SCENARIOS-Children go to front of flu vaccine line

By Maggie Fox, Health and Science Editor

WASHINGTON, July 10 (Reuters) - The World Health Organization plans to release guidance next week on the need for a vaccine against the new H1N1 swine flu virus.

Companies are already working to make one.

Few experts doubt there will be an H1N1 vaccination campaign but questions remain about how much companies should be prepared to make, whether to try to stretch the supply using ingredients called adjuvants and who should get the vaccine.

Here are some potential scenarios:

FULL SPEED AHEAD

WHO could advise companies to switch to full-speed production of an H1N1 swine flu vaccine for the northern hemisphere's autumn, with the goal of getting a maximum 4.9 billion doses ready. More doses would be needed if clinical trials show that people will need two shots a few weeks apart to get the best protection.

Even more doses would be needed if children aged 2 to 9 turn out to need three doses. Currently, unvaccinated children under 9 need two influenza vaccinations the first year to get full protection from seasonal flu.

It is possible WHO will recommend that all age groups be immunized against the new H1N1 virus. Usually, countries issue guidelines for specific age groups, depending on who is worst hit and balanced against vaccine availability.

The elderly and the very young are usually at the front of the line for influenza vaccines because they are most likely to die or develop serious complications.

Between 250,000 and 500,000 people die every year from seasonal flu, but this new virus targets older children, young adults and people with conditions such as asthma, pregnancy and heart disease.

LIMITED PRODUCTION

WHO may advise companies to proceed with limited and cautious production of a vaccine, keeping in reserve the possibility of a stepped-up seasonal flu vaccination campaign.

Vaccine makers such as Sanofi-Aventis (SASY.PA), Novartis (NOVN.VX), Baxter (BAX.N), Schering-Plough SGP.N subsidiary Nobilon, GlaxoSmithKline (GSK.L) and Solvay (SOLB.BR) and AstraZeneca's (AZN.L) MedImmune subsidiary have finished making seasonal flu vaccines for this year.

The seasonal formula is a cocktail of three different strains -- the seasonal H1N1 strain, a very distant cousin of the pandemic strain, an H3N2 virus and an influenza B virus.

H1N1 is spreading in places where seasonal flu is circulating and scientists say it has the potential to mix in a flu patient's body and emerge as a new, unpredictable strain.