Flu vaccine makers anxiously await the decision, scheduled to be announced at 5 a.m. EST/1000 GMT, because they need this guidance to start formulating vaccines for the Northern Hemisphere's fall vaccine mix.

The decision is more complicated this year because of the pandemic of H1N1 swine flu. Although the pandemic is on the wane, the virus is still infecting and killing people.

Other strains of flu are still circulating but at far lower levels than usually seen in February.

Here are some possible outcomes of the WHO meeting:

SWINE FLU REPLACES H1N1 IN VACCINE

The pandemic strain of H1N1 could simply replace the older H1N1 component of the vaccine. Influenza vaccines are trivalent, meaning they contain three different strains. They are an H1N1 strain, and H3N2 strain and an influenza B strain.

Every year these strains drift slightly and the vaccine is reformulated when one drifts enough to justify changing it. Because flu viruses constantly mutate, monitoring these changes is an ongoing process. The WHO advisers could potentially vote to have the pandemic H1N1 strain substitute for the current H1N1 seasonal strain.

This would be good news for vaccine makers and governments that have made and stockpiled doses of H1N1 vaccine that have not been used and could become part of the seasonal flu vaccine mix.

ADD H1N1 TO THE SEASONAL VACCINE

Advisers may be reluctant to completely replace the H1N1 component with the pandemic strain. Although there is not yet much seasonal H1N1 circulating, during the 2008-2009 season it became resistant to the antiviral drug oseltamivir, Roche AG's Tamiflu.

Flu experts have been debating whether it is possible to have a quadrivalent seasonal flu vaccine -- one with four components instead of three. There could be two H1N1 strains in such a vaccine -- seasonal and pandemic.

KEEP TWO SEPARATE VACCINES

The advisers may decide to keep the trivalent formula for seasonal flu vaccine and also continue to recommend the pandemic H1N1 swine flu vaccine as a separate immunization.

The H1N1 swine flu vaccine will not be dropped. The current pandemic virus, the H1N1 virus, is by far, the most common virus being isolated for influenza viruses around the world, WHO's Dr. Keiji Fukuda told reporters last week.

It shows no signs of disappearing and ... that there is a very good bet that we will expect to see this virus around for quite a while.

U.S. health officials said H1N1 swine flu has killed as many as 17,000 Americans, including 1,800 children. Globally, 15,000 deaths have been officially reported but WHO says it will take years to get the true, much higher, toll.

Medicine