Drug makers are gearing up to make vaccines against the new H1N1 virus, starting test batches and pledging free doses for poor countries.

Governments are pledging billions for the vaccine - the United States has set aside $1 billion, the Netherlands ordered 34 million doses and Australia has ordered 10 million doses as the World Health Organization declared a pandemic.

Yet no one is sure yet whether even to give the vaccine to anyone and if so, who should get it.

Usually the guidelines for influenza vaccination are clear - give it to the oldest, the youngest and the sickest. In an average year, influenza is a factor in 250,000 to 500,000 deaths, and in developed countries, 90 percent are the elderly.

But the new H1N1 swine flu virus is not behaving typically. It has dragged the Northern Hemisphere’s flu season well into June, although influenza usually dies way down by April.

“The fact that we are seeing ongoing transmission now indicates that we are seeing something different,” Dr. Daniel Jernigan of the U.S. Centers for Disease Control and Prevention told reporters this week.

Most cases are in people aged 5 to 24 - usually the last group to get seriously ill from flu and the last group vaccinated during a normal flu season.

There have been few serious H1N1 cases among the elderly, perhaps because they have some pre-existing immunity to a similar H1N1 that circulated in the past, or maybe because their immune system response is different - flu experts do not know yet.

To decide what to do, WHO officials will have to watch and see what this new strain does.

REAL-TIME EXPERIMENT

As WHO Director-General Dr. Margaret Chan said at the beginning of the pandemic last month, the world is watching it unfold in real time - and much of humanity is taking part in a real-time experiment.

If the virus continues to affect mostly older children and young adults, the strategy should be clear - vaccinate those groups. And at least one study supports this idea for other reasons.

This week a team at Britain’s University of Warwick said computer models suggest that vaccinating children aged 6 months to 18 years may be the best way of using limited supplies to control the pandemic.

Because schools can be epicenters for spreading flu infection, including the new H1N1, this might make more sense than trying more general approaches. Vaccinating schoolchildren would prevent them from spreading it to two more vulnerable groups - the elderly and the very young.

It may also protect those clearly more at risk from both H1N1 and seasonal flu - people with asthma, diabetes and heart disease, suppressed immune systems and pregnant women.

By Friday, WHO had confirmed 44,287 cases of the new flu and 180 deaths, with Laos, Oman, Papua New Guinea, South Africa all reporting their first cases.

The United States alone has more than 21,000 cases with officials saying there are likely at least 10 times that many, and 87 deaths.

WHO estimates vaccine makers could produce up to 4.9 billion pandemic flu shots a year at most. That would depend on how many doses people needed and whether vaccine boosters called adjuvants could be used to stretch out the doses.

Even this best-case scenario would leave more than a billion people unvaccinated.

Vaccine maker Sanofi-Aventis, which says it makes 40 percent of the world’s supply of flu vaccine, says at full capacity its two Pennsylvania factories can make 150 million doses of seasonal influenza vaccine a year. Its French plant can make 120 million doses per year.

This would triple for H1N1, as the seasonal vaccine is a cocktail of three different viruses.

Other companies that make flu vaccine include Novartis AG, Baxter International Inc, GlaxoSmithKline and Solvay.

By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters)