Xiaoqian Sun,Changhong Zheng,Sebastian Wandelt,Anming Zhang
Air transportation and COVID-19: A tale with three episodes
Aviation,Pandemic,Recovery,Data Science
The COVID-19 pandemic has highlighted the extreme vulnerability of our aviation system towards external disruptions. While there have been several earlier aviation-related crises, the impact of COVID-19 is unmatched in the history of modern aviation. Accordingly, a better understanding of the mechanisms and ramifications of this pandemic is instrumental for preparing towards future external disruptions. The contribution of our study is threefold. First, we dissect the disruptive impact of the pandemic on the scientific literature and extract the major trends and insights. Given the wide range of related venues and the extent of disruption, there have been many studies published in the last 2–3 years. Second, we perform a data-driven analysis of the full disruption cycle containing three episodes, starting with the epidemic shock early in the year 2020, over the pandemic stalemate, towards the endemic-induced recovery in the year 2022. Third, we summarize the major insights and derive a set of policy recommendations and future research directions which we consider essential on the way towards what we call pandemic-resilient aviation.
Air transportation and COVID-19: A tale with three episodes

The COVID-19 pandemic has highlighted the extreme vulnerability of our aviation system towards external disruptions. While there have been several earlier aviation-related crises, the impact of COVID-19 is unmatched in the history of modern aviation. Accordingly, a better understanding of the mechanisms and ramifications of this pandemic is instrumental for preparing towards future external disruptions. The contribution of our study is threefold. First, we dissect the disruptive impact of the pandemic on the scientific literature and extract the major trends and insights. Given the wide range of related venues and the extent of disruption, there have been many studies published in the last 2–3 years. Second, we perform a data-driven analysis of the full disruption cycle containing three episodes, starting with the epidemic shock early in the year 2020, over the pandemic stalemate, towards the endemic-induced recovery in the year 2022. Third, we summarize the major insights and derive a set of policy recommendations and future research directions which we consider essential on the way towards what we call pandemic-resilient aviation.

The aviation industry has been continuously growing in the last century, surviving from a number of major shocks, including the Arab oil embargo in 1973, Gulf War in 1991, the 9/11 terrorist attacks in 2001, the Global Financial Crisis in 2008, and several local/regional disease outbreaks, which highlights the resilience of our aviation system. The spread of earlier diseases, e.g., SARS in 2003, MERS in 2012 and Ebola in 2014, was controlled at the regional level due to specific disease properties and timely responses of local governments and international organizations, leading to limited effects on air transportation. It seems, however, that the aviation industry has not learned its lessons from these earlier near-misses, culminating into the devastating impact on the industry by the current Corona Virus Disease 2019 (COVID-19). This disease, caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), was first reported at the end of the year 2019. Declared as a formal pandemic by the World Health Organization (WHO) on March 11th, 2020, COVID-19 has endangered people’s health and lives, and caused severe socio- and conomic consequences by sweeping the world rapidly. Globally, as of February 6th, 2024, there had been 672 million confirmed cases of COVID-19, including 6.84 million deaths, according to the WHO. The pandemic has caused the worst global recession since the Great Depression in the 1930s; according to the United Nations (UN), the number of people living in extreme poverty had risen from 812 million in 2019–889 million in 2021. Aviation is one of the industries hardest hit by the COVID-19 pandemic, with unprecedented global declines in passenger volume and airline revenue. Pending the available vaccines, government authorities implemented non-pharmaceutical measures to control local epidemic in early stages. It was shown that air transportation played an important role in spreading the COVID-19 [13], [39], [61]; as a consequence, flight bans were executed by countries and regions against the first waves of the pandemic. According to IATA [26], the greatest disconnection of the worldwide air transport since the World War II has occurred in April 2020, when passenger traffic was reduced by 94 % compared to the year 2019. Total operating revenues for the aviation industry were down by 62 % in 2020. Many governments established aid reliefs and support programs for the aviation industry [1], [60]. For example, the United States passed a $2.2 trillion stimulus bill, called the Coronavirus Aid, Relief, and Economic Security Act (CARES), to mitigate impacts of economic downturn on various sectors, including $25 billion for passenger airlines. Australia announced several hundred- million-dollar packages at different scales to support the aviation industry. Several airlines went bankrupt despite financial assistance, fleet re-arrangements and layoffs, while others still struggle in the predicaments. Meanwhile, up-stream and downstream industry