TORONTO

Health Canada has approved a vaccine that helps prevent painful shingles outbreaks in older people who had chickenpox earlier in life.

The vaccine, which has been available in the United States since about mid-2006, boosts immunity to the varicella zoster virus -- the bug that causes chickenpox -- which lives in latent form in the nervous system of people who have had the disease.

Reactivation of that latent virus occurs later in life in about 15 per cent to 20 per cent of people who suffered through chickenpox, according to data on the Public Health Agency of Canada's website.

The reactivation produces a painful rash that typically lasts a couple of weeks; it's a condition known by the names shingles, herpes zoster or just zoster. In some cases the pain endures for months and even years after the rash subsides, a condition known as post-herpetic neuralgia.

"It (shingles) occurs in all age groups but it increases in incidence slowly and the curve starts to increase more rapidly somewhere around (age) 50,'' explained Dr. Allison McGeer, an infectious diseases specialist at Mount Sinai Hospital in Toronto.

"And then the increase gets steeper and steeper as you get older and older.''

The Public Health Agency doesn't estimate how many cases of shingles occur in Canada yearly, saying only that "given that the risk of having at least one reactivation to herpes zoster is 15 per cent to 20 per cent, there are likely a significant number of zoster cases occurring each year in Canada.''

The U.S. Centers for Disease Control estimates about one million cases of shingles occur in that country every year.

The vaccine, called Zostavax, is licensed for people 60 and older. Its manufacturer, Merck Frosst Canada, declined to give an estimate of what the one-shot vaccine will cost when it becomes available to Canadians later next year.

A herpes virus expert from the CDC explained that after chickenpox infection, the varicella zoster virus remains latent in nerve cells in the ganglia, clusters of nerve tissue located near the spinal cord.

Dr. Rafael Harpaz said that for reasons that aren't completely understood, in some cases later in life the virus will essentially re-awake, start to multiply and travel down the nerve cells to the skin where it will trigger the characteristic shingles rash.