key: cord-0836057-ah9z9luw authors: Freedman, David O; Wilder-Smith, Annelies title: In-flight transmission of SARS-CoV-2: a review of the attack rates and available data on the efficacy of face masks date: 2020-09-25 journal: J Travel Med DOI: 10.1093/jtm/taaa178 sha: c758deb6b2ef6f33224e8835ae16f60961aa3c55 doc_id: 836057 cord_uid: ah9z9luw The absence of large numbers of published in-flight transmissions of SARS-CoV-2 is not definitive evidence of safety. All peer-reviewed publications of flights with possible transmission are categorized by the quantity of transmission. Three mass transmission flights without masking are contrasted to 5 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; 1 conversely, several other carefully studied flights resulted in no transmission. SARS-CoV-2, the novel coronavirus that shares 86% homology with SARS -CoV, differs in having both significant transmission from pre-symptomatic 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 3 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: (i) in the days before departure from the flight origination point; (ii) en-route to the airport; (iii) while at the airport; (iv) on the flight or even (v) on/after arrival at the destination airport. This review presents a comprehensive table summarizing 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. The Table is ordered and categorized by the quantity and certainty of transmission. The order is not chronologic due to variation in intensity and pandemic onset date in the various flight origin countries; a separate column describes SARS-CoV-2 incidence in the origin country at the time of the flight. Generally, quantitation of risk is imprecise and must account for many variables, including differing incidence rates of SARS-Co-V-2 at origin and destination, intensity of viral load in index cases, flight duration, masking practices onboard, preflight screening and passenger spacing. In the disruption of the pandemic, the opportunities for rigorous studies have been few, experienced that public health epidemiologists with experience with in-flight outbreak investigations have been otherwise occupied, and the sparse published literature is confounded by limited formal documentation of needed epidemiological facts around apparent secondary cases. Foremost, the human and financial resources to trace, interview and test hundreds of passengers from a flight have been lacking. Governments and the very large travel industry may face economic and political considerations in supporting overly detailed investigations. Four well-documented flights (Table 1 ) describe mass transmission events. Flights A and C present sophisticated proof from whole genome sequencing and provide essentially indisputable evidence of in-flight transmission to 11 and 2 secondary cases, respectively. Immediately pre-flight infection of the secondary cases is theoretically possible but highly unlikely given identical sequences with the index cases. Flight B with at least 15 secondary cases lacks genetic proof, but at the time (March 2) of the London-Hanoi flight, neither the UK nor Vietnam had more than a handful of sporadic cases. Flight D arrived in Hong Kong, which since April has had mandatory arrival PCR testing on Day 0 and Day 14 with quarantine in between; 27 passengers were PCR+ on Day 0 (11 September 2020, date last accessed). Two likely secondary cases (one seated in Row 40 with 5 index cases) had negative Day 0 PCR testing and were PCR+ on Day 14; pre-flight transmission shortly before the relatively short flight cannot be ruled out. Of note, Emirates airlines had extremely strict pre-flight screening, in-transit screening and masking procedures on board (meals were served) in place at the time of this flight with an enormous number of COVID-19 cases during an 8-hour flight. Once international border controls began, thousands of chartered evacuation flights with >1.7 million passengers were organized mostly by the repatriating government or a cruise line. Since 29th January, the US Government alone has helped to coordinate the repatriation of at least 85 000 Americans on 881 flights . A number of these flights have carried COVID-19 cases, 5 but no national databases or unified international registries documenting evacuation flights or their passenger loads are publicly available, and few data have been published to date. The Korean CDC (Flights E and F) managed such flights meticulously and has published well-documented data on these. The one secondary case from a clearly documented total of 6 index cases on Flight E had quarantined alone for 3 weeks prior to the flight, and her socially distanced path to the aircraft from home was managed by the Korean CDC. Onboard, 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 2 PCR+ index cases on a small jet (Flight G) repatriating 9 other masked PCR negative Israeli evacuees from the Diamond Princess. No follow-up data are yet available for any possible secondary cases during the repatriation of 300 masked US evacuees from the Diamond Princess (Flight H) where 14 PCR+ evacuees were seated in a separate section of the aircraft or from the Costa Luminosa (Flight I). Incomplete epidemiologic evidence to determine likelihood for 3 proposed in-flight transmissions (Flights J, K and L) is available. Very early in the pandemic, a flight from Wuhan to Toronto (Flight M) with 2 passengers of 350 PCR+ on arrival had no secondary transmission; however, only active follow-up of flight passengers for symptom development and not systematic PCR was undertaken. 