WASHINGTON The pandemic of H1N1 swine flu has not yet peaked, a committee of experts advised the World Health Organization on Tuesday.
"The committee advised that it was premature to conclude that all parts of the world have experienced peak transmission of the H1N1 pandemic influenza and that additional time and information was needed to provide expert advice on the status of the pandemic," WHO spokesman Gregory Hartl said by e-mail.
WHO planned a news conference for Wednesday.
The United Nations agency declared last June that the new virus was causing the first influenza pandemic in more than 40 years after it spread around the world from Mexico and the United States in just six weeks.
Under WHO rules the emergency committee, composed of 15 experts and headed by Australian John MacKenzie, makes confidential recommendations to WHO director-general Dr. Margaret Chan.
She is then required to inform the health ministries of WHO's 192 member states and the Vatican of her decision. The WHO's decision will be announced formally by the WHO's top flu expert Dr. Keiji Fukuda on Wednesday at 1000 GMT.
WHO has confirmed the virus has killed 16,000 people but notes this is a gross underestimate, as hardly any patients are diagnosed or tested. It will take a year or two after the pandemic ends to establish the true death toll, the WHO says.
The Centers for Disease Control and Prevention does projections based on testing and patterns of disease reports and projects that H1N1 has killed up to 17,000 people in the United States alone, and put as many as 370,000 into the hospital with serious illness.
In contrast, seasonal influenza kills 250,000 to 500,000 people globally but most are frail and elderly. H1N1 has attacked young adults and children.
The pandemic sparked a race to develop new vaccines by drug makers including GlaxoSmithKline and Sanofi-Aventis but has proved to be of moderate severity, and many people failed to take the new vaccine.
Previous influenza pandemics have had waves of disease activity spread over months, meaning the post-peak period could last quite a while, according to the WHO.
The final stage, called the post-pandemic period, is when disease activity returns to levels normally seen for seasonal influenza, it says.
"There is no on and off switch for a pandemic. It's not a single event. What we have to see is that the behavior of the H1N1 virus becomes like the behavior of other seasonal viruses," Hartl said earlier on Tuesday.
"At the moment, it is still causing substantial outbreaks of disease outside the normal influenza seasons and affecting groups who are not normally affected by seasonal influenza. So as long as that continues, it does not behave like seasonal influenza."
Younger people, especially those with chronic medical conditions, and pregnant women continue to be at a higher risk of infection and viral pneumonia from the H1N1 virus, Fukuda told reporters last week.
The WHO has cautioned that the H1N1 virus could still mutate or mix with the more deadly bird flu virus, which remains endemic in poultry in many Asian countries.
(Reporting by Stephanie Nebehay in Geneva and Maggie Fox in Washington; editing by Paul Simao)