The World Health Organization on Thursday acknowledged what many health experts have been saying for weeks: The outbreak of the novel H1N1 virus is a pandemic.
"The world is now at the start of the 2009 influenza pandemic," Dr. Margaret Chan, director-general of the WHO, said in a Geneva teleconference. "This virus is now unstoppable."
In a letter sent to WHO member countries, Chan said that she was raising the agency's infectious disease alert to Phase 6, its highest level, in recognition of the fact that the virus is undergoing communitywide transmission in Australia as well as in North America. Such spread in two distinct regions of the world is the primary criterion for raising the alert level.
But the agency said that the pandemic is only "moderate in severity" and cautioned against overreaction to the increased alert level.
The announcement marks the advent of the first global influenza epidemic in 41 years. The last one was the Hong Kong flu epidemic of 1968, which killed an estimated 1 million people worldwide.
The WHO had delayed declaring an H1N1 pandemic in part because some member nations had feared that it would trigger panic. Dr. Keiji Fukuda, assistant director-general, said Tuesday that the agency had used the time to conduct an educational campaign in an effort to minimize overreaction. It remains to be seen whether that effort will prove successful.
Most experts viewed the declaration as anticlimactic.
"It's been pandemic all along; nothing is different," said Dr. Charles Ericsson of the University of Texas Medical School at Houston. "Don't panic. The virus is still mild."
So far, the H1N1 flu, or so-called swine flu, pandemic this year has accounted for 28,774 laboratory-confirmed cases and 144 deaths in 74 countries, although health officials believe many times that number of people have been infected but have not been tested because their illnesses have been mild.
A normal seasonal flu outbreak kills about 250,000 to 500,000 people worldwide.
In most industrialized countries, the step up in the alert level will have little practical effect. In the United States, where there have been more than 13,000 cases, more than 1,000 hospitalizations and at least 27 deaths, "we have been reacting as though it were a pandemic already," said Dr. Anne Schuchat, director of the Centers for Disease Control and Prevention's National Center for Immunization and Respiratory Diseases. The same has been true in many other countries.
The announcement will have a greater effect on Third World countries, freeing additional funds for treatment and prevention and helping to make stocks of antiviral drugs more readily available.
Chan said the agency had already distributed more than 5 million doses of the antiviral drug Tamiflu to 121 developing nations, which will stockpile the medicine for use "should the virus arrive at their doorstep." The agency will now begin distributing 5.65 million additional doses that have been donated by Tamiflu's manufacturer, Roche Holding of Geneva, including doses specially formulated for children.
The World Health Organization had hesitated to raise the alert level out of concern that such an announcement would be misconstrued as an indication that the virus has become more pathogenic.
The declaration "does not mean that there is any difference in the level of severity" of the virus, said Dr. Thomas R. Frieden, who became CDC director Monday. "This is not at this point a flu that is anywhere near as severe as the 1918 Spanish flu. There is no change in the behavior of the virus, only that it is spreading in more parts of the world."
In fact, all evidence to date is overwhelming that the virus is mild in its effects. About 2% of victims have been hospitalized, Chan said. Most infections have been in people younger than 25, which is a marked difference from the seasonal flu, which has its greatest effect on the elderly and frail. About one-half to two-thirds of deaths have occurred in people with chronic underlying conditions, such as respiratory problems like asthma, cardiovascular disease and autoimmune disease. Those who are obese or pregnant are also at increased risk.
Experts fear, however, that as it passes through populations, the virus could mutate to become more lethal and return with increased force in the winter influenza season. That may have been what happened with the Spanish flu, which killed millions worldwide.
"The virus writes the rules, and this one, like all influenza viruses, can change the rules, without rhyme or reason, at any time," Chan said.
Officials had previously said they feared that the announcement would lead frightened people who are not seriously ill to overrun hospital emergency rooms, impairing the healthcare system's ability to treat the truly sick. That has happened in previous outbreaks, and there is some evidence that it is happening in South America, particularly in Argentina, where the numbers of infected have been growing.
Chan and United Nations Secretary-General Ban Ki-moon urged member countries to avoid the imposition of travel restrictions, border closings and bans on imported food -- all of which have already happened in the earlier stages of the outbreak.
"We must guard against rash and discriminatory actions such as travel bans or trade restrictions," Ban told a news conference at U.N. headquarters. "Our response to any pandemic must be grounded in science."