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+ patients are displayed with arrival date, flight number and date of the positive PCR test. Between 16th June and 4th July, 5 separate Emirates airlines flights with 7 or more passengers with positive PCR tests on Day 0 arrived in Hong Kong (Flights N-R). No secondary cases were identified on Day 14 screening despite 58 passengers who were PCR+ on the 5 flights each of 8-hour duration with a total of ∼1500-2000 passengers. At the time of these flights, Emirates had strict in-flight masking protocols (meals were served). The Hong Kong database consists of single passenger case reports for hundreds of flights with passengers who tested positive at Day 0 or Day 14 and should be the subject of further analysis. As two examples, Canada and Australia have long public lists each containing >1000 flights with a documentation of having retrospectively known COVID-19 cases on board. In each of these countries, the flight information and seat row numbers of known cases are kept live for 2 weeks in order to encourage other passengers who self-identify to self-isolate or get tested; however, no available information on any secondary cases in other passengers is posted. Public health authorities in other countries have similar lists, but analyses of these databases have yet to be published. The USA presents a more difficult landscape for such analysis, as high background infection rates obscures the determination of place of acquisition. US CDC has stated awareness of 1600 cases on US flights and 11 000 contacts within 2 rows but has not yet published in-flight transmission estimates. Data on known cases in flight crew are mostly available only to the airline medical departments and infection may have been acquired anywhere off or on-duty, but such data are regarded as protected private health information by most airlines. No aggregate data with de-identified statistics for flight crew have been published. The 3 major, and best documented in-flight transmission events, Flights A-C had clear case clustering (see Table 1 for details). Cases in flight A were restricted to a small area of the midcabin on an A330 widebody aircraft. On Flight B, the single index case sat in Business Class and the attack rate for the remaining passengers, 11/12 of whom were sitting within 2 rows was 62%. On Flight C, both index cases were in Business Class and transmitted to flight crew. Seat plans were not available for all flights in the table, but a minority occurred more than 3 rows from any index case; the 2-row rule for contact tracing may need to be re-visited. This review focuses on the epidemiology of actual documented human transmission. Cabin airflows, cabin aerosolization and filtration parameters of aircraft ventilation systems are beyond our scope. The overall published data, as Table 1 . Summary data on peer-reviewed or public health publication of flights with suspected or possible in-flight SARS-CoV-2 transmission from January 2020 to 21 September 2020 incomplete as it is, support the concept of proximity to a SARS-Co-V infected person as a key factor in in-flight transmission. On Flights A-C, with mass transmission events, masking was not mandated in any way and, according to the published reports, was rarely practiced. On Flight D, with 25 passengers PCR+ on arrival but with rigid masking, there were only 2 transmissions and 1 was seated in Row 40 next to 5 index cases. On flights N-R with the rigid masking policies (meals served) of Emirates Airlines, no secondary cases were identified on Day 14 screening despite 58 passengers who were PCR+ on a total of 5 flights of 8 hours each with ∼1500-2000 passengers. Inflight masking became mandatory in Canada on 4th June and in Australia on 22nd July. Even with the incomplete contact tracing and testing to detect secondary cases available, aggregate figures on in-flight transmission before and after masking would be informative. The absence of large numbers of confirmed and published inflight transmissions of SARS-CoV-2 is encouraging but is not definitive evidence that fliers are safe. Limited data dissemination to date, which may be partly related to current economic or political circumstances, has resulted in only a slow return towards a normal volume of commercial flights. The circumstances for robust study have been too few among the disruption of the pandemic to know with precision the risk and factors needed to quantitate transmission under widely varying circumstances. At present, based on circumstantial data, strict use of masks appears to be protective. Structured prospective studies to quantitate transmission risk on flight with rigid masking protocols are now most pressing. At the same time, those with robust data on inflight transmission in the days prior to on-board masking should come forward and publish these data. Transmission of the severe acute respiratory syndrome on aircraft Flight-associated SARS-CoV-2 transmission from cruise ship passengers during a medium-haul Australian domestic flight supported by whole genome sequencing Transmission of severe acute respiratory syndrome coronavirus 2 during long flight In-flight transmission of severe acute respiratory syndrome coronavirus 2 SARS-CoV-2 infection prevalence on repatriation flights from Wuhan City Asymptomatic transmission of SARS-CoV-2 on evacuation. Flight Emerg Infect Dis Absence of in-flight transmission of SARS-CoV-2 likely due to use of face masks on board Potential transmission of SARS-CoV-2 on a flight from Singapore to Hangzhou, China: an epidemiological investigation Assessment of SARS-CoV-2 transmission on an international flight and among a tourist group Lack of COVID-19 transmission on an international flight We would like to thank Sarah Houghwout for research assistance. None.