Q: Do I really need the H1N1 swine flu vaccine? I read that this flu is actually weaker than the usual seasonal flu. And there are no new cases in my community.

A: It's still too soon to call the 2009 H1N1 influenza A strain a weaker virus than the usual influenza virus. Yes, the total number of deaths caused by this flu is slightly below what many public health officials had expected. However, the scary fact is that the deaths are occurring in younger people. About 80 percent of the deaths have been in people ages 18 to 64.

There are a couple of reasons why younger people have a higher risk of complications from this new H1N1 virus.

First, no similar virus similar to this one has circled the globe in more than 60 years. So those under the age of 65 have virtually no natural immunity against this strain.

Second, younger people have stronger immune systems. This might not seem to make sense. If they have stronger immune systems, shouldn't they be able to fight off the virus more easily? The problem is that sometimes the immune system overreacts. In rare cases, the immune system goes wild. It can cause widespread inflammation and organ damage throughout the body in an attempt to fight off the virus.

I also worry that an estimated 85 percent of people have not been infected. Even if 20 percent of Americans have had the vaccine against H1N1, that still leaves almost two-thirds at risk of this infection.

My other worry is that this H1N1 virus could evolve to become even more contagious and to cause more severe disease. Typically, flu viruses thrive in cold, dry air. This virus appeared in the warmer months. It stayed active through a relatively warm November in North America. We are just now getting into the very cold weather.

If you haven't done so already, get vaccinated against H1N1

The seasonal flu vaccine does not provide protection against this flu.

It is great to hear that the number of H1N1 infections seems to be slowing down. But this does not mean that the pandemic is over. We can put those odds in our favor if everyone eligible for the vaccine gets it.

Howard LeWine, M.D. is a practicing internist at Brigham and Women's Hospital in Boston. He serves as Chief Medical Editor of Internet Publishing at Harvard Health Publications, Harvard Medical School.

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(c) 2009 U.S. News & World Report