People with an asymptomatic coronavirus infection may also be able to transmit it unlike previously thought, according to a new study. It identified a case of Covid-19 transmission by an asymptomatic person on an airplane during an evacuation flight.

The study included 310 passengers who boarded an evacuation flight from Milan, Italy, to South Korea. Before boarding, a group of 11 symptomatic people was identified and removed from the flight, while the remaining 299 people were allowed to board. Physical examinations and temperature checks were performed before boarding.

All the passengers were provided with N95 respirators, remaining two meters apart for physical distancing during preboarding. Most wore the respirators at all times except during mealtimes and when using the toilet during the flight. After the flight, everyone was quarantined for two weeks at a government facility.

Among the 299 passengers, six had a confirmed positive result for SARS-CoV-2 on quarantine day 1 and were transferred immediately to the hospital, the study showed. At 14 days after the positive test, the six patients reported no symptoms and were categorized as asymptomatic. All 18 members of the cabin crew and medical staff were negative for SARS-CoV-2 on both occasions.

But what was most surprising for the researchers was the case of a 28-year old woman with no underlying disease that had a confirmed positive test result for Covid-19 on day 14. She wore the mask during the whole flight, except when she used the toilet, which was shared by other passengers, including an asymptomatic patient.

“Given that she did not go outside and had self-quarantined for 3 weeks alone at her home in Italy before the flight and did not use public transportation to get to the airport, it is highly likely that her infection was transmitted in the flight via indirect contact with an asymptomatic patient,” the researchers wrote.

To reinforce the results, the researchers did an external validation using a different dataset. They looked at another flight from Milan to South Korea, also under strict infection control procedures. Among the passengers, there were 3 asymptomatic cases. One person also tested negative on day 1 but positive on day 14.

Other, less likely, explanations for the transmission are previous SARS-CoV-2 exposure, longer incubation period, and other unevaluated situations, the authors argue.

Previous studies of inflight transmission for other respiratory infectious diseases, such as influenza and severe acute respiratory syndrome, revealed that sitting near a person with respiratory infectious disease is a major risk factor for transmission, similar to the findings of this study.

“Considering the difficulty of airborne infection transmission inflight because of high-efficiency particulate-arresting filters used in aircraft ventilation systems, contact with contaminated surfaces or infected persons when boarding, moving, or disembarking from the aircraft may play a critical role in the inflight transmission of infectious diseases,” the researchers wrote.

The findings suggest the following strategies for the prevention of SARS-CoV-2 transmission on an aircraft, the researchers considered. This includes wearing masks during the flight, maintaining physical distance before boarding and after disembarking and hand hygene to prevent infections.

The study was published in the journal Emerging Infectious Diseases.