About half the U.S. population should get vaccinated against H1N1 influenza and pregnant women and healthcare workers should be at the front of the line, U.S. health advisers agreed on Wednesday.

Up to 160 million doses of flu vaccine will be available for the start of a vaccination campaign planned for mid-October. The Advisory Committee on Immunization Practices recommended that state and local health officials prepare to vaccinate as many as 160 million people.

Each person will likely need two flu vaccine doses and officials said it was not clear exactly how much vaccine would be available when.

“The main message is that it’s half the population (who are the priority to be vaccinated). And it’s the younger half of the population, as well as healthcare workers,” Kathy Neuzil, the advisory committee’s influenza work group chairwoman, said in an interview.

The group nearly unanimously accepted advice from the U.S. Centers for Disease Control and Prevention. The Health and Human Services Department almost always follow the advice of the committee.

The recommendations said pregnant women, people who care for babies, and healthcare workers should be the first protected against the virus - a total of around 41 million people - in the event that not enough vaccine is available.

Children between the ages of 6 months and four years were also included in that group. The vaccine was not recommended for infants under six months.

People at risk of serious complications from catching the flu should follow, including those with asthma, diabetes and heart disease - and then healthy young adults aged 19 to 24, the panel said.

Members of the panel said young adults should be a priority because they are more likely to become infected and tend to work in places that would accelerate the flu’s spread.

Pregnant women are at special risk from the new strain and vaccinating them protects their newborns, too, the CDC’s Dr. Anthony Fiore told the committee. Surveys show that people over 65 are at lower risk of contracting H1N1.

A CDC report released earlier on Wednesday showed pregnant women were four times as likely as other people to suffer severe complications and even die from H1N1 infection.

Five companies are making H1N1 vaccine for the U.S. market - AstraZeneca’s MedImmune unit, Australia’s CSL Ltd, GlaxoSmithKline Plc, Novartis AG and Sanofi-Aventis SA.

It is not clear how many doses of vaccine will be available right away but the United States would need 600 million doses to immunize everyone.

SUPPLY AND DEMAND

The panel struggled to keep a balance between getting limited supplies of vaccine to people who need it the most urgently and making sure that there was enough demand for it.

In the past, influenza vaccines have been thrown away at the end of the flu season because people lost interest in being vaccinated.

It also sought to provide clear guidance to state and local health officials attempting to explain to the public who is a priority for H1N1 - as opposed to seasonal flu - vaccination.

At present, only 20 to 25 percent of people in high-risk groups are vaccinated against seasonal flu, Fiore said in an indication of the size of the task facing doctors and clinics.

The U.S. government has taken delivery of 20 million doses of a vaccine against the new strain, has ordered 195 million doses and should be ready to start an immunization campaign in October, said Robin Robinson of the U.S. Department of Health and Human Services.

Data from human trials of the new vaccine, which have just begun, will not be available until late September, officials said.

H1N1 swine flu is now so widespread that the World Health Organization has stopped counting individual cases. Health experts are afraid it could worsen, especially when the Northern Hemisphere’s influenza season starts in the autumn.

Fiore also released new CDC data showing that obese people do not have an especially high risk of death or complications from swine flu, as some earlier studies had suggested.

Fiore said the working group was assuming that people will need two doses of vaccine to be fully protected, but the clinical trials are also looking at this issue.

By Matthew Bigg

ATLANTA (Reuters)