Should you still get shot for H1N1? Well, that depends

H1N1 swine-flu cases are beginning to wane in many, though not all, parts of the U.S. The flu is now widespread in half the states, down from nearly every state a few weeks ago.

At the same time, the H1N1 vaccine finally is plentiful enough that Illinois health officials have dropped rules restricting who has priority for the shot.

Many people may wonder if it's worth getting vaccinated now.

And the answer is that it depends.

It depends on whether a third wave of the novel flu storms in after the holidays, which many experts fear may happen - as it has in previous pandemics.

All the ingredients for another round seem to be present - people mingling and traveling and getting run down; colder, stormier weather driving more people indoors; low winter humidity levels kicking in, which studies show make it easier for flu virus to survive and spread.

The federal Centers for Disease Control and Prevention recommends vaccination, especially for those at high risk of complications - pregnant women, caregivers of children under 6 months, people who are 6 months to 24 years, health care and emergency workers, people with chronic illnesses.

At the same time, some experts think most Americans already have been exposed to the flu and picked up immunity to the virus without experiencing any symptoms. But no widespread testing for antibodies has been done to confirm this suspicion.

It depends also on whether the HIN1 strain rearranges or mingles with another virus in some way that produces more severe illness.

There's no evidence that this has happened, but even if it does, "it's likely that the vaccine that we currently have would be effective," said Dr. Thomas Frieden, director of the CDC.

It depends too on whether the flu virus continues to behave as it has in healthy adults. One recent study analyzed similarities between 2009 H1N1 and similar seasonal-flu strains that have been in circulation since 1988. Researchers found enough viral common ground to suggest that most adults have at least some immune-system memory that helps protect against the new flu.

The CDC estimates that some 5,000 people have died from the flu, and a new model published by researchers at Harvard and other institutions this week anticipates the final toll will likely be 10,000 to 15,000 - about a third fewer deaths than occur during a "typical" flu season.

Most of the deaths have occurred in people under age 65 - a reverse of regular flu seasons - and largely among people with underlying medical conditions, although a minority had been in apparent good health.

One recent report based on autopsy reports of 34 H1N1 victims from New York and Milwaukee early in the outbreak found that in those fatal cases, the virus caused damage to both the upper respiratory system and deep into the lungs, similar to the damage recorded in victims of the flu in 1918 and 1957. More than half of the most recent victims also had bacterial infections in the lungs. And 91 percent of them had underlying medical problems like heart disease or respiratory problems.

Frieden points out that while there could be additional waves of swine flu, it's also likely that some other seasonal-flu strains will pick up now through February. The "official" flu season doesn't actually start until January. Millions of people who were able to get the regular flu shot this fall, but didn't qualify until now for a swine-flu shot now have a chance to be more fully protected.

But for those who don't get the H1N1 vaccine this time around, odds are also pretty good that swine flu will also be covered in next year's flu shot.