Researchers in Romania, the UK, and Canada have systematically reviewed the evidence available on the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) aboard aircraft.
The SARS-CoV-2 virus is the agent responsible for the ongoing coronavirus disease 2019 (COVID-19) pandemic that continues to pose a threat to global public health and has now caused more than 3.73 million deaths worldwide.
The researchers say the review findings are consistent with the suggestion that SARS-CoV-2 transmission can occur in aircraft but is a relatively rare event.
However, the data published so far do not permit any conclusive assessment of the likelihood and extent of transmission, and the evidence from most studies is of low quality, they add. The ability to compare findings across the studies is also limited by variation in the study designs and methodologies used
Standardized guidelines are needed for future studies of SARS-CoV-2 transmission on aircraft, says the team.
“To our knowledge, no other systematic review of the literature has been undertaken to assess the evidence for transmission of SARS-CoV-2 aboard aircraft,” says Elena Cecilia Rosca from Victor Babeș University of Medicine and Pharmacy in Timișoara, Romania and colleagues.
A pre-print version of the research paper is available on the medRxiv* server, while the article undergoes peer review.
More about the risk that air travel poses
The overarching aim of efforts to control the control COVID-19 pandemic is to suppress SARS-CoV-2 transmission to prevent subsequent illness and death.
However, the transmission of SARS-CoV-2 and many aspects of the illness it causes are incompletely understood, and the measures introduced to restrict transmission are based on the best information available.
Air travel may be associated with the transmission of the virus, given the high number of passengers and the close proximity between passengers and crew.
As in other contained or semi-contained settings, the onboard transmission of viruses can occur through direct person-to-person contact, contact with contaminated surfaces, and droplet transmission.
“Research is ongoing to understand SARS-CoV-2 modes of transmission, with a continuous array of new publications,” says Rosca and colleagues.
“As a result, there is a need to continuously and systematically conduct reviews of available studies with the latest knowledge to inform recommendations using the most up-to-date information,” they write.
What did the researchers do?
The researchers set out to evaluate relevant data on SARS-CoV-2 transmission aboard aircraft, report any important implications for policymaking and highlight areas where further research is urgently needed.
Between February 1st, 2020, and January 27th, 2021, the team searched LitCovid, medRxiv, Google Scholar, and the WHO Covid-19 database for studies on SARS-CoV-2 transmission aboard aircraft and screened for additional studies using the reference lists of relevant articles.
Rosca and colleagues identified 753 studies, of which 20 were considered eligible. Eighteen studies assessed the in-flight transmission of SARS-CoV-2, representing 130 individual flights, and two studies investigated the presence of the virus in wastewater from aircraft.
What did they find?
The team says the overall quality of reporting across the studies was low.
The two wastewater studies reported samples that were positive for SARS-CoV-2 by polymerase chain reaction (PCR,) but the cycle threshold (Ct) values were relatively high, ranging from 36 to 40. The Ct value refers to the number of times a sample needed to be amplified before the virus could be detected, with a high value indicating a low viral load and a low value indicating a strong viral load.
The researchers also say the definition of an “index case” was highly varied across the studies, and the proportion of contacts who were traced ranged from 0.68% to 100%.
Overall, 2,800 of 19,729 passengers, 140 of 180 crew members, and eight out of eight medical staff were successfully traced.
In total, 273 index cases and 64 secondary cases were reported and among the studies that followed up 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 across short-, medium-, and long-duration flights. One study of a short flight lasting around two hours 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 the hypothesis that the risk of transmission increases with the length of flight due to higher exposure needs further investigation.
The studies also reported on the possibility of SARS-CoV-2 transmission from asymptomatic, pre-symptomatic, and symptomatic individuals.
“However, a major limitation of most studies consisted of the possibility of asymptomatic index cases transmitting the infection and of asymptomatic secondary cases not being investigated due to lack of any symptoms, lowering the quality of case ascertainment,” says Rosca and colleagues.
Higher quality studies pointed to potential transmission
Four studies that performed genome sequencing and phylogenetic analysis reported higher-quality, reliable evidence and indicated that the aircraft setting may be associated with SARS-CoV-2 transmission.
Furthermore, viral cultures of index cases that were performed in two studies generated ten positive results.
“The positive viral cultures of index cases indicate that infectious virus was present, with potential for transmission to the secondary cases,” writes the team.
“Standardized guidelines for the reporting of future research should be developed”
Rosca and colleagues say the current evidence indicates that SARS-CoV-2 transmission aboard aircraft is low. Still, the data published do not permit any conclusive assessment of the likelihood and extent of transmission.
“Standardized guidelines for the reporting of future research should be developed,” writes the team. “Future studies should aim for a comprehensive assessment of passengers and crew, with a complete follow-up strategy.”
The researchers say that factors that may influence transmissions, such as infectivity of the index case, the susceptibility of passengers, and the effectiveness of exposure, should be consistently assessed across studies.
“Furthermore, new studies should take into account other factors that might impact transmission patterns, including natural immunity and vaccination coverage,” they conclude.
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.
Rosca EC, et al. Transmission of SARS-CoV-2 associated with aircraft 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