Global health officials stepped up efforts to prepare for quick vaccination against the H1N1 pandemic virus, saying it appeared now to be affecting older age groups spared earlier in the pandemic.

The World Health Organization and the US Centers for Disease Control and Prevention both said on Friday they can only estimate how many people have been infected but the swine flu virus was still spreading quickly.

'As the disease expands broadly into communities, the average age of the cases is appearing to increase slightly,' the WHO said in a statement.

'This may reflect the situation in many countries where the earliest cases often occurred as school outbreaks but later cases were occurring in the community.'

The virus has been notable for affecting older children and young adults, groups normally not hard-hit by influenza.

The CDC said summer camps and military facilities were affected, but both agencies said there was no evidence the virus was mutating into drug-resistant or more virulent forms.

The CDC broadened its recommendations for seasonal flu vaccine - saying all children over the age of 6 months should get one, in part to lower the overall burden of respiratory disease when autumn and winter come.

WHO said vaccination against H1N1 might start in weeks, even though clinical trials to test the safety, efficacy and needed dosage of H1N1 vaccines have barely started.

'Manufacturers are expected to have vaccines for use around September. A number of companies are working on the pandemic vaccine production and have different timelines,' WHO said.

At least 50 governments have placed orders or are currently negotiating with pharmaceutical companies to secure supplies of H1N1 vaccines, which are still being developed.

WHO is trying to ensure that health workers in poor countries can be vaccinated so hospitals can stay open if the flu becomes more debilitating as it spreads. Sanofi-Aventis and GlaxoSmithKline have promised to donate 150 million doses to this aim to date.

Other leading flu vaccine makers include Novartis, Baxter, AstraZeneca's MedImmune arm and Solvay.

'We are continuing to see transmission here in the United States in places like summer camps, some military academies and similar settings where people from different parts of the country come together,' the CDC's Dr. Anne Schuchat told reporters in a telephone briefing.

'This is very unusual to have this much transmission of influenza during the (summer) and I think it's a testament to how susceptible people are to this virus,' she added.-Reuters