"We may end up averting a crisis. That's our hope," said President Obama, who took time away from the G-8 summit in Italy to telephone another summit back home - the 500 state and local health officials meeting to prepare for swine flu's fall threat.

No final decision has been made on whether to vaccinate Americans, Health and Human Services Secretary Kathleen Sebelius stressed. That depends largely on studies with experimental batches that are set to start the first week of August - to see if they are safe and effective, and to learn whether they require one or two doses.

But if all goes well, the federal government will buy vaccine from manufacturers and share it for free among the states, which must then "try and get this in the arms of the targeted population as soon as possible," said Sebelius, who announced $350 million in grants to help states and hospitals prepare.

First in line for shots probably will be school-age children, young adults with risky conditions such as asthma, pregnant women, and health workers, she said. Unlike seasonal flu, the swine flu seems more dangerous to these groups than to older people.

"Schools are natural places" to offer those vaccines, Education Secretary Arne Duncan said.

Go home and get schools, mayors, and other community leaders to spread that message, Sebelius said.

"The last thing we want is millions of parents to be surprised," she said, the day the get-your-kid-vaccinated-at-school note comes home.

Swine flu is almost certain to return in the fall given its continued spread here - at least 50 outbreaks in children's summer camps so far - and abroad.