Readers have sent me many follow-up questions after my recent shingles vaccine column. It's great to see your interest, and I wanted to answer the most common follow-up questions I received.

To recap, shingles is caused by the reactivation of the virus that also causes chickenpox, varicella-zoster. Most people are infected with this virus in childhood and remain infected for the rest of their lives.

In most of us, the virus lives "asleep" inside some of our nerve cells, causing no symptoms. However, it can "awaken," or reactivate, and start making copies of itself. When it does this, it can cause shingles -- a severe, painful rash on your body or face.

The shingles vaccine contains a greatly weakened version of the virus. The vaccine causes the immune system to attack the virus when it reactivates, to quiet it down and prevent it from causing symptoms.

-- Does the shingles vaccine help if you've already had shingles?

A past episode of shingles boosts your immunity to the virus. So people who have had shingles are less likely to have a second case. But some people do get recurrent attacks.

No study has been done that shows the shingles vaccine prevents second episodes. It's a reasonable assumption that it would, but the research hasn't been done. Most experts recommend the vaccine for people over 50, whether they've had shingles or not.

-- Who should not get the vaccine?

The shingles vaccine is not recommended for people with weakened immune systems. That includes people who are having cancer treatments such as radiation and chemotherapy, or people with diseases that can weaken the immune system (such as leukemia, lymphoma or HIV). Finally, the same goes for anyone being treated with drugs that affect the immune system, like steroids. And people who have had a life-threatening allergic reaction to gelatin should not get the vaccine.

-- Should people with rheumatoid arthritis avoid the shingles vaccine?

Many drugs used to treat rheumatoid arthritis suppress the immune system. They include steroids, drugs called disease-modifying antirheumatic drugs (DMARDs) and medications called anti-TNF drugs. Anyone with rheumatoid arthritis who is taking these drugs should not be vaccinated for shingles.

-- Should pregnant women get the vaccine?

Definitely not. A pregnant woman's immune system does not fight viruses as effectively. This is rarely an issue, since the vaccine is recommended for people over 50 years of age.

-- Can a person who's received the vaccine infect others with weakened immune systems?

According to the CDC, it's safe for a person who has been vaccinated for shingles to be around people with weakened immune systems, as well as infants, children and pregnant women. There are no documented cases of a person getting chickenpox from someone who had been vaccinated.

-- How often should I get the shingles vaccine?

According to current research, the shingles vaccine is effective for at least six years. It's also possible that it protects for much longer than that. Researchers are working to determine exactly how long the vaccine provides protection. Maybe we'll get lucky and need to have it only once.

(Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: www.AskDoctorK.com.)

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