The airline has so far resisted mounting pressure to provide a full explanation, after the 41-year-old pilot was taken to Princess Margaret Hospital in Kwai Chung to be treated for measles.

It is against aviation law for anyone to be part of a flight crew “if they know or reasonably suspect their physical or mental condition renders them temporarily or permanently unfit to perform such functions”.

Microbiologist Dr Ho Pak-leung, an international expert on infectious diseases at the University of Hong Kong, questioned if the pilot had broken International Air Transport Association rules by flying while sick.

According to the association’s medical manual, pilots are required to obtain a doctor’s opinion on flying during the contagious stage of measles.

“There is a need for the Hong Kong Airport Authority to look into in this,” Ho said.

He also said the scale of the current outbreak had been small. “But I expect more cases to surface, and there could be a second round of infections,” Ho said.

The Airport Authority, meanwhile, has set up an additional vaccination station for staff in consultation with the Centre for Health Protection and the Department of Health, and about 850 people have been vaccinated since last Friday.

Officials said they would leave it to Cathay to explain what measures it was taking.

“We will wait for the company to make a response or follow up,” said Undersecretary for Food and Health Dr Chui Tak-yi.

If necessary, he added, the government would contact the measles vaccine manufacturer to ramp up supply.

As for existing precautions being taken at the airport, Centre for Health Protection head Dr Wong Ka-hing confirmed that departing airline employees or passengers did not have to go through temperature screening.

Such an arrangement was only necessary during the 2003 Sars outbreak, he noted, and in any case, temperature sensors would not necessarily have picked out the pilot if he had no fever.

“For instance, he could have taken some medicine and was not having a fever,” Wong said.

Lawmaker Ben Chan Han-pan, of the pro-government Democratic Alliance for the Betterment and Progress of Hong Kong, said the pilot’s case had exposed “major loopholes” in the system.

Chan, who chairs the Legislative Council’s transport panel, questioned whether the pilot had informed Cathay management of his illness.

“If he told his company and they still sent him out, that’s wrong,” Chan said.

Cathay would only say workers who felt unwell “should stay at home”.

“We encourage our employees to be immunised or seek medical help if they are not feeling well,” a spokesman said, adding that anyone with measles should not return to work without being cleared by a doctor.

A source from Cathay’s pilots union said there were no internal guidelines specifying when a pilot should report sick.

The pilot in question was one of five airport and airline employees infected with the disease this month.

According to the Centre for Health Protection, the 41-year-old first developed a fever and rash on March 12. Before that, he was on two flights between Hong Kong and Manila, on March 8 and 9.

Despite falling ill, officials said, the pilot took seven other flights between Hong Kong, Singapore and Bangkok between March 13 and 16.

He only sought medical treatment at North Lantau Hospital last Thursday, and was later transferred to Princess Margaret Hospital, where he tested positive for measles.

A 23-year-old Cathay flight attendant who flew on Tokyo and Cebu routes was also infected with the virus.

The pilot told the centre he did not come into close contact with the flight attendant. Health authorities said he had limited exposure to the public, and his family did not show signs of infection.

Additional reporting by Danny Lee