SCENARIOS-Children go to front of flu vaccine line

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.

Controlling seasonal flu may be at least as important as controlling H1N1.

LIMITED PRODUCTION

This may be recommended if it appears the virus has started to change, making a vaccine less protective. Infectious disease experts say there is little sign of this so far.

WHO may also recommend limited production while clinical trials -- tests in people -- get underway. U.S. health officials said this week those trials will get started in August and will give more information about whether the vaccine is safe and what kind of immunity it produces.

Using an adjuvant to boost immune response to a vaccine is possible as well. Such a vaccine would not be licensed but U.S. regulatory authorities may allow it under emergency use provisions. Adjuvants can be mixed with vaccine after it is made so their availability would not affect production.

WHAT IS KNOWN

Most people have limited immunity to the new H1N1 swine flu virus. It is a mixture of two pig viruses, one of which is itself a mixture of pig, human and bird viruses.

There is some suggestion that people over the age of 50 may have some immunity to the virus, perhaps because it is similar to H1N1 strains that circulated before the 1957 pandemic that killed 2 million people globally. In that pandemic, H1N1 strains were replaced within months by a new strain called H2N2, and H1N1 did not reappear again until the 1970s.

Health officials already agree that vaccination campaigns should focus on school-age children, as they are not only among those most at risk of infection but schools serve as breeding grounds where viruses of all types spread.

In addition, people with underlying conditions, including pregnant women, are among groups that would be at the head of the line, as well as healthcare workers who must stay healthy to care for the sick and administer vaccines.