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The chicken pox vaccine loses its effectiveness over time, meaning that a second dose is required to stave off the disease over the long term, new research suggests.

But, for now at least, public health officials in Canada say they will concentrate their efforts on getting all Canadian toddlers a single dose of the vaccine.

"Currently, we have a one-dose recommendation, and that approach has proven very effective in reducing cases of chicken pox," Theresa Tam, director of the immunization and respiratory infections division of the Public Health Agency of Canada, said in an interview.

She said that, in the future, Canadian public health officials may consider adding a second dose, but only after the chicken pox vaccine has been widely used for a number of years and the benefits of a booster become more clear.

But David Scheifele, director of the vaccine evaluation centre at B.C. Children's Hospital in Vancouver, said waiting would be a mistake, and that Canada should move swiftly to introduce booster shots to limit the ravages of the disease.

"What has been described in this paper is a fundamental limitation of the vaccine," he said in an interview. "A single dose worked very well, but not entirely satisfactorily. One can do better with a second dose," Dr. Scheifele said.

Currently in Canada, it is recommended that all children receive the vaccine for chicken pox (also known as varicella) between 12 months and 18 months of age. Those shots are provided free of charge in all provinces and territories. Children over 12 and adults who want the vaccine require two shots.

In the United States, it is recommended that children be vaccinated at 12 months to 18 months, and that they receive a booster shot at four to six years of age, just before starting school.

The study, published in today's edition of the New England Journal of Medicine, shows the vaccine is very effective, but that effectiveness wanes within five years.

To do the study, researchers from the Los Angeles Department of Health Services and the U.S. Centers for Disease Control and Prevention in Atlanta tracked the incidence of the disease in 350,000 children in California over the past decade.

A total of 11,356 of those children contracted chicken pox in that period, of which 1,080 cases were among children who had been vaccinated.

Children 8 to 12 who had been vaccinated at least five years previously were significantly more likely to contract chicken pox than those whose vaccinations were more recent.

The risk of so-called breakthrough varicella increased to 58.2 cases out every 1,000 children who received the shots five years after vaccination, from a mere 1.6 cases in a 1,000 at one year.

While it was obvious from the outset that there was breakthrough disease, the new research shows that cases are far more prevalent and severe over time, a key factor in the U.S. decision to give boosters.

Chicken pox, which is caused by a virus called varicella zoster, was, until recently, one of the most common childhood illnesses. Before the vaccine came along, more than 350,000 children a year contracted chicken pox in Canada.

While most children suffered mild symptoms, about 10 per cent of cases required medical treatment or hospitalization for complications, including encephalitis and flesh-eating disease, and about a dozen children died annually.

Its symptoms include a moderate fever and characteristic spots. Tiny lesions, or blisters, develop on top of the area of redness and become itchy, open sores. The sores form crusts that usually fall off in a week, sometimes leaving crater-like scars.

In the United States, upward of 80 per cent of children are now vaccinated against chicken pox and the stated intent of public health officials is to eradicate the disease.

In Canada, chicken pox is still prevalent in the general population, and it is unclear what percentage of children have been immunized, because there are no national surveillance figures.

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Vaccine versus the virus

History of the vaccine: The chicken pox vaccine, sold under the brand name Varivax, began being used widely in the United States in 1995 and in Canada in 2000. But it was not until 2006 that all Canadian provinces and territories provided the vaccine free of charge.

How Canada and the United States compare: The Canadian National Advisory Committee on Immunization recommends that all children be vaccinated against chicken pox, caused by the varicella zoster virus, between 12 and 18 months of age. Canada uses a stand-alone vaccine, meaning it isn't combined with other childhood vaccines in a single shot. In the United States, it is recommended that all children be vaccinated against chicken pox between 12 to 18 months. It is given in combination with the vaccines for measles, mumps and rubella. The United States also recommends a booster shot at ages four to six years, before children begin school. In both countries, however, it is recommended that children over 12 and adults receive two doses. As with many vaccines, adolescents and adults usually require a higher dose than children.

Key findings of study: The new study shows that the ability of the vaccine to protect children against chicken pox wanes, meaning they become susceptible to contracting chicken pox again about four to five years after the initial vaccine.

Chicken pox is not kids' stuff: The chicken pox vaccine was slow to be accepted because of a mistaken belief that chicken pox is benign and harmless. While most children have mild symptoms, about 10 per cent of cases require hospitalization, and about a dozen children died of chicken pox each year in Canada before the vaccine. For adults, chicken pox can be life-threatening. Children who contract chicken pox are also far more likely to suffer from shingles, an extremely painful condition, later in life.

TEXT: ANDRÉ PICARD