of aviation and other related sectors have also been affected. Airports shut down or became parking lots for aircraft. Aircraft manufacturers, oil suppliers and maintenance providers lost orders. Global tourism, along with accommodations and catering, has been slowed down significantly, especially for parts of the tourist industry that rely heavily on air connections [19]. With the ongoing progress in the fight against the pandemic, several vaccines are distributed worldwide to help public with acquiring necessary levels of immunity, which seems to pave the path for a global aviation recovery. Meanwhile, several new disease waves were observed for SARS-CoV- 2 variants with enhanced transmissibility, more serious illness, or decreased efficacy of diagnostic identification, medications, vaccinations and antibodies [57]. These last three years with the COVID-19 pandemic have highlighted the vulnerability of our aviation system and the society as a whole. Accordingly, it is instrumental to better understand the mechanisms and ramifications of COVID-19 for preparing towards future external disruptions. In this study, we perform an investigation of the interactions between the COVID-19 pandemic and aviation. Our data-driven analysis is divided into three stages based on the major periods of COVID-19, see Fig. 1. The first period in this study (The epidemic shock) was defined based on the excessive number of flight reductions in March 2020 – June 2020, which has been reported in the extant literature, see, e.g., Sun et al. [44]. The transition between Period 2 (The pandemic stalemate) and Period 3 (The endemic recovery) was set to June / July 2021, given the total number of passengers in global aviation. While the exact time point is hard to determine – and depends on the region and formal definition of recovery, the main purpose of these categories is to serve as a foundation for a clearer dissection and discussion. Future work could provide a formal analysis of the transition points between the major stages with COVID-19. The remainder of our study is structured as follows. Section 2 reviews scientific literature on aviation and COVID-19. Section 3 introduces the epidemic shock from March 2020 to June 2020. Section 4 describes the pandemic.

stalemate from July 2020 to June 2021. Section 5 reports the endemic recovery from July 2021 to June 2022. Section 6 concludes our study and provides several recommendations for future work. Analysis of the disruptive impact of COVID-19 is unparalleled in scientific literature, particularly in the aviation domain. Air transportation plays dual roles in the COVID-19 pandemic by being among the major victims, and by accelerating the virus spread through its global and highly efficient transportation system. Various analyses of aviation and COVID-19 related studies have emerged in the past three years, published in many distinct venues, including aviation-specific journals and conferences, as well as transportation, economics, health, environment and law outlets. The existing literature on aviation-pandemic remains disorganized, however. The main reasons are as follows: (1) Inconsistent terms are used in relevant papers. (2) The innovations from researchers in various disciplines are sometimes hard to identify. (3) There is lack of clear key messages in some early COVID-19 papers. Therefore, review articles/literature surveys can help to go through the excessive number of papers and summarize novel insights. We provide an overview on these review articles below. Some overview work mainly focused on the aviation industry itself. Khatib et al. [27] identified the transmission risk of in-flight commercial air travel, involving operations/actions of staff and passengers at airports and in planes. Bielecki et al. [7] reviewed air passenger transportation and containment strategies in the early stage on the following aspects: traveler numbers, pre- and in-flight prevention, testing recommendations, photo-epidemiology of mask use, and quarantine measures and their effectiveness. Sun et al. [51] collected more than 110 papers on aviation and COVID-19 published in 2020, analyzing global air transport system, impacts on passenger- centric flight experience and long-term impacts on aviation. Bidaisee [6] discussed impacts of the pandemic on aviation and various challenges, e.g., airlines, cargo transportation, aircraft and pilots. Khatib et al. [28] reviewed literature on air travel safety during the pandemic, with recurring themes including the role of passenger travel in geographic spread of COVID-19, in-flight transmission risk, effectiveness of travel controls and flight-related prevention, and challenges of vaccine passports. Thepchalerm and Ho [53] summarized operational, market, and financial impacts of the pandemic on airline business from organization reports and academic literature. Amankwah-Amoah [2] outlined innovation activities of the global airline industry during the pandemic, involving new disinfection and deep-cleaning approaches, touchless technologies at air- ports, blocking middle seats policies, in-flight social distancing, and biometric check-in. Other review articles covered broader topics. Ko et al. [29] emphasized the intermediate and long-term sequelae of COVID-19 with aeromedical significance and proposed corresponding recommendations for the aeromedical examiner to evaluate pilots and their performance. Li [31] summarized interactions of air transportation and COVID-19, introduced