UPDATE 1-Companies start initial work on pandemic vaccine

* Genetic code of virus available, virus samples awaited

* Production process will take months

* WHO will only call for new vaccine if pandemic imminent

* Making pandemic vaccine means sacrificing seasonal supply

(Adds comment from WHO spokesman, analysts)

By Ben Hirschler

LONDON, April 28 (Reuters) - Manufacturers have taken the first preparatory steps in developing a vaccine against a new strain of flu that has killed up to 149 people in Mexico and threatens to become a pandemic.

But the World Health Organisation (WHO) said it would only call for large-scale production of such a pandemic vaccine if it strongly believed the world was on the edge of an unstoppable global outbreak of flu.

Switzerland's Novartis AG NOVN.VX said on Tuesday it had received the genetic code of the new virus strain, enabling it to start work on evaluating production, and it hoped to receive the actual virus in its laboratories "in the near future".

Still, it is likely to take between five and six months before a vaccine is commercially available, according to the WHO.

The delay is not for want of trying.

The U.S. Centers for Disease Control and Prevention has already taken a sample of the H1N1 virus causing the disease, produced a vaccine virus strain and is growing it up, marking the first stage of the production process.

These samples, which can then be distributed to companies, must be grown in specially produced chicken eggs in a cumbersome system that experts agree is outdated. New and more efficient technologies based on cell cultures are, however, still a few years away.

While the race is on to prepare a pandemic vaccine, health authorities and companies still have to make the tricky decision of when, or indeed whether, to switch capacity from producing seasonal vaccines to making the new shot.

“If you go to pandemic flu production, people won’t get normal flu shots -- it is a huge consideration,” said WHO spokesman Gregory Hartl.

At least 20 companies, in addition to Novartis, make flu vaccines, including Sanofi-Aventis SASY.PA, GlaxoSmithKline GSK.L, Baxter International BAX.N, Australia's CSL CSL.AX and nasal spray maker MedImmune, now part of AstraZeneca AZN.L.

They are currently geared to producing season flu shots, which vaccine specialists say will not stop the new form of swine flu spreading around the world, although people who have been immunised may end up with milder symptoms.

So-called swine flu is a variant of the H1N1 form of the human influenza virus, but tests show the H1N1 component of the current seasonal flu vaccine does not protect against the new strain.

A big challenge facing manufacturers will be making sufficient quantities of vaccine from a limited supply of active ingredient, or antigen.

One option to extend supply is to use an additive, known as an adjuvant, to increase the body’s immune response and reduce the amount of antigen needed in each shot.

Several companies are working on this approach. Glaxo, in particular, said it believed its novel adjuvant technology could make a significant contribution to antigen sparing.

The British company previously developed a vaccine against the H5N1 strain of bird flu -- originally thought to pose the biggest pandemic threat -- using a special adjuvant that allowed an extremely low dose of active ingredient to be used.

Credit Suisse analysts said companies with the know-how to make pandemic flu vaccines stood to benefit from additional sales, adding there could also be an increase in seasonal flu vaccine demand as disease awareness increases.

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