A study highlights how strategic use of masks and seating arrangements can significantly reduce the risk of COVID-19 spread during flights, offering crucial insights for safer air travel in future pandemics.

Transmission of SARS-CoV-2 on aircraft: A scoping review. Image Credit: ronstik / Shutterstock

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

In a recent study posted to the medRxiv preprint* server, researchers reviewed data on the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in airplanes to understand how the virus spreads among passengers and crew within the aircraft. By analyzing reports of contact tracing, they identified factors such as seating proximity and mask usage that influence infection rates, providing valuable insights for managing future respiratory pandemics.

Background

SARS-CoV-2 can spread onboard aircraft through droplet and aerosol transmission and contact with infected surfaces. Factors such as the congregation of passengers during embarking and disembarking may also increase the risk of transmission. In response, airlines adopted a variety of safety measures during the coronavirus disease 2019 (COVID-19) pandemic, such as the use of masks, restrictions in in-flight service, physical distancing, and the installation of advanced high-efficiency particulate air (HEPA) filters. Past research on similar respiratory illnesses, like SARS-CoV-1 and influenza, provided some insights, but recent epidemiological data is crucial to understanding SARS-CoV-2 transmissibility on aircraft.

About the Study

The present review aimed to evaluate contact-tracing reports, in-flight transmission factors, and secondary attack rates, which is the probability of a susceptible person within a group getting infected, to assess the factors influencing the transmission of the virus during flights. The study also utilized a nine-point tool to evaluate the quality of data, categorizing studies as low, medium, or high quality, ensuring a transparent assessment of the available evidence. The researchers believe that the insights from this study will be useful for future public health strategies and improve pandemic preparedness in air travel.

The review included peer-reviewed studies published between 2020 and late 2023, using a predefined set of inclusion and exclusion criteria based on the Population, Concept, Context, or PCC framework. The review focused on studies that involved passengers and crew to assess the transmission of SARS-CoV-2 inside aircraft settings. In addition to the flight period, the analysis extended to pre-boarding, post-boarding, and transit times at the airport, providing a comprehensive view of flight-associated transmission. Secondary transmission rates, such as secondary attack rates, were assessed, and flight-associated transmission was identified as infection due to no other known causes.

The data extracted from the studies included details on study design, international or domestic flights, flight characteristics, and the inclusion of both passengers and crew. Additionally, the number of cases, identification of index cases and the contacts, non-pharmaceutical interventions implemented before, during, and after the flight, as well as the variant of SARS-CoV-2 involved were also extracted from the studies. The researchers also obtained numerical outcomes from the studies, such as secondary attack rates, risk ratios, and odds ratios. This systematic approach enabled the team to draw more reliable conclusions from the diverse data.

Results

The review found that the transmission of SARS-CoV-2 within flights occurred sporadically, with secondary attack rates falling between 0% and 10%. The study’s findings align with previous research, which reported similar variations in secondary attack rates, ranging from 0% to 16%. Mask-wearing within flights was linked to lower transmission rates, with the studies indicating that crew and passengers who wore masks had lower secondary attack rates than those who did not. Short periods of mask removal, such as during meals, significantly increased infection risks, with studies showing an increase of up to 59% in such instances.

Another important factor that increased the transmission risk was proximity to an infected individual within the flight. The review found that passengers seated within two rows of the infected individual were more likely to be infected than those seated further away. These findings were consistent with earlier research on other respiratory viruses, such as SARS-CoV-1, where close contact significantly increased infection rates.

Although the review did not specifically quantify the impact of HEPA filtration, the general findings indicated that HEPA filters substantially reduced the airborne transmission of the virus and limited its spread to close contacts. The study observed that both short and long flights could have cases, suggesting that flight duration alone does not clearly influence transmission risk. This conclusion aligns with previous studies that reported similar findings.

Most of the cases reported from flights involved the Kappa variant, although the researchers believe this reflects the dominant strain in circulation at the time rather than the variant’s increased transmissibility. Some cases occurred despite mask use and negative polymerase chain reaction (PCR) tests, highlighting that infections can still occur even with precautions.

Conclusions

Overall, the study highlighted the sporadic nature of in-flight transmission of SARS-CoV-2 and reported that mask-wearing and proximity to infected individuals were two of the key factors that determined infection risk. The researchers emphasize the importance of consistent and standardized reporting for future studies to better inform public health strategies. This comprehensive analysis provides valuable insights that can aid in developing more effective guidelines for air travel during future pandemics.

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.