aviation bailout and its effectiveness, and discussed relationships between aviation and economic growth. Arena and Aprea [3] analyzed major consequences of the pandemic on air transport, especially about negative economy impacts on different markets and positive contributions of travel reduction to air pollution. Sun et al. [48] collected and dissected more than 100 aviation-pandemic papers published in 2021 by eight categories: airlines, airports, passengers, workforce and education, markets, contagion, sustainability, and economics and law. Salesi et al. [42] reviewed literature on travel-related strategies for controlling adverse impacts of recent pandemics to maintain the sustainable development of air transport and tourism sectors in the South Pacific region. Troyer and Bidaisee [54] discussed effects of considerable revenue decreases on airlines, impacts on air cargo transportation, the airworthiness of aircraft, the maintenance of medical certificate and flying currencies for pilots, and other challenges. It is crucial for stakeholders to develop informed strategies and policies, in order to contain local spreading, particularly at an early stage. Meanwhile, flexible adaptations are vital in a world without constancy. Changes in capacity and frequency of flights relate to local outbreak situations and government policies. The degree and time spans of flight reductions vary by countries and regions, given different execution and periods of travel restrictions. Ding et al. [16] proposed a decision recommendation framework of Transportation Lock-down and Quarantine Problem (TLQP), based on a SEIR model with a highly efficient heuristic ground on Effective Distance and Cost-Effective Lazy Forward algorithm. Mo et al. [33] modeled disease dynamics of COVID-19 in Singapore and evaluate several public transit policies by a time-varying weighted public transit encountered network. Qian and Ukkusuri [38] studied the coupling between the epidemic spreading and the mobility dynamics in public transportation systems by the Trans-SEIR model, and showed effective entrance control with optimal allocation of limited resources. Hale et al. [23] introduced the Oxford COVID-19 Government Response Tracker (OxCGRT) to capture the number and degree of policy measures from over 180 countries. Ding et al. [15] proposed a Flight-SEIR model, enabling estimations of the imported cases by air in early phases, the reproduction number and impact of travel restrictions and reopening. Zhu et al. [62] utilized Causal Bayesian Network model to analyze the causal relationship in the post-recovery process of commercial flights and assess possible consequences of non-pharmaceutical policies. Another interesting stream in the literature concerns the impact of the COVID-19 pandemic on air travel intention. Several recent studies have elaborated on the use of Zoom meetings, Work From Home (WFH) options, additional travel requirements, trust in airlines that might reduce the air travel intention, see [10], [9]; Garaus and Hudáková [20] for excellent studies on this subject.

China was one of the earliest countries affected by COVID-19. As more confirmed cases reported in Wuhan, this city with 11 million inhabitants was locked down on January 23rd, 2020. With. timely and wider restrictions and lockdowns, the spread of COVID-19 in China was under control. Although many domestic and international flights have been canceled since late January 2020. This regional epidemic outbreak turned into a full pandemic. The lockdown of Wuhan deferred the transmission of COVID-19 by three to five days in mainland China, and contributed to slowing down the worldwide spread of SARS-CoV-2 until mid-February 2020 [11]. Fig. 2 shows the spatio-temporal evolution of COVID-19 from March 2020 to June 2020 (Episode I). COVID-19 had spread to more than 60 countries on March 1st, 2020, where eight infected countries with over 100 cases were mainly located in Asia and Europe. More countries and regions imposed non-pharmaceutical measures to control local epidemic in early stages. The number of global flights declined significantly after the declaration of the pandemic outbreak by the WHO on March 11th, 2020. However, most country borders were closed too late, missing the best time window to safeguard their people from encroachments of COVID-19. Since May 1st, 2020, confirmed cases were reported almost all over the world, with only 20 countries untouched. The air connectivity of Southern hemisphere descended much more than the Northern part in the middle of May 2020, and the number of daily origin-destination pair dropped by 70 % compared with the beginning of 2020. Epicenters of COVID-19 shifted to America, Brazil, Russia, India, and other countries at the end of Episode I. Fig. 3 (left) shows the weekly pandemic situation and reductions in air passenger during Episode I. As more global cases reported, the total capacity of passenger transportation (orange line) drastically reduced in March 2020 and showed signs of recovery after two months of standstill. With the effective implementation of mitigation strategies, the domestic capacity (blue line) has been on the upturn since May despite some fluctuations. The international market was affected much more. The significant difference between these two markets was mainly resulted from stringent. flight restrictions on border entry/exit policies. Some early studies calculated risks of importation and exportation of COVID-19 