In-Flight Transmission of SARS-CoV-2, Does it Occur?

An interesting article published in the Journal of Travel Medicine September 2020 by David Freedman and Annelies Wilder-Smith looked at this vexing question.

They highlighted that the absence of large numbers of published in-flight transmissions of SARS-CoV-2 is not a definitive evidence of safety. They looked at peer reviewed publications of flights with possible transmission and categorised them by the quantity of transmission. Three mass transmission flights without masking were contrasted to five with strict masking and 58 cases with zero transmission.

In-flight transmission of SARS-CoV had previously been demonstrated during the symptomatic but not asymptomatic phase of illness.

In 2003, up to 22 transmissions occurred on a single flight from a single index case; conversely, several other carefully studied flights resulted in no transmission.

SARS-CoV-2 differs in having both significant transmission from presymptomatic and asymptomatic persons as well as secondary cases that may remain asymptomatic even with a 14 day follow-up period.

At the same time, cases secondary to in-flight transmission may be detected in as few as three days post flight.

As timing is so critical, the burden of absolute proof for ascertaining in-flight transmission risk is high.

A possible secondary case, who presents with COVID-19 symptoms, or is detected as an asymptomatic person with a positive COVID-19 PCR several days after arriving at their destination, could have been infected:

  1. In the days before departure from the flight origination point. 2. On route to the airport. 3. Whilst at the airport. 4. On the flight or even. 5. On or after arrival at the destination airport.

This paper reviewed all peer reviewed or public health publication of flights with likely, possible or unproven in-flight SARS-CoV-2 transmission from 24 January 2020 to 21 September 2020.

Following review of the published papers, they categorised flights as follows.

Evacuation/repatriation flights. As soon as international border controls were closed, thousands of chartered evacuation flights with more than 1.7 million passengers were organised, mostly by repatriating governments or cruise liners.

Since 29 January, the US government alone had helped to coordinate the repatriation of at least 85,000 Americans on 881 flights.

A number of these flights have carried COVID-19 cases, but no national databases or unified international register documenting evacuation flights or their passenger loads are publicly available and few data has been published to date.

However, the Korean CDC managed such flights meticulously and has published well documented data on these.

One secondary case from a clearly documented total of six index cases on a flight who had quarantined alone for three weeks prior to the flight, and her socially distanced part of the aircraft from home was managed by the Korean CDC. On board, she and all other passengers were masked except for meals but she did use a specific lavatory that had been used by an index case.

No transmission was found from two PCR index cases on a small jet repatriating nine other masked PCR negative Israeli evacuees from the Diamond Princess.

There is as yet no follow-up data available for any possible secondary cases during the repatriation of 300 masked US evacuees from the Diamond Princess. On this flight, there were 14 PCR positive evacuees seated in a separate section of the aircraft.

Possible Single Transmission with Weak Evidence

Incomplete epidemiological evidence exists to determine the likelihood for three proposed in-flight transmissions from three flights which occurred from Wuhan, Israel and from New York.

High Risk Flights with no Evidence of Transmission

Very early in the pandemic, a flight from Wuhan to Toronto, which had two passengers PCR positive on arrival out of 350 passengers showed no secondary transmission whatsoever. However, there was only active follow-up of flight passengers for symptom development and not systematic PCR testing.

The strongest evidence that in-flight transmission is not inevitable even with large numbers of infected persons aboard comes from a unique public database maintained by the government of Hong Kong.

All PCR positive patients are displayed with their arrival date, flight number and date of the positive PCR test. Between 16 June and 4 July, five separate Emirates Airline flights with seven or more passengers with positive PCR tests on day 0 arrived in Hong Kong.

On day 14, screening showed no secondary cases to be identified, despite the fact that 58 passengers were PCR positive on the flights, which had a duration of more than eight hours and a total of 1500 to 2000 passengers.

At the time of these flights, Emirates had a strict in-flight masking protocols, albeit meals were served. This Hong Kong database consists of only a single passenger case report for hundreds of flights with passengers who tested positive at day 0 of day 14 and requires further analysis.

Lack of Published or Public Data on Flights with Proven Covid-19 Cases

Canada and Australia have long public lists each containing more than 1000 flights with documented evidence of retrospectively containing patients with known COVID19 on board.

In each of these countries, the flight information and seat row numbers of known cases have been kept live for two weeks in order to encourage other persons who self-identify to self-isolate or get tested. Unfortunately, there is no available information on any secondary cases.

Flight crew are another example but unfortunately airlines keep this information to themselves and within their medical departments and as such, no aggregate data with de-identified statistics for flight crew have been published.

Case Clustering Proximity to Index Cases

The three major and best documented in-flight transmission events showed that the cases in the flights have been clustered together.

For example, in one flight cases were restricted to a small area of the mid cabin on an A330 wide body aircraft.

In a further flight, the single index case sat in business class and the transmission rate for the remaining passengers, 11/12 of whom were sitting within two rows was 62%.

In another case, both index cases were in business class and transmitted to the flight crew. Although seat plans were not available for all the flights in the group studies it
would appear that a minority occurred more than three rows from any index case and it would appear that the two row rule for contact tracing should be revisited.

Masking

On the various flights with mass transmission events, masking was not mandated in any way and according to the published reports was not practiced.

On a flight with 25 passengers PCR positive on arrival, there was rigid masking and on this fight, there were only two transmissions and one was seated next to five index cases.

In a further two flights where rigid masking policies apart from meals on an Emirates Airline, there were no secondary cases identified on day 14 screening despite 58 passengers being PCR positive out of the total of five flights of eight hours with over 1500 to 2000 passengers.

Discussion

The absence of large numbers of confirmed and published in-flight transmissions of SARS-CoV-2 is encouraging but it is not definitive evidence that fliers are safe. This limited available data probably related to current economic or political circumstances has resulted in a slow return towards a normal volume of commercial flights.

At present, based on the evidence above, strict use of masks appears to be protective when undertaking flights.

The authors go on to suggest that any data which could be available would help in assessing whether flying is currently safe during this COVID-19 pandemic.

The London General Practice has always evidenced and encouraged face masking. The practice is open and PPE is strictly worn by all medical personnel.