An influenza pandemic, when it arrives, will be an immediate threat to the health of nearly everyone on Earth, but very little is being done to prevent its potential devastation, say experts who met this week at the World Health Organization’s headquarters in Switzerland.

A vaccine is unlikely to prevent the global spread of a pandemic strain of flu virus, but it could save millions of lives. To do so, however, the world must be ready to make, test, pay for, distribute and probably share what will be a scarce supply, the experts concluded.

“We have a unique window of opportunity now to get our homework done to ensure that that when it matters most, vaccine production can happen immediately,” said Klaus Stohr, the leader of WHO’s influenza activities who led the two-day, closed-door meeting that ended Friday. “That’s our chance, and we don’t want to miss that chance.”

Arlene King, an official with Canada’s public health agency, said a pandemic, or global epidemic, would be “a national health-security issue. It will be the largest public health infectious disease emergency we ever face in most countries, and certainly globally.”

Avian influenza — “bird flu — is the latest strain knocking on humanity’s door. The so-called A/H5N1 virus, which has spread widely in chickens and ducks, also has infected 44 people in Thailand and Vietnam this year, killing 32. Should it adapt fully to humans and be capable of easy person-to-person spread, it would probably spread worldwide in three to six months.

“We have in Asia an H5N1 virus which is ready to cause a pandemic,” Stohr said.

If the three flu pandemics in the 20th century are models, one-fifth to one-third of the world population might be infected in the next one. Even if only 1 percent were to die, as some experts predict, it would cause huge social and economic disruption. The “Spanish flu” pandemic of 1918-1919 killed at least 50 million people.

The meeting, attended by about 50 people from national health ministries, drug regulatory agencies and vaccine companies, was held to lay out the obstacles to developing a pandemic vaccine and begin the work of overcoming them. WHO has no authority to compel countries or companies to act. There was general consensus that not nearly enough is being spent on planning for a pandemic — and that governments must lead.