at regional level [8], [22], [21], [30], [59] as well as global level [55], [34], [11] at the early stage of the pandemic. Fig. 3 (right) depicts year-on-year changes in the number of flights for selected countries during the first half of 2020. Most countries canceled their flights vastly in April and May, especially France, Germany and Spain with over 90% reduction compared to the 2019 level. As flights canceled and aircraft grounded, European airlines rearranged their fleets for further cost savings and efficiency to avoid further losses from the ongoing COVID-19 crisis. In April 2020, Lufthansa announced that seven A340–600 s, six A380s and five B747–400 s would be decommissioned, as well as eleven A320s from short-haul operations. Austrian Airlines intended to retire three B767s and all its A319s by 2022. In June 2020, Iberia Airlines confirmed that it would accelerate the retirement of all A340–600s by the end of 2020, three of which has been decommissioned before the onset of the pandemic in 2019. Meanwhile, financial reliefs were sent by European governments. Sweden provided 455 million Euro loan guarantee scheme for those airlines holding Swedish commercial aviation licenses on April 11th, 2020; France offered a loan package of seven billion Euro for Air France on May 5th, 2020; Germany granted an aid package of nine billion Euro to Lufthansa on June 25th, 2020 [41]. The responses of Australia and Japan, as well as Thailand and South Korea, to COVID-19 slightly lagged behind the initial slow growth of confirmed cases, but upgraded timely to flatten their epidemic curves with relatively low stringency of measures [32]. Australia, with a population of 25.7 million, imposed restrictions on air travel more heavily than Japan with 126 million people, although the numbers and trends of confirmed cases in these two countries were similar. In April 2020, the daily infection rate in Australia had declined significantly, due to over 86 % drops in the number of flights along with other COVID-19 prevention strategies, which showed that Australian policies of pandemic-related interventions seemed to be successful. Its aviation sector would restart mainly depending on domestic market, and other public transport would restore confidence by further expedients [5]. Despite a comparatively dense aging population, the downward trend in the number of flights in Japan was relatively low. with a maximum descent of less than 50 % among others. Such aviation responses probably owed to the Japanese emergency law, limiting the power of the national and local government to enforce lockdowns [52]. Nevertheless, this pandemic caused substantial negative impacts on Japanese aviation, including airline yield, scheduled capacity and frequency, these effects also varied across mainline and regional routes [35]. As the first epicenter of COVID-19, China was the first victim as well as the first revived market, with a series of mitigation strategies including checks on negative test, quarantine for two weeks (or more), contact tracing and flight bans. To reduce further transmission, the Civil Aviation Administration of China (CAAC) required that strict prevention should be taken on flights arriving at and departing from China, and that the load factor should be less than 75 % from late March. Strict traffic bans on inbound flights since the end of March 2020 reduced the risk of case importations by half, although this risk rose due to the global spread and changes of pandemic centers changing [59]. CAAC adjusted its Five-One policy1 on scheduled international passenger flights including Reward and Circuit Breaker measures and issued the first order in June 2020. For a single inbound flight of an airline, if all passengers test negative for COVID-19 for three consecutive weeks, the airline’s operation on that specific route shall be rewarded one flight per week with a maximum of two flights per week; if there were five or ten passengers with positive test, this flight shall be suspended for one week or four weeks, respectively. China achieved a quick recovery to 80 % of the pre-COVID-19 level in June 2020. Notwithstanding, such a fast recovery might not be possible in countries with “curve flattening” strategies, instead of full control of COVID-19 [14]. The subsequent hotspots of COVID-19 implemented different policies confronted with high infection rates. The United States seemed to be reactive until its confirmed cases soared, with a comparatively small decrease in the number of flights and a recovery in June 2020. The lagged responses might be resulted from American federalism, fracturing the authority and capacity of policy makers to tackle the pandemic [37]. Despite relatively proactive containment in Brazil, Russia, and India, these countries failed to curb the spreading of COVID-19 in a short time. Brazil partially shut down some non-essential economic activities and then imposed the full lockdown in April 2020, where the increasing cases was probably due to politically mixed messages or the size and density of its population [32]. Russia closed its borders to foreigners at the end of March 2020, but then the federal and regional authorities issued some contradictory orders [4]. The nationwide lockdowns in India led to flight declines with a maximum of over 82 % in April 2020. The aviation market revived in May 2020, probably due to lifting some restrictions, but it slightly dropped again in June 2020 due to the increase of infections.

