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Supplies of the swine flu vaccine are now plentiful, and federal officials are encouraging people of all ages to get the shots to prevent a potential third wave of illness and deaths. Those now eligible include healthy adults age 25 to 64 and anyone 65 or older—groups initially given lowest priority to receive the vaccine by the U.S. Centers for Disease Control and Prevention (CDC).

The new policy comes amid steadily increasing supplies and moderate demand—in polling, only about half of Americans say they want the H1N1 shot—with most localities now lifting restrictions on who can get the vaccine.

The first wave of the new, pandemic strain of H1N1 flu peaked early last summer. Now the much-anticipated second wave, which peaked in October, also has subsided. But the traditional cold-weather flu season still has a few months to go, and no one knows for sure what those months will bring. The big question is, will there be a third wave of swine flu? And if so, will it be serious enough to cause a new spate of hospitalizations and deaths?

Experts are divided on both questions. But federal officials, citing 20th-century pandemics involving multiple waves, say another surge in H1N1 cases before spring is a distinct possibility. Health and Human Services (HHS) Secretary Kathleen Sebelius said in a recent press conference that during the 1957 Asian flu pandemic, for example, a fall outbreak died down but was followed by another wave in January and February.

“I think complacency is probably our top enemy right now,” Anne Schuchat, M.D., head of the CDC’s National Center for Immunization and Respiratory Diseases, told reporters Thursday.

With an effective H1N1 vaccine now widely available, officials are urging Americans to take advantage of the lull—and potentially blunt the impact of a third wave—by getting vaccinated. They hope to drive home that message with the launch of National Influenza Vaccination Week, January 10-16.

“There’s plenty of vaccine,” said Schuchat. Most of the roughly 60 million doses already administered have gone to children, she said. “It’s the time now for adults and seniors to be vaccinated.”

The CDC’s priority system based on risk has so far been right on target. Of the nearly 10,000 U.S. deaths from H1N1 between April and November, some 8,600 were among young adults and children, a far higher percentage than would be expected in a typical flu season. A recent study looking at the first 99 hospitalizations for H1N1 in New York City found that three-quarters had at least one underlying chronic condition like asthma or heart disease—another criterion for priority access to the shot.

Still, said Allison McGeer, M.D., a microbiologist and a top influenza expert at Mount Sinai Hospital in Toronto, none of these numbers should lead older people to dismiss the pandemic influenza. “It’s not as if it’s trivial,” she said. “There’s a couple [who contracted the illness] here, ages 70 and 71, and she died and he’s been in the hospital for four months.”

Where to go for shots

As in many areas across the country, at some Los Angeles County clinics the wait for swine flu shots stretched for hours this fall. The focus then had to be on those most at risk. “I tried to make sure that I was catching people as they were coming onto the lines, to avoid them having to wait and then find out they weren’t high-priority,” said Robert Kim-Farley, M.D., director of communicable disease control and prevention for the county’s public health department. But in mid-December, the county announced the shot could now be offered to all comers, and authorized its release to retail pharmacies and more doctors’ offices. In some states like California the decision to open vaccination to the general public is being made by local public health departments; in many cases state health authorities are lifting the restrictions statewide. (Your state health department will have more information. To find a swine flu shot provider near you, go to the HHS website

Seasonal flu shots hard to find

Health officials likewise encourage just about everyone—especially older adults—to get a seasonal flu shot. But those who haven’t already done so are likely to find it difficult to find a dose now. To make way for H1N1 vaccinations, clinics and other providers pushed out the seasonal flu vaccine early this year, and heightened attention to flu in general seems to have caused a spike in demand. The “vast majority” of seasonal flu vaccine doses have already been used, according to the CDC’s Schuchat.

As of early January, though, there’s been very little seasonal flu activity in North America. It’s certainly not too late for an ordinary flu strain to begin making the rounds, but in many years flu activity has already peaked by February, said McGeer. “The later we go, the more hopeful I get that it’s going to be quiet.”

“My betting,” added McGeer, “is that we are going to see flu activity in February and March, and that it’s mostly going to be the pandemic strain.”

It’s possible the new H1N1 flu could actually displace other strains for a period of time, or it could circulate along with them, according to Derek Cummings, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health in Baltimore.

So far, the pandemic flu, though highly infectious and easily spread, has proved no more deadly than a typical seasonal flu, with an enormous number of Americans—roughly one in six—thought to have been infected, but only about 200,000 getting sick enough to land in the hospital. Indeed, as more people gain immunity to the pandemic flu through vaccination and infection, the distinction between this new strain and ordinary seasonal flu strains may fade away.

Katharine Greider lives in New York and writes about health and medicine.