New flu "unstoppable", WHO says, calls for vaccine

*Healthcare workers go to front of line for vaccine

*Companies complain of difficulty in making vaccine

*Obesity now a clear risk factor for complications

*Studies confirm this new virus is different

By Maggie Fox, Health and Science Editor

WASHINGTON, July 13 (Reuters) - Saying the new H1N1 virus is "unstoppable", the World Health Organization gave drug makers a full go-ahead to manufacture vaccines against the pandemic influenza strain on Monday and said healthcare workers should be the first to get one.

Every country will need to vaccinate citizens against the swine flu virus and must choose who else would get priority after nurses, doctors and technicians, said Dr. Marie-Paule Kieny, WHO director of the Initiative for Vaccine Research. [LD54719]

Several reports showed the new virus attacks people differently than seasonal flu -- affecting younger people, the severely obese and seemingly healthy adults, and causing disease deep in the lungs.

Kieny briefed reporters on the findings of the WHO's Strategic Advisory Group of Experts on Immunization, or SAGE. "The committee recognized that the H1N1 pandemic ... is unstoppable and therefore that all countries need access to vaccine," Kieney said.

"The SAGE recognized first that healthcare workers should be immunized in all countries in order to retain a functional health system as the virus evolves," she added.

After that, each country should decide who is next in line, based on the virus's unusual behavior.

Seasonal influenza is deadly enough -- each year it is involved in 250,000 to 500,000 deaths globally. But most are the elderly or those with some kind of chronic disease that makes them more vulnerable to flu, such as asthma.

ELDERLY ADVANTAGE

The elderly seem to have some extra immunity to this new H1N1, which is a mixture of two swine viruses, one of which also contains genetic material from birds and humans. It is a very distant cousin of the H1N1 virus that caused the 1918 pandemic that killed 50 million to 100 million people.

A study published in the journal Nature on Monday confirmed that the blood of people born before 1920 carries antibodies to the 1918 strain, suggesting their immune systems remember a childhood infection. [N13166187]

The work by Dr. Yoshihiro Kawaoka also supports other studies that this new H1N1 strain does not stay in the nose and throat, as do most seasonal viruses.