Any decision to downgrade the outbreak to post-pandemic will be based on recommendations by influenza experts after a review earlier in the day, they said.
Japanese health ministry official Kensuke Nakajima told Reuters that the WHO is likely to decide to downgrade: We have heard the information that they are trying to say something on the lines of it is (now) a post-pandemic period.
Angus Nicoll, a flu specialist at the European Center for Disease Prevention and Control (ECDC), who is not a member of the WHO's Emergency Committee, told Reuters: The data we are seeing is consistent with such a decision of a post-pandemic period moving on to a new seasonal influenza.
The U.N. agency has been heavily criticized for its handling of the first pandemic in more than 40 years, which turned out to be milder than expected in most countries.
The WHO has also firmly rejected allegations that it acted under the influence of drug companies in declaring a pandemic.
The Emergency Committee of experts gave its assessment to the WHO and its director-general Margaret Chan, after reviewing epidemiological data about current cases, mainly from the southern hemisphere, spokesman Gregory Hartl said.
The Emergency Committee made its recommendation to the director-general who is in the process of finalizing the wording of the recommendation, Hartl told a news briefing.
POST PEAK OR POST PANDEMIC
Chan, who participated in the experts' debate from her native Hong Kong, will make the announcement at a virtual news conference starting at 1300 GMT, a WHO statement said.
The experts' discussions by teleconference -- shrouded in secrecy -- lasted nearly three hours on Tuesday, Hartl said, and advised Chan on whether the virus has moved either to a post-peak or post-pandemic phase.
In June 2009, the WHO said a new swine flu virus, H1N1, that emerged in the United States and Mexico and spread around the world in six weeks, was the first pandemic since 1968. A full pandemic corresponds to phase 6 on the WHO's six-point scale for measuring the spread of a disease.
Experts analyzed the level of infections in the southern hemisphere, where it is winter, and examined whether H1N1 is behaving more like a seasonal flu.
The WHO's assessment of whether the disease is a pandemic or not is important for national health authorities, and would affect government plans to stockpile and distribute vaccines.
Dozens of companies make influenza vaccines, including Sanofi-Aventis, GlaxoSmithKline, Novartis, AstraZeneca and CSL.
More than 18,449 people worldwide are confirmed to have died from H1N1 infections, including many pregnant women and young people. But the WHO says that it will take at least a year after the pandemic ends to determine the true death toll, which is likely to be much higher.
John MacKenzie, an Australian expert, chairs the committee of 15 external international experts. The identities of the others are kept secret to shield them from possible influence from the drugs industry, governments and other interest groups.
Seasonal flu kills up to an estimated 500,000 people a year, 90 percent of them frail elderly people, according to the WHO. The 1957 and 1968 pandemics killed about two million and one million people, respectively, it says.