Viruses, including SARS-CoV-2, evolve over time as the genetic code changes during replication of the genome. Impacts of most changes are negligible, while some may affect phenotypic characteristics of viruses. A variant of concern (VOC), probably with enhanced transmissibility, upsurged disease severity, or lessened effectiveness of diagnostics, treatments and vaccinations, can circulate in multiple countries even around the world and increase its proportion of new confirmed cases. 1This policy stated that Chinese airlines could fly just once a week on one route to any country, while foreign airlines could operate just one flight a week to China. Variants may have spread widely in the time interval between the dates of earliest documented samples and the dates of designation as VOCs ( Fig. 4, left), even since the beginning of the COVID-19 pandemic. In general, variants are more likely to appear in countries with more infected cases, such as Alpha first reported in the UK, Gamma in Brazil, and Delta in India, except Beta and Omicron in South Africa. At the beginning of Episode II, the domestic and international aviation market rebounded gradually, given regional demand of air passenger traffic and the summer holiday (Fig. 4, right). Member States of the EU, for example, reopened their borders for non-essential travel from 15 countries in a coordinated manner as from July 1, 2020, which improved European aviation in latter half of 2020. With a year-over-year loss of 40 % in September 2020, the market of Eastern and Central Europe underpinned by Russia, performed better than the market of Western Europe [40]. Nevertheless, emerging SARS-CoV-2 variants became dominant in the pandemic, undulating the recovery trend of this industry. Surges of newly confirmed cases due to variants resulted in reversals of aviation markets by forcing governments to implement temporary restrictions on air travel. The synchrony in the same direction between the number of new cases and capacity of passenger transport revealed that flight restrictions lagged behind the pandemic situation, and that delayed reactions of aviation aggravated the COVID-19 pandemic. Flight restrictions are effective to restrain the initial transmissions of variants; otherwise, they can only defer the arrival time of variants into other countries [45]. Several air transportation stakeholders expected that the ongoing process of vaccination and travel bubbles would accelerate the return of aviation industry to normalcy. As vaccines available to most countries for the second half of Episode II, the aviation industry further rebounded. Nonetheless, there remained some challenges, e.g., inequitable access to vaccines for people in lower-income countries, emerging cases of immune escape and introduction of vaccination passports for air traffic. The COVID-19 vaccination passport/certificate proves that its holder is theoretically immune to this disease, which can relieve the holder from some epidemic-related process and promote international mobility. Haque et al. [25] proposed an architectural framework for vaccination passports based on blockchain (VacciFi), with integrity, verifiability, traceability and trustworthiness. Such documents in both paper versions and electronic versions, however, receive scientific, ethical and le- gal concerns [43], [17], [24], [36]. Broader challenges of vaccination passports at a larger scale concerning five aspects: competition, epidemiology, technology, ethics, and politics, are discussed in Sun et al. [47]. Travel bubbles, corridors or bridges were proposed by some countries to keep their air connectivity for specific destinations, where passengers can be exempted from containment measures. Such strategies can be regarded as an effective and important first attempt to resume the international market, if there are high levels of mutuality in country/regions pairs during and post COVID-19. It is vital to consider epidemic situations, economic benefits and sustainability for countries involved to construct these bubbles/corridors [58]. However, establishment and maintenance of travel bubbles/corridors can be dilemmas for authorities under volatility. The travel bubble between Australia and New Zealand was suspended before implementation due to outbreaks in Australia’s Victoria. Countries and regions have been repeatedly added and removed from the list of UK travel corridor since its execution on July 3rd, 2020. On the other hand, there were a few obvious travel bubbles identified by the actual flight data, including country pairs comprising Russia with Ukraine, France with Kenya, UK with Sri Lanka, and Singapore with Switzerland [50]. Fig. 5(a) shows aviation capacities of ten selected country markets in the last week of Episode II (June 21–27, 2021) versus the 26th week in 2019 (June 24–30, 2019). USA ranked first despite a reduction of 18.8 % compared with the 2019 level, followed by China with a decline of only 0.3 %. The Russian market showed a rare increase of 2.9 % among others, while the aviation capacities of Spain, Japan, Germany, and United Arab Emirates (UAE) decreased by more than 50 % compared to the year 2019. Air traffic of passenger transport in China’s domestic market had revived to pre-pandemic level under a series of management expedients, including the adjustment of Circuit Breaker measures. Yu and Chen [56] utilized non-recursive structural equation models to evaluate the single/joint executions of three air traffic strategies in China, revealing that the conjunction of Circuit Breaker and double negative tests contributed to curbing the importation risk of COVID-19. This study, however, analyzed only one airport for the first six months after imposing Circuit Breaker. Despite safeguarding the domestic market recovery in China, this policy resulted in short supplies in international flights and upsurges of their ticket price. Fig. 5(b) shows the US market during Episode II. Domestic flights were less affected for a shorter period compared with international flights, probably due to no alternative to air transportation in America, e.g., well-established high-speed railway systems are missing. Restrictions and recovery of international flights from the US varied by destinations. Air connections to Africa and Oceania revived much more strongly than to Asia owing to disconnecting with China, and flights to Europe and South America recovered gradually throughout the first half of Episode II. Capacity of passenger transportation has significantly increased since March 2021.

