It’s Official: H1N1 Flu Is a Pandemic

World Health Organization (WHO) officials declared a pandemic of H1N1 influenza on Thursday, two months after the first cases of the new flu virus were reported in Mexico. It is the first flu pandemic in 41 years, since the 1968 Hong Kong flu.

The WHO’s decision to raise the pandemic alert to Phase 6, the highest level, was based on specific criteria, most significantly that the disease is now widespread, sustaining transmission in more than two regions around the world — including the Americas (Mexico and the U.S., which have so far borne the greatest brunt of the new flu) and Australia, where cases have risen sharply. The latest data show there are nearly 30,000 cases in 74 countries, with 144 confirmed deaths.

(See the top 5 swine flu don’ts.)

The long-anticipated announcement by the WHO, which followed an emergency meeting of flu experts in Geneva, signals to health officials around the globe that the new influenza strain will continue spreading quickly to more countries. It also adds urgency to their preparedness plans, which include acquiring and distributing antiviral medications and a vaccine, if and when one becomes available. The WHO has dispatched antivirals to 121 nations and asked vaccine makers, who are now completing their production of seasonal flu shots, to begin testing and production of a vaccine for H1N1.

Although the pandemic alert is a formal sounding of the alarm for H1N1, it does not reflect any increase in the severity of illness. The alert criteria drawn up by the WHO specifically include transmissibility of a new virus but not severity, since it is difficult to gauge accurately the deadliness of a newly emerging infection in real time. “The declaration of a pandemic does not suggest that there has been any change in the behavior of the virus,” said Dr. Thomas Frieden in his first press conference as director of the Centers for Disease Control and Prevention (CDC). “It only means that it is spreading in more parts of the world.”

(See pictures of thermal scanners hunting for swine flu.)

WHO director-general Dr. Margaret Chan stressed that the agency was declaring the pandemic a “moderate” one, given that the majority of infections are controllable with proper antiviral treatments. But the health agency has met with criticism recently for delaying the pandemic declaration for fear, in part, of stoking public panic.

Frieden noted also that since the first swine flu cases emerged, the U.S. has essentially been operating under the assumption of a pandemic. “In the U.S. and the Americas, we have already had widespread and continued transmission of the virus for some time, so this doesn’t change any of our actions,” he said. “Our key goal is determining where the virus is spreading and to reduce its impact, particularly on those who are most vulnerable.”

Unlike seasonal flu, which typically infects the very young, the very old and the immuno-compromised, the new flu strain seems to affect young, often healthy people; 57% of the cases reported in the U.S. have occurred in those 5 to 24 years old. Why that’s so is still unknown, as is the origin of the virus, although a genetic analysis published today in the journal Nature suggests that H1N1 may have been circulating among humans as early as last August. So far, in the U.S., 13,000 cases of H1N1 have been confirmed, including more than 1,000 hospitalizations and 27 deaths.

“Here in the U.S., the Phase 6 declaration isn’t going to change what we do day to day,” said Dr. Anne Schuchat, director of the CDC’s National Center for Immunization and Respiratory Diseases. The Department of Health and Human Services has allocated $1 billion to the development, testing and production of an H1N1 vaccine, and five different companies, including GlaxoSmithKline and Sanofi-Aventis, currently have the seed virus from CDC that will form the core of the immunization. But a vaccine won’t be ready until September at the earliest, and even then, it won’t be clear whether that shot will be a good match against the H1N1 influenza virus that will be circulating in the fall, since it may mutate between now and then.

(Read “Battling Swine Flu: The Lessons from SARS.”)

Still, as Chan pointed out, “No previous pandemic has been detected so early, watched so closely in real time right at the very beginning. The world can now reap the benefits of pandemic-preparedness investments made over the last five years.”

Raising the pandemic alert is only a reminder that despite our best efforts to prevent or detect new diseases early, simple vigilance is the best defense against something as unpredictable as influenza. “We have a head start,” Chan said. “But like all influenza viruses, this one can change the rules.”

See more about swine flu.

See pictures of the swine flu outbreak in Mexico.