The number of active severe cases in that age group gradually increased from five around the beginning of the fourth wave, to a high of 62 on December 12, accounting for 56 per cent of the 110 seriously ill patients in public hospitals at that time.  

At the peak of the third wave, which ran from July 22 to August 7, that age group accounted for around 40 to 50 per cent of the seriously or critically ill patients, and 47 such active cases at the worst point.

Those aged 80 and above account for less than 20 per cent of the seriously or critically ill patients in the fourth wave so far, lower than the high of 36 per cent observed in the studied period of the previous wave.

The highest number of seriously ill patients aged 20 to 49 in hospital on a single day was seven in the fourth wave, compared to five in the third.

Hong Kong has so far recorded 7,803 confirmed infections and 123 related deaths.

Pro-establishment activist Leticia Lee See-yin was believed to be among the latest Covid-19 victims. Sources said Lee, 56, tested preliminary-positive after being certified dead on arrival at Pok Oi Hospital on Wednesday.

A source from the Hospital Authority’s senior management said there had been a “significant increase” in the number of patients – including relatively younger ones – requiring treatment in intensive care units.

“This is an unusual and worrying sign, so we believe we should alert the public about this,” the source said.

Dr Linda Yu Wa-ling, a chief manager at the authority, said on Wednesday that among the critical and serious patients, several were newly confirmed cases.

“No matter old or young, or how healthy you are usually, your condition could deteriorate quickly after contracting this infection,” Yu warned.

According to Dr Kenny Chan King-chung, a member of the authority’s coordinating committee for intensive care, in some of the recent cases, the patients arrived in hospital in critical condition and immediately required intubation to assist with breathing. These patients tended to have a high viral load, which was considered more infectious, Chan has previously said.

Some patients also had silent hypoxia, a condition in which oxygen saturation in their body plummeted to abnormally low levels, but they developed no signs of shortness of breath, according to Dr Wu Tak-chiu, of the authority’s task force on clinical management of infections.

Professor David Hui Shu-cheong, a respiratory medicine expert from Chinese University, said the rise in overall infections in the fourth wave was much faster than in the past.

There was a 38 per cent increase – from 812 to 1,118 – in the hospitalisation of patients in the first 12 days of this month. But there was a fivefold increase in the number of seriously ill patients – from 21 to 110.

The occupancy rate of intensive care unit beds in public hospitals was around 70 per cent.

A senior intensivist, speaking on condition of anonymity, said the increase in seriously or critically ill middle-aged patients might explain why intensive care units were under more pressure during the current wave.

“For those aged in their 50s and 60s and without major chronic illnesses, if their lungs are severely damaged, they must be rescued at all costs,” the doctor said. “While elderly people and those with long-term illnesses are less likely to be treated in intensive care units.”

He said some old and fragile patients would opt for conservative treatments rather than being intubated in intensive care units.

Frail elderly patients were among the most affected by the coronavirus during the third wave, as around 20 care homes reported infections, with at least four hit by major outbreaks.

The fourth wave was accelerated by the dance venue cluster, which has now ballooned to more than 720 cases and extended to people of various social backgrounds.

Universities are racing to investigate why younger patients are developing more serious symptoms, when they were thought to be less likely to develop severe complications compared with the elderly.

Dr Gilman Siu Kit-hang, a Polytechnic University academic who studied the coronavirus strains in the city, said no mutation had been seen in the current type, which was believed to have stemmed from infected travellers from Nepal in October.

“The virus strain of those severely ill patients was the same as [the one in October],” Siu said.

He said animal studies would be conducted to see whether the strain currently circulating in Hong Kong was by nature more virulent than those in the previous waves.

He believed the recent clinical observations made by doctors might be due to the physical conditions of patients rather than changes in the virus.

“Would there be any difference in vital indexes or if treatments had been consistent for all patients?” he asked.

Hui warned that delays in treatment could result in serious complications, even among younger people who were deemed fit.

Additional reporting by Robbie Hu