Aviation gradually recovered and reached close to the pre-COVID-19 level in Episode III, i.e., July 2021-June 2022, despite some setbacks when new waves of COVID-19 emerged ( Fig. 6(a)). Reverse synchronization between the number of new cases and air capacity indicated that aviation response was timelier than before. Domestic market was sensitive to the increases of new confirmed cases probably due to endemic situations, while international market had quick reflexes in front of the severe pandemic. Omicron variant, earliest identified in South Africa on November 9th, 2021, was designated as a VOC by the WHO on November 26th, 2021. After this official declaration, many countries, including the UK, Germany, Italy, the Netherlands, Singapore, Canada, the US, South Korea and Australia, imposed restrictions on travelers from some southern African countries to prevent Omicron variant. Israel and Japan announced that they would close their borders and prohibit all foreign tourists from entering at the end of November 2021. Nevertheless, Omicron with higher infectivity and immune escape has spread worldwide rapidly since late 2021 and hit the recovery of aviation in early 2022. As global transitions from the COVID-19 pandemic phase to the local epidemic in 2022, many governments have lifted travel restrictions, eased health-related measures and simplified air travel experience, which has promoted passenger demand increasing and endemic aviation recovering. Fig. 6(b) shows top 10 domestic markets in June 2022. The US remained the largest domestic aviation market according to capacity, with 6.9% below the 2019 level and 9.5% above 2021 level. China (CN), the second largest domestic market, was 4.8% above June 2019 but 8.5% below June 2021. Others also recovered to around the pre-COVID-19 levels. Fig. 7 (left) reports changes in flight frequency of top 20 airlines in June 2022 versus different periods, including six airlines from the United States: American Airlines (AA), Delta Air Lines (DL), United Airlines (UA), Southwest Airlines (WN), Alaska Airlines (AS) and JetBlue Airways Corporation (B6). Ryanair (FR) and IndiGo (6E) achieved year-over-year frequency growths of about 20 % in June 2022 compared with the year of 2019, though most airlines were below their June 2019 levels. On the other hand, flight frequencies of many airlines increased in June 2022 compared with the same period of the previous year, especially Easyjet (U2), Air Canada (AC) and British Airways (BA) with significant volume rises of more than 2.5 times. Airline frequency monthly growth in June 2022 was dominated by three Chinese carriers: Air China (CA) increased flights by nearly 60 %, China Eastern (MU) by nearly half and China Southern (CZ) by almost a third. Business models influenced the performance of airlines during the COVID-19 pandemic. besides the disease dynamics and local policies. Low Cost Carriers fared better than Full Service Carriers to deal with impacts of unexpected crises [18]. Fig. 7 (right) shows top 20 airports in term of capacity in June 2022, including eight airports from the United States, among which five ranked top seven. Similarly, capacities of most airports in June 2022 reduced in comparison with 2019 levels, except Delhi Airport (DEL) and Las Vegas Airport (LAS) with increases of 2 % and 7 % respectively. Nevertheless, the number of passengers served by all these airports was more than the year of 2021, particularly London Heathrow Airport (LHR), Paris Charles de Gaulle Airport (CDG), Amsterdam Airport (AMS) and Frankfurt Airport (FRA). Concerning monthly increments, Guangzhou Airport (CAN), adding back significant vol- umes of 0.8 million more seats in June 2022, was the fastest growing one with an increase of 37.2 %. Charlotte Airport (CLT) is the second largest, whose month-on-month growth was 9 %, i.e., 0.21 million seats. However, only six out of top 20 airports involved in top ten domestic routes and top ten in- ternational routes ( Fig. 8). Busiest domestic routes were mainly in Asian countries, and busiest international routes were mainly across Europe and Western Asia, despite eight out of top 20 air- ports located in the United States. There were three airports serving up to three busiest routs. Tokyo Haneda Airport (HND), ranking eighth in the top 20 airports, served three busiest domes- tic routes: Sapporo New Chitose-Tokyo Haneda (CTS-HND) with 872,780 seats, Fukuoka-Tokyo Haneda (FUK-HND) with 858,229 seats, and Tokyo Haneda-Okinawa Naha (HND-OKA) with 565,846 seats. Dubai International Airport (DXB), ranking eleventh in the top 20 airports, served three busiest international routes: Dubai-Riyadh (DXB-RUH) with 235,700 seats, Mumbai-Dubai (BOM-DXB) with 200,573 seats, and Dubai-London Heathrow (DXB-LHR) with 199,200 seats. Riyadh Airport (RUH) served one busiest domestic route, i.e., Jeddah-Riyadh (JED-RUH) with 618,246 seats, and two busiest international routes: Dubai-Riyadh (DXB-RUH) with 235,700 seats, and Cairo-Riyadh (CAI-RUH) with 187,722 seats.

