Risk of SARS-CoV-2 transmission on airplanes is low: a review of the evidence

Researchers in Romania, the UK and Canada have systematically reviewed the available evidence on the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on airplanes.

The SARS-CoV-2 virus is the causative agent of the current 2019 coronavirus disease (COVID-19) pandemic which continues to pose a threat to global public health and has caused more than 3.73 million deaths in the world.

The researchers say the examination results are consistent with the suggestion that transmission of SARS-CoV-2 can occur on airplanes, but that it is a relatively rare event.

However, the data published so far do not allow a conclusive assessment of the likelihood and extent of transmission, and the evidence in most studies is of low quality, they add. The ability to compare results between studies is also limited by variation in study designs and methodologies used.

Standardized guidelines are needed for future studies on the transmission of SARS-CoV-2 on airplanes, according to the team.

“To our knowledge, no other systematic review of the literature has been undertaken to assess the evidence for the transmission of SARS-CoV-2 on board airplanes,” says Elena Cecilia Rosca of the Victor University of Medicine and Pharmacy Babeș in Timișoara, Romania and his colleagues.

A pre-printed version of the research paper is available on the site medRxiv* server, while the article is subject to peer review.

Learn more about the risk of air travel

The overarching goal of efforts to control the COVID-19 pandemic is to suppress the transmission of SARS-CoV-2 to prevent subsequent illness and death.

However, the transmission of SARS-CoV-2 and many aspects of the disease it causes are not fully understood, and the measures introduced to restrict transmission are based on the best information available.

Air travel may be associated with transmission of the virus, given the high number of passengers and the close proximity between passengers and crew.

As in other confined or semi-confined environments, transmission of viruses on board can occur through direct person-to-person contact, contact with contaminated surfaces and transmission of droplets.

“Research is underway to understand the modes of transmission of SARS-CoV-2, with a continuing array of new publications,” says Rosca and colleagues.

“Accordingly, there is a need for continuous and systematic reviews of available studies with the latest knowledge to inform recommendations using the most recent information,” they write.

What did the researchers do?

The researchers set out to assess relevant data on the transmission of SARS-CoV-2 on board airplanes, to point out any important policy implications, and to highlight areas where further research is needed. emergency.

Between February 1st, 2020 and January 27e, 2021, the team searched LitCovid, medRxiv, Google Scholar and the WHO Covid-19 database for studies on the transmission of SARS-CoV-2 on board airplanes and searched for additional studies using the reference lists of relevant articles.

Rosca and his colleagues identified 753 studies, of which 20 were considered eligible. Eighteen studies evaluated in-flight transmission of SARS-CoV-2, representing 130 individual flights, and two studies examined the presence of the virus in aircraft wastewater.

What did they find?

The team says the overall quality of reporting in the studies was low.

Both wastewater studies reported positive samples for SARS-CoV-2 by polymerase chain reaction (PCR), but the cycle cutoff (Ct) values ​​were relatively high, ranging from 36 to 40. La Ct value refers to the number of times a sample had to be amplified before the virus could be detected, with a high value indicating a low viral load and a low value indicating a high viral load.

The researchers also say that the definition of an “index case” varied greatly from one study to another and that the proportion of contacts found varied from 0.68% to 100%.

A total of 2,800 of the 19,729 passengers, 140 of the 180 crew and eight of the eight medical personnel were successfully located.

A total of 273 index cases and 64 secondary cases were reported and among studies that followed more than 80% of passengers and crew, the secondary attack rate ranged from 0% to 8.2%.

The secondary attack rate ranged from low to high on short, medium and long duration flights. One study of an approximately two hour short flight reported two index cases and five secondary cases, while another study of an 18 hour flight reported two index cases and four secondary cases.

The researchers say that the hypothesis that the risk of transmission increases with the duration of the flight due to higher exposure requires further investigation.

Studies have also reported the possibility of transmission of SARS-CoV-2 from asymptomatic, pre-symptomatic and symptomatic individuals.

“However, a major limitation of most studies was the possibility that asymptomatic index cases transmit the infection and that asymptomatic secondary cases were not studied due to the absence of symptoms, reducing the quality of the determination of the disease. cases, ”says Rosca and colleagues.

Better quality studies have shown potential transmission

Four studies that performed genome sequencing and phylogenetic analysis reported better and reliable evidence and indicated that airplane tuning may be associated with the transmission of SARS-CoV-2.

In addition, viral cultures of index cases that were performed in two studies yielded ten positive results.

“Positive viral cultures from index cases indicate that infectious virus was present, with potential for transmission to secondary cases,” the team writes.

“Standardized guidelines for reporting future research should be developed”

Rosca and his colleagues say current evidence indicates that the transmission of SARS-CoV-2 on airplanes is low. Yet the published data do not allow a conclusive assessment of the likelihood and extent of transmission.

“Standardized guidelines for reporting future research should be developed,” the team writes. “Future studies should aim for a comprehensive passenger and crew assessment, with a comprehensive follow-up strategy.”

The researchers say that factors that may influence transmissions, such as index case infectivity, passenger susceptibility and efficacy of exposure, should be evaluated consistently across studies.

“In addition, the new studies should take into account other factors that may have an impact on modes of transmission, including natural immunity and vaccine coverage,” they conclude.

*Important Notice

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

Journal reference:

Rosca EC, et al. Transmission of SARS-CoV-2 associated with air travel: a systematic review (Version 1). medRxiv, 2021. doi: https://doi.org/10.1101/2021.06.03.21258274, https://www.medrxiv.org/content/10.1101/2021.06.03.21258274v1

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