HONG KONG—Rashida Fathima’s anxiety levels spiked as she boarded the red-eye flight from New Delhi to Hong Kong with her family. Covid-19 cases were surging in India, and the plane was packed almost to capacity.

Within two weeks of landing, Mrs. Fathima, her husband and two children tested positive for the coronavirus at their quarantine hotel. More than a third of the passengers on flight UK6395—52 so far—have tested positive, the most from any plane arriving in the city. The cluster is stirring debate among health experts in Hong Kong over how they got infected, and highlights the struggle facing the aviation industry as it seeks to get people traveling again.

Speaking from the hospital, Mrs. Fathima said she feared her family picked up the infections on the April 3 journey, despite wearing masks almost the entire time and avoiding using the restrooms on board.

Some passengers—including one in the same row—coughed repeatedly during the six-hour flight, people took masks off to eat, and some parents walked their crying children up and down the aisle, she said.

The airline and the organizers of the chartered flight told The Wall Street Journal that they did everything possible to minimize potential transmission.

Vistara airline, run by Tata SIA Airlines Ltd., said it ensures “strict compliance with all guidelines issued by the Indian as well as the destination countries’ authorities for all flights.”

Experts say that plane travel is generally safe. But even with the best precautions there are risks associated, as coronavirus cases continue to soar in some parts of the world and with young children not yet eligible for vaccination.

There could, for example, have been passengers with particularly virulent strains of the virus. Being packed in tightly, no matter how good the ventilation, puts certain passengers in the way of potential infection through respiratory particles from people sitting nearby or on surfaces in restrooms. And there is the risk of the virus spreading at airports before and after flying.

Other possibilities being discussed by health experts include whether travelers became infected at quarantine hotels in Hong Kong after arrival, or a failure by India’s overloaded health system to accurately detect cases before the passengers left—though only four tested positive on arrival.

Rashida Fathima’s 2-year-old son and 8-year-old daughter on a morning connecting flight from Chennai to New Delhi, before their flight to Hong Kong.


Rashida Fathima

India is in the grips of a startling second wave of coronavirus infections. On Friday, the country reported more than 332,000 new infections, the highest one-day tally recorded in any country since the pandemic began. It would also be possible, epidemiologists say, that some passengers on the plane got infected in the country after their preflight virus test.

Hong Kong suspended flights from India for two weeks on Tuesday and upgraded India to the list of extremely high-risk countries for arrivals. The city has one of the most stringent testing, tracing and quarantine systems in the world, mandating 21 days of hotel quarantine for almost all inbound international travelers and subjecting them to several rounds of Covid testing over the period. This offers a more detailed glimpse of travelers’ infection status. Most of the cases on the flight from New Delhi weren’t discovered until days after the group had landed in the city.

University of Hong Kong scientists who work with the city’s health department are sequencing genomes from the passenger cluster and examining information to discern whether they were infected on board.

Hong Kong-based GC Nanda & Sons Ltd., known as Nanda Travel, which organized the charter flight said the Airbus A321neo plane was at 85% capacity, with 146 passengers and 7 infants.

All passengers produced evidence of a negative Covid-19 test from reputable labs taken within the 72 hours before the plane took off, the company said. Six passengers were denied boarding by the charter operator as their test-results certificates didn’t meet the travel company’s standards.

Passengers were required to wear masks except for when eating prepackaged meals. Alcohol wipes and hand sanitizer were provided.

“We’re shocked by this,” said Poonam Nanda, the director of Nanda Travel. “This one flight appears to be an astonishing outlier and we are all confounded by these numbers.”

Of 29 charter flights the company has operated to bring stranded travelers back to Hong Kong in the past few months, less than 1% of passengers tested positive on arrival and on many flights there have been zero subsequent cases, she said. The company will enhance its protocols, such as requiring negative Covid-19 test certificates from not more than 48 hours before departure on future flights, she said.

“The risks of air travel are generally quite manageable with staggered seating and consistent mask usage, especially when using toilet facilities,” said Siddharth Sridhar, a clinical virologist at the University of Hong Kong’s Department of Microbiology.

As India suffers an overwhelming second wave of coronavirus infections, scientists are trying to understand a new ‘double-mutant’ variant that was first detected in the country. Photo: Rafiq Maqbool/Associated Press

Ventilation and air filtration further reduce risk of virus transmission on planes, Dr. Sridhar said. Yet there are many variables that could have an impact, including whether people wore masks consistently on board, whether meals were served, how many restrooms were operational, and how many of the passengers were part of single family units, he said.

Blocking the middle seat on airplanes can reduce coronavirus risks to passengers by 23% to 57%, a Centers for Disease Control and Prevention lab-based study published this month suggested. Despite that, most carriers in the U.S. have resumed selling middle-seat tickets.

The real issue is how passengers comply with regulations, said Leo Poon, a professor at the University of Hong Kong’s School of Public Health. A study he co-wrote last year showed that SARS-CoV-2—the virus that causes Covid-19—can be transmitted on airplanes, highlighting the importance of continuing infection control.

Mrs. Fathima, who runs an import-export business in Hong Kong, said a morning connecting flight from Chennai to New Delhi and the diaper-changing room at the Hong Kong airport could also have been potential places where she, her husband, 8-year-old daughter and 2-year-old son picked up the virus.

“We were so careful when we were in India,” Mrs. Fathima said, adding that the family stayed home for a week before traveling. “I never would have imagined catching the virus on the journey.”

Write to Natasha Khan at [email protected]

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