The vaccine has become sufficiently plentiful, prompting at least 24 states and some other communities to lift restrictions and open distribution to everyone. And some pharmacies are starting to get the vaccine for general distribution. Earlier, supplies had been targeted at those most at risk, including children and pregnant women.
Health officials' biggest fear now is that, with the perception that the pandemic is waning, many people will decide they don't need the vaccine.
Sebelius, who got her shot Wednesday, urged everyone to get vaccinated.
"We have a wonderful window of opportunity to prevent or lessen a third wave," she said at a news conference. In the 1957 Asian flu pandemic, she noted, the fall outbreak tailed off only to be followed by another wave of disease after the first of the year, perhaps triggered in part by holiday travel.
The first wave of swine flu began in April, when the strain was discovered. A larger wave started in late summer. Through mid-November, about one in six Americans caught H1N1, and about 10,000 have died, according to estimates from the Centers for Disease Control and Prevention. The strain has been especially virulent in children and young adults.
Although the pandemic is ebbing in the United States - at least for now - there is still a great deal of flu elsewhere in the world, Dr. Keiji Fukuda of the World Health Organization said. Flu continues at high levels in parts of Europe, such as France, Switzerland and the Czech Republic, and in parts of central Asia, such as Kazakhstan and Kyrgyzstan, as well as in Russia.
"One of the common questions coming to us is, 'Is the pandemic over? Is it time to call it?' And, really, the answer is that it's too early to make such a call," he said.
Fukuda, special advisor on pandemic influenza to the World Health Organization's director-general, said the agency plans to begin distributing 180 million pledged doses of swine flu vaccine to developing countries, hopefully within days. The first shipments will go to Azerbaijan, Afghanistan and Mongolia because they are in the Northern Hemisphere, where the outbreak is currently most severe.
The doses have been pledged by the five major manufacturers and 12 countries, although it is not clear how many have followed through. The United States, for example, has pledged 10% of its supply to the WHO but has not yet shipped any.
The WHO's goal is to ship 200 million doses to 95 countries that would not otherwise have access to the vaccine. The shots will go primarily to healthcare providers and first responders.
But more vaccine may become available soon because many countries placed orders on the assumption that two doses would be required to immunize most people. Now that it is clear that one dose is sufficient for everyone except children younger than 10, there is likely to be a surplus.
The U.S., for example, ordered the ingredients for 251 million doses, and it seems unlikely that all those will be used.
Germany and Spain have already been negotiating with vaccine manufacturers to reduce their deliveries and have tried to sell some of their vaccine on the open market.