By Maggie Fox, Health and Science Editor

WASHINGTON, Oct 22 (Reuters) - Deployed military troops, emergency workers, pregnant women and children will be among the first to get scarce vaccinations if a pandemic strain of flu breaks out, U.S. officials said.

A long-awaited report to be issued on Tuesday lays out who would be first in line to get vaccinated against H5N1 bird flu or any other strain of pandemic influenza.

The Health and Human Services report proposes creating four categories of people, and vaccinating the top tier of each category first. The categories are homeland and national security, critical infrastructure, health and community support services and the general population.

"Certain military personnel like deployed forces would get vaccinated before certain other military personnel," HHS science adviser William Raub said in a telephone interview.

Virtually all health experts agree that the world is overdue for a pandemic of some sort of influenza.

No one can predict when one might come, how bad it would be or which strain of influenza virus may be responsible. But the H5N1 bird flu virus that has infected 331 people since 2003, killing 203 of them, is the current chief candidate.

Companies are working to make a vaccine against H5N1 but the process takes months and it is not clear if vaccines formulated to match the current strain would protect well against whatever mutated version emerges to cause a pandemic.

"We won't be able to start making a true vaccine against the actual pandemic virus until that virus appears, until we have samples," Raub said.

"No matter how rapidly we build up our production capacity we won't have 300 million doses of vaccine overnight. The material will come off the line in successive lots.

"So therefore it means we need to have in hand a well-thought-out, transparent framework as to how we hope states and communities would make the decision about who gets vaccinated first."

REVISIONS PENDING

The plan, to be released at public meetings and on the Internet at http://pandemicflu.gov, is not final and will be revised after comments from experts and members of the general public, Raub said.

Much depends on how severe the pandemic actually is. The 1918 pandemic killed hundreds of millions of people globally, but two others, in 1957 and 1968, were much milder.

Raub said it also is not clear if people who are vaccinated with the so-called pre-pandemic vaccines being prepared now should go to the back of the line.

Level "A" people to get vaccinated first include "deployed and mission-critical personnel" under the homeland and national security category. Level A in the critical infrastructure category include emergency medical services workers, law enforcement and fire personnel, flu vaccine and drug makers and key government leaders.

Under health, those first in line include public health and direct health care providers, while under the general public category, pregnant women and infants and toddlers come first.

"In past pandemics, groups at increased risk for serious illness and death have differed by age and health status," the report reads.

"Because the high-risk groups in the next pandemic are not known, this guidance will be reassessed and may be modified at the time of the pandemic."

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