At least 49 passengers on a flight from New Delhi to Hong Kong have tested positive for Covid-19, authorities said on Tuesday, as a two-week ban on all flights from India, as well as Pakistan, and the Philippines took effect.

All of the passengers who tested positive flew into Hong Kong on a flight run by Indian operator Vistara on April 4.

The positive tests are significant as Hong Kong has been recording fewer daily cases than the total detected on the flight since it brought a fourth wave under control in January.

India meanwhile is battling skyrocketing infections, with hospitals running out of beds and the government reimposing painful restrictions, including a seven-day lockdown that began on Monday in the capital, New Delhi.

As many as 188 passengers could have travelled on the Vistara flight, but Hong Kong authorities did not say how many people were onboard the aircraft. The positive Covid-19 results surfaced during Hong Kong’s mandatory three-week quarantine period, one of the strictest entry regimes in the world.

47 passengers on Vistara flight UK6395 from New Delhi to Hong Kong on April 4th have now tested positive for COVID-19. pic.twitter.com/Mb4wCEPMPl

— 1B002Aaron 💉 💉 🇭🇰 (@tripperhead) April 18, 2021

Two more COVID-19 cases added today (including one positive for the N501Y mutation).

Total COVID-19 cases from Vistara UK6395 arriving in Hong Kong April 4th: 49

(case number, seat number, quarantine hotel and age of the 49 confirmed cases) pic.twitter.com/9Nji58EZyj

— 1B002Aaron 💉 💉 🇭🇰 (@tripperhead) April 19, 2021

All travellers flying into the territory must also provide a negative test within 72 hours prior to departure.

Several cases have also been detected on flights arriving in Hong Kong from Mumbai, authorities said.

Eric Feigl-Ding, an epidemiologist at the Federation of American Scientists, said on Twitter that only eight cases from the Vistara flight were detected before hotel quarantine began. The rest were only identified during the time in quarantine.

“If it weren’t for hotel quarantine – [Hong Kong] would have completely allowed these [positive cases] to begin spreading in the community!” he said. “This is why border quarantines are critical.”

Feigl-Ding also said it was possible that not all transmission occurred during the flight.

“Some may have occurred after the initial 72 hour pre-departure time window after they got tested. India cases increased 60 % each week for the last 2 weeks. But this is why our border testing is leaky,” he added.

5) So, it could also be hotel transmission to some degree too. Hotel room cross infections have been observed in Australia’s hotel quarantine system too. https://t.co/ZHKkk0bjzr

— Eric Feigl-Ding (@DrEricDing) April 19, 2021

A spokesman for Vistara, responding to the cluster on its flight, said the company “ensures strict compliance with all guidelines issued by the Indian as well as the destination countries’ authorities for all flights, including all charter flights to and from Hong Kong”.

Authorities in Hong Kong announced late on Sunday a ban on all flights from India, Pakistan and the Philippines from April 20 – May 3, categorising the countries as “extremely high risk” after detecting the N501Y mutant coronavirus strain in the city for the first time.

Scientists say the N501Y mutation makes the virus more transmissible.

Dr Leung Chi-chiu, a specialist in respiratory medicine, told Hong Kong’s RTHK broadcaster the two-week ban was not enough and that people who have spent time in India, Pakistan, and the Philippines should be banned from coming to Hong Kong for as long as it takes for the Covid-19 situation in those places to improve.

“We will have to keep this ban unless the situation there improves or unless we have devised other measures to reduce the chance of importation of these dangerous variants into Hong Kong,” he said.

Densely populated Hong Kong was one of the first places to be hit by the coronavirus but the tough entry conditions, strict social-distancing measures, and universal mask-wearing have helped keep infections to just over 11,000 with 209 deaths.

Approximately 9.8 percent of the 7.5 million people who live in the city have been vaccinated so far.