GENEVA - Global health officials stepped up efforts to prepare for quick vaccination against the H1N1 pandemic virus, saying on Friday it appeared now to be affecting older age groups spared earlier in the pandemic.
The World Health Organization and the U.S. Centers for Disease Control and Prevention both said 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.
The pandemic spread globally in less than two months and has infected people in 160 countries.
WHO has said the pandemic is unstoppable but some countries are still trying to limit its spread, in part by detaining travelers. Britain's Foreign Office said at least 160 Britons are being held in quarantine in China, Singapore, India and Egypt.
Schuchat said there is no indication the virus is any worse in one country than another.
There are differences in reporting. In some places, we're hearing about only the severe cases. In other places, we're hearing about illness that's in the community, she said.
She declined to call the pandemic mild and noted that people had died and many others had spent weeks in hospitals, sometimes on ventilators.
She said the CDC was also watching for more cases of seizures. The agency reported on Thursday on four children who suffered seizures from H1N1 infection but recovered.
Schuchat advised against summer camps offering the antiviral drug oseltamivir -- Roche AG and Gilead Sciences Inc's Tamiflu -- to prevent infection among children.
The drugs should be reserved for people at high risk of complications who have been in close contact with a known case, she said.
(With additional reporting by Michael Kahn and Avril Ormsby in London; Writing by Maggie Fox, editing by Alan Elsner)