Health Canada has approved a vaccine to help prevent painful shingles in people 60 or older who had chickenpox earlier in life.

Zostavax, made by Merck Frosst Canada, should be available through doctors and pharmacies starting sometime next year, the company announced Tuesday.

Shingles occurs when the chickenpox virus reactivates after lying dormant in nerve cells, sometimes for decades, and starts reproducing again. In other cases, the virus may stay dormant indefinitely.

Shingles, also called herpes zoster, causes a painful red rash and sensations of tingling, itching and burning. The rash can lead to scarring and the pain can persist in some people for months or years. Up to 20 per cent of adults who have had chickenpox will get shingles later in life.

Older age, suppressed immune status and lack of re-exposure to the varicella virus seem to increase one's risk of developing shingles, said Dr. Allison McGeer, an infectious diseases specialist at Mount Sinai Hospital in Toronto.

Previously, adults may have gained a natural booster by being exposed to children infected with chickenpox. But as more young children in North America are vaccinated against chickenpox, it is theorized that fewer adults could be gaining that natural boost to their immune system that helps keep the virus dormant.

Since studies from Alberta, Manitoba and British Columbia reported increased shingles rates before chickenpox vaccine campaigns began in those provinces, there is also some evidence suggesting the incidence of shingles was on the rise before the chickenpox vaccine, said Dr. Rafael Harpaz, a herpes virus expert from the U.S. Centers for Disease Control.

Merck has not yet said how much the vaccine will sell for in Canada.

The vaccine was approved for use in the U.S. in 2006.

When the U.S. approval was announced, the company said the long-term effectiveness of the vaccine wasn't clear. Merck's research shows it works for at least four years, and the company plans to follow patients for 10 years to track the vaccine's effectiveness.

It's also not yet clear whether seniors will need more than one injection, or how well the vaccine works in people with weakened immune systems and are at greater risk of developing shingles, McGeer said.