While the endemic recovery across major aviation markets has been obvious - some airlines even reported new passenger or profit records – exceeding pre-COVID-19 levels – there are challenges remaining for the aviation industry. Moreover, we hope that all stakeholders have made use of the opportunity for constructing a pandemic-resilient aviation system, enabling a better handling of future epidemic risks. We discuss a few relevant challenges below and provide an outlook into potential directions for future work. Capital-intensive and profit-driven, airlines may need to reconsider their business models ac- cording to unprecedented and extensive impacts of COVID-19. Advanced technology, e.g., data science and artificial intelligence, may offer tools for airlines to improve their crisis-responses from the over-reliance on historical models. During the COVID-19 pandemic, some airlines converted passenger aircraft to freight aircraft or hybrid aircraft for transporting medical supplies and other goods in order to maintain revenue, which open up new business models and markets in the future. Moreover, this pandemic or COVID-X may create chances for airline startups to enter the market with modern technology and supportive polices from regulatory authorities and international organizations [49]. Pandemic-related measures in airports can hide large heterogeneity, involving process, criteria, target audience and cost, which calls for a consistent standard from international authorities [12]. Disinfection of public areas, screening operations, passenger flow control and optimized process of baggage-claim were added to daily work for mitigating infection probability, massive concentration and long waiting time over two to three years. However, there have been debates over the effectiveness of additional measures in literature, including disinfection equipment, temperature screening, checks on negative-test and/or vaccination certifications, and configurations of security control lanes check-in counters. New operations at airports may bring complaints from passengers and put pressure on aircraft turnaround times. User experience can be utilized as a basis for better implementation and more efficient management. Solutions for aviation workforce in emergencies should be further discussed. Due to extreme declines in aviation companies’ revenue, a large number of workers were laid off during the pandemic, as well as human resources restructuring and organizational closures. Job insecurity may increase. pressure and negative emotions of crew, and affected pilots flight proficiency, possibly culminating in flight safety incidents. Nevertheless, pandemic-induced occupations especially on techniques, have emerged in aviation and related sectors, which may help the industry get prepared when hit by future crises [46]. Future work on COVID-19 and aviation needs to consider a wider spectrum of the aviation industry, including freight transportation, airports and their internal retailers, aircraft manufacturers and maintenance providers. Emerging technologies, e.g., blockchain, cloud computing, Internet of Things (IoT), and biometrics, could be another focus to improve the resilience of aviation against external crises, which should be applied to multiple scenarios for analysis, prediction and (auxiliary) decision making, as well as monitoring, detection, and disinfection. Here, we would like to highlight the critical role of effective and efficient airport security protocols for a successful fight against future epidemic outbreaks. The collection and management of passengers’ health and travel information will be essential for better controlling passenger flows and reducing the risks of transmission, without the use of excessive border closures and lockdowns at a global scale. The latter is presumably one of the key insights we can learn from the COVID-19 pandemic. Modern technologies will also help here but need to be applied with a ubiquitous sense of privacy and data security concerns.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.