As Canada moves slowly toward immunizing all children against chicken pox, research is casting some doubt on the effectiveness of a one-shot vaccine.

The study, based on an outbreak of chicken pox (also called varicella) at a daycare centre in the United States, found that the vaccine protected 44 per cent of children from contracting the common childhood disease. But it was 86-per-cent effective in preventing severe complications.

"In this outbreak, vaccination provided poor protection against varicella, although there was good protection against moderate or severe disease," said Karin Galil, a epidemiologist at the U.S. Centers for Disease Control and Prevention, and the principal author.

She said the protection fell short of the findings of clinical trials, where Varivax, as it is called, was found 71 to 100 per cent effective in preventing chicken pox, and 95 to 100 per cent effective in preventing severe cases. It was believed that a single shot would provide life-long protection.

The research, published in The New England Journal of Medicine, found that the longer the period between vaccination and exposure to the virus, the greater the child's risk of contracting chicken pox.

Galil said this suggests the "current vaccination strategy may not protect children adequately," meaning a booster shot may be necessary. This would drive up costs and, some fear, imperil plans to immunize all children.

Christine Homsy, a spokeswoman for Merck Frosst Canada & Co., the manufacturer, said the company wanted to review the data before commenting.

But Dele Davies, an infectious-disease consultant at Alberta Children's Hospital in Calgary, said it is important to keep the findings in perspective. "The good news here is that, even though the vaccine wasn't working as well as expected, it was still 86 per cent effective in preventing severe complications."

Davies also stressed that the outbreak studied, in which 25 of 88 children attending a New Hampshire daycare contracted chicken pox, is noteworthy only because widespread use of the vaccine has been so successful in reducing incidence of the disease in the United States. "These findings shouldn't deter us at all from using the varicella vaccine."

Chicken pox is a highly infectious disease that is caused by a virus called varicella zoster. More than 350,000 Canadian children contract the disease each year.

In the United States, 68 per cent of children are now vaccinated against chicken pox. Canada's coverage rate is believed to be a fraction of what it is in the U.S.

To date, chicken pox is part of the routine childhood immunization schedule only in Prince Edward Island, Alberta, Nunavut and the Northwest Territories. Nova Scotia will begin in January. In other provinces, parents can purchase the vaccine for about $75.

The Canadian Paediatric Society had decried this inequality of access across the country and said all childhood vaccines should be available free of charge.

While most children suffer mild symptoms, chicken pox is not benign. In about 10 per cent of cases, there can be complications that require medical treatment or admission to a hospital; these include encephalitis, pneumonia and flesh-eating disease. About a dozen children die of chicken pox in Canada each year. Contracting chicken pox in childhood also makes it more likely that, later in life, a person will have shingles, a painful and debilitating condition.