By Karin Zeitvogel

BETHESDA, Maryland - US President Barack Obama joined top officials Thursday in urging Americans to ramp up preparations for a return of swine fall in the fall, amid pledges of a huge vaccination campaign.

'I think it's clear that although we were fortunate not to see a more serious situation in the spring when we first got news of this outbreak, the potential for a significant outbreak in the fall is looming,' Obama said. He told Americans to be prepared for a more severe outbreak of the A(H1N1) virus than that which swept the United States in the spring in remarks piped into a meeting from the G8 summit in L'Aquila, Italy.

'We want to make sure that we are not promoting panic, but we are promoting vigilance and preparation,' he said.

Health Secretary Kathleen Sebelius warned that (A)H1N1 flu 'is not gone, it's continuing to spread, it's in every state in this country' and she said Americans should not let down their guard.

'The administration has been actively preparing for H1N1 scenarios that may develop over the coming months,' she told health officials meeting at the National Institutes of Health in Maryland.

And she urged Americans to also prepare on an individual level at the Bethesda summit, which also included Education Secretary Arne Duncan and Homeland Security chief Janet Napolitano.

The United States is one of the countries the worst hit by the swine flu, with 170 dead, and in Mexico, where the outbreak began in April, 121 people have died from the illness.

US officials have predicted that a million people may already have been infected with the disease, which the World Health Organization (WHO) said last month had become a pandemic.

Sebelius announced that 'preparedness grants' worth 350 million dollars will be made available to state and local public health authorities to allow them to step up their preparations.

All the officials, including Obama, said that a key tool in the US arsenal against swine flu was the development of an effective vaccine, followed by a targeted vaccination program.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said clinical trials on a first candidate vaccine were expected to begin next month.

Scientists would pay attention in the tests to see how quickly the vaccine triggers an immune response, he said, saying the knowledge would be crucial in the event of the disease returning in the fall flu season in a more virulent form and able to spread rapidly.

Usually, the immune response from flu vaccines kicks in in around three weeks, he said, but the trials would test the (A)H1N1 vaccine candidates to see if they trigger an effective immune response within one week or a fortnight.

Sebelius told the meeting that 'the current estimate is that some vaccine will be ready for distribution in mid-October.' Fauci said there would probably be tens of millions of doses by then.

That would not be enough for a blanket vaccination campaign, so populations who have been shown to be at the greatest risk from the new strain of swine flu - children, pregnant women and health workers who are dealing with the virus on a daily basis - would likely be vaccinated in a first round of immunizations, the officials said.

A handful of pharmaceutical companies around the world are working to develop a vaccine against (A)H1N1 influenza, which WHO says has infected 100,000 people in 137 countries and territories, and caused 440 deaths around the world.