HONG KONG — A top Hong Kong scientist said today a flu outbreak that prompted authorities to keep more than 500,000 primary and kindergarten students home wasn’t deadlier than past viruses.

Yuen Kwok-yung and a panel of experts studied two flu patients who died amid the outbreak — a 7-year-old boy and a 3-year-old girl — and found that the flu virus hadn’t spread beyond their lungs, which suggests the virus isn’t exceptionally virulent.

“If it is a more virulent virus, we should be able to find the flu virus in other organs,” Yuen told reporters after a four-hour meeting with his colleagues.

Earlier today, the World Health Organization also said there was no sign that the situation in Hong Kong was anything but a regular seasonal flu outbreak.

The outbreak has not been linked to bird flu, which has been detected in birds in Hong Kong. Bird flu remains difficult for humans to catch, though scientists fear the virus that causes it could eventually mutate into a form that spreads easily among humans.

But the new flu outbreak raised memories of the deadly severe acute respiratory syndrome, or SARS, which surfaced in southern China in November 2002 and later killed 299 people in Hong Kong. That disease ground this bustling financial capital to a halt as locals avoided going out, while the government was criticized for not responding quickly enough.

The government has ordered all kindergartens, primary and special education schools closed for two weeks starting today, Health Secretary York Chow said. It was the first time the government has closed schools for public health reasons since SARS.

But some students and their parents missed the late Wednesday announcement and went to school anyway.

At the Cho Yiu Catholic Primary School, nine students arrived early Thursday. School staff took their temperature and gave them masks. The students read picture books instead of attending class.

“I’m worried that I’ll get infected. Many classmates have been infected with the flu. I’m afraid,” said 9-year-old Winnie Ting, who was already wearing a mask before arriving at school.

Principal Chan Kwok-ming said on average 20 students took sick leave every day in recent days.

Explaining his decision on Thursday, Chow said he acted quickly because “when you wait until you have all the data, it’s too late.”

The closure will affect nearly 560,000 students at 1,745 schools, according to enrollment figures from the 2006-2007 academic year.

There was no sign yet of the major public panic that followed the SARS outbreak five years ago. Residents have not started wearing masks en masse as they did during SARS.

Since March 6, health officials have recorded nine confirmed flu outbreaks and 65 suspected ones in Hong Kong, mostly at schools, affecting 532 people in the territory of nearly 7 million. At least three children have died amid the outbreak, two of whom were confirmed with the flu.

WHO spokesman Gregory Hartl said the first child who died tested negative for all types of influenza viruses. The second child tested positive for the flu but also suffered from an underlying metabolic disease. The third — a 7-year-old boy who died Tuesday — had both the flu and encephalitis.

Another WHO spokesman, Peter Cordingley, said Thursday from Manila that “this is just regular seasonal flu.”