An outbreak at a school is defined as three flu cases in four days.

Wong said the decision was made after an interdepartmental government meeting on Thursday.

This came as a four-year-old boy was reported on Friday night to be in critical condition after being infected with influenza A. He was in the paediatric intensive care unit at United Christian Hospital in Kwun Tong.

According to the latest statistics, at least 275 outbreaks have been reported at schools and other institutions, such as elderly care homes, since the beginning of the winter flu season on December 30.

Most cases were at kindergartens and childcare centres, where there was a sharp increase to 87 last week from zero the previous week. This week there had been 105 as of Thursday.

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Outbreaks at primary schools meanwhile were said to be at a “medium” level as opposed to very high at kindergartens.

On Friday Wong would not rule out taking further action to curb the spread of flu.

“We still have not reached the peak of the influenza season. We expect the number of cases to rise,” he said.

Top infectious diseases expert Professor Yuen Kwok-yung said he saw no need to close all kindergartens and childcare centres, as no child deaths from flu had been recorded this year.

The occupancy rate at hospital paediatric wards has not reached the level seen in February last year, when there was a citywide suspension of classes at kindergartens, primary schools and special-needs schools as their Lunar New Year holiday was brought forward.

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But public hospitals are still seeing a spike in their workload this flu season. Figures from the Hospital Authority show the number of extra beds added per day stood at between 1,700 and 1,800, and the average occupancy rate for paediatric beds had reached 100 per cent.

A one-off 10 per cent increase in an allowance for the authority’s frontline staff has been endorsed and will come into effect on January 28 as a way to encourage them to provide extra help over the winter flu season. The new rate will be in effect for 12 weeks until the end of April.

Lam Chui-ling, principal of Tsuen Wan Trade Association Chu Cheong Kindergarten, said her facility had not been told to suspend classes because it had only one reported flu case. But she said the government should consider bringing forward the start of the Lunar New Year holiday for kindergartens again, to as early as next week to avoid any further spread of the illness.

“The major infection source is through contact at school such as with saliva,” Lam said. “I think advancing the holiday could cut off the source of contagion.”

She said her kindergarten would require children to wear surgical masks from next week. About 90 per cent of the children had been vaccinated, she added.

Education sector lawmaker Ip Kin-yuen suggested authorities could have managed the problem better.

He said suspensions could be chaotic for schools and parents, as both had to make contingency plans.

“I think right now, kindergartens have just received the notification from the government, and at the same time parents are anxious to find out whether their children are affected, so they’re probably calling the schools to find out, but schools are under tremendous pressure,” the legislator said.

He urged the administration to publish the list of kindergartens affected to avoid confusion.