rate of 0.3%. When mapping where passengers sat during the flight, 4 of the 6 patients sat in the same row, though the reasons this occurred were not stated in the paper, and the person who was positive of Day 14 shared a bathroom with an asymptomatic case who tested positive on Day 1, which the authors hypothesize may have been a point of indirect transmission. The attack rate of new infection likely due to in-flight exposure for this cohort is 0.3% (1 of 293). In their replication, of 205 passengers, they found 4 positive cases, for a similar period prevalence of 2.0% and likely attack rate of 0.5% (1/202) among the among susceptible passengers.

Study Strengths

The study mapped the seating positions of people on the flight, which helps to identify potential points of contact where transmission may have occurred. They also used a replication dataset which showed a similar level of transmission on another long-haul flight from Milan, Italy to South Korea on April 3, 2020. Their testing protocol allowed them to identify asymptomatic cases who were otherwise missed via symptom screening. Also, by including the medical staff and crew in their monitoring and testing, they were able to show that transmission did not occur from the staff to the passengers, but rather new transmission was more likely to be passenger-to-passenger.

Limitations

Because individuals were not tested before they boarded the flight, authors were unable to determine whether the asymptomatic cases were exposed and infected during the flight or before. It is unlikely that all 6 who tested positive on day 1 developed infection from the flight due to the incubation period of disease. Additionally, while the authors hypothesize that the bathroom may have been an indirect point of transmission, it is impossible to know if this was the case, or whether this additional case had been infected just prior to the flight as well.

Value added

This paper shows that there are very low rates of transmission on long-haul flights with pre-flight screening by trained medical professionals and high-quality mask use at nearly all times.

This review was posted on: 15 September 2020