WASHINGTON (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.

Scientists work on developing the H1N1 (Influenza A) vaccine inside a Bio-safety Level 3 (BSL-3) lab at Sillapakorn University on the outskirts of Bangkok, July 12, 2009. REUTERS/Chaiwat Subprasom

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.

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.

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.

“The H1N1 virus replicates significantly better in the lungs,” Kawaoka said. Other studies have also shown it can cause gastrointestinal effects, and that it targets people not usually thought of as being at high risk.

“Obesity has been observed to be one of the risk factors for more severe reaction to H1N1” -- something never before seen, Kieny added. It is not clear if obese people may have undiagnosed health problems that make them susceptible, or if obesity in and of itself is a risk.

Slideshow ( 7 images )

On Friday, a team at the U.S. Centers for Disease Control and Prevention and the University of Michigan reported that nine out of 10 patients treated in an intensive care unit there were obese. They also had unusual symptoms such as blood clots in the lungs and multiple organ failure.

None have recovered and three died.

The CDC estimates at least a million people are infected in the United States alone and clinics everywhere are advised not to test each and every patient, so keeping an accurate count of cases will be impossible. The United States has documented 211 deaths and WHO counted 429 early last week.

Kieny said WHO would also work to get better viruses for companies from which to make vaccines. She said the strains that had been distributed did not grow very well in chicken eggs -- used to make all flu vaccines.

One exception -- AstraZeneca’s MedImmune unit makes a live virus vaccine that is squirted up the nose and it is easier to produce, Kieny said.

WHO said countries should continue with their normal vaccination programs against seasonal flu. Kieny said the seasonal H3N2 strain was also very active now in the southern hemisphere’s winter.

Sanofi-Aventis, Novartis, Baxter, Schering-Plough’s Nobilon, GlaxoSmithKline, Solvay, CSL and AstraZeneca’s MedImmune are among those working on flu vaccines.