HONG KONG — A leading virus expert urged health authorities around the world Sunday to stay vigilant even though the recent swine flu pandemic was less deadly than expected, warning that bird flu could spark the next global outbreak.

A World Health Organization official also defended the U.N.’s health body against accusations that it wasted governments’ money and enriched pharmaceutical companies with its strong warnings during the swine flu outbreak’s early days last year.

WHO declared the swine flu pandemic over last month. The latest death toll is just over 18,600 — far below the millions that were once predicted. The head of the global health body has credited good preparation and luck, since the H1N1 swine flu virus didn’t mutate as some had feared.

But speaking to reporters on the sidelines of an influenza conference in Hong Kong, researcher Robert Webster warned against complacency.

“We may think we can relax and influenza is no longer a problem. I want to assure you that that is not the case,” said Webster, chairman of the virology and molecular biology department at St. Jude Children’s Research Hospital in Memphis, Tennessee.

Webster predicted that the next pandemic could be sparked by a virus that spreads from water fowl to pigs and then onto humans — such as the H5N1 strain of bird flu, which has killed 300 people over the past seven years. He noted that after several years of decline, the number of bird flu cases in humans increased in 2009, lifted by an uptick of cases in Egypt.

“H5N1 can kill 61 percent of humans infected, but it doesn’t know how to spread from human to human. But don’t trust it because it could acquire that capacity. So we must stay vigilant,” he said.

Sylvie Briand, head of WHO’s global influenza program, said its surveillance has shown that the bird flu strain isn’t capable now of jumping between humans except in rare cases of close personal contact, but echoing Webster, warned: “These are viruses that are evolving. They are changing all the time.”

Both experts said it was difficult to predict when — or if — bird flu might set off a new pandemic.

“We don’t understand enough about the virus to make predictions,” Webster said.

University of Hong Kong microbiologist Malik Peiris said scientists are closely monitoring mutations by influenza viruses — including bird flu viruses — but it’s hard to determine which mutations are most likely to spread among humans.

Briand also defended WHO against accusations that it hyped the swine flu pandemic, saying it was acting with limited information when the virus first surfaced and adjusted its response when it became clear the outbreak wasn’t as serious as expected.

WHO received at least $170 million from member states to deal with the outbreak, some of which was invested in immunization programs long after the virus strain was known to provoke only mild illness in most of those infected. Governments spent many times that amount buying vaccines and antiviral medicines that are now being junked.

European legislators have repeatedly accused WHO of overstating the danger of swine flu and playing into the hands of the pharmaceutical industry, which has earned millions from the outbreak since it began in April 2009.

“We prepared for the worst and hoped for the best. And as the information became available, there was adaptation of the plan and adaptation of the recommendations in order to really tailor the response to the reality,” Briand said. “But it took some time.”