By Michael Kahn

LONDON (Reuters) - Current flu vaccines will not stop a deadly virus spreading around the world but people who have had their annual shots may end up with milder symptoms, vaccine specialists said on Monday.

The flu virus has killed as many as 149 people in Mexico and has spread into the United States, Canada and Europe, prompting fears the new strain could mark the start of a pandemic.

So-called swine flu is a variant of the H1N1 form of the human influenza virus. Mutations of this strain have been circulating in the human population for years and the current seasonal flu vaccine is designed to protect against H1N1.

But tests show the H1N1 component of the current seasonal flu vaccine does not protect against the new strain although the jabs may still offer some benefit, experts say.

"There may well be some immunity to H1N1 at the population level that will provide a degree of protection," said Dr. John McConnell, editor of the journal Lancet Infectious Diseases.

While the virus has so far has killed no one outside Mexico, it spreads quickly between humans which makes health officials fear it could cause the flu pandemic that scientists say is long overdue.

Experts agree the process for making vaccines is clumsy and outdated, but new and more efficient technologies are still a few years away.

At least 20 companies make flu vaccines including Sanofi Pasteur, Australia's CSL Ltd, GlaxoSmithKline Plc, Novartis AG, Baxter and nasal spray maker MedImmune, acquired by AstraZeneca Plc.

"Clearly if this virus evolves into a pandemic, the first wave will come and go before a vaccine can be produced," Karl Nicholson, a vaccine expert at Leicester University in Britain said in a telephone interview.

"There has been this realization we can't make a vaccine in time so we should be priming populations."

Another problem is many people do not get yearly flu vaccines because they live in poor countries or do not see it as a priority.

U.S. health officials recommend that 261 million Americans -- 85 percent of the population -- should be vaccinated against flu but a RAND Corp. study in December showed only about a third of those who should have did get the vaccine.

Other experts noted that because swine flu is only partly related to human strains, vaccines already in use will not be effective in producing antibodies that provide protection.

But there may be some benefit when it comes to white blood cells, which can shorten the period of infection, Ian Jones, a researcher at the University of Reading said.

"The most likely result is that a current vaccination would not stop you from being infected but might reduce the severity of the symptoms," he said.

(Reporting by Michael Kahn; Editing by Maggie Fox and Robert Woodward)

• More News on H1N1 Flu (Swine Flu)
• More News on Immunization

Related MedlinePlus Pages:

• H1N1 Flu (Swine Flu)
• Immunization