A new study has found that the chickenpox vaccine given to children wears off -- suggesting that a second dose is required to ward off a more severe case of the disease over the long term.

The authors of the study, which can be found in The New England Journal of Medicine, examined 10 years of surveillance data linked to chickenpox cases in 350,000 subjects.

The research found that children between the ages of eight and 12 who were vaccinated at least five years before getting chickenpox were significantly more likely to have moderate or severe symptoms. Those who were vaccinated before the five-year mark were more likely to develop mild symptoms.

The study also found that the number of cases of chickenpox in kids who had been vaccinated increased with each year that passed after they received the vaccination -- again suggesting the vaccine doesn't provide once-and-for-all protection.

"The annual rate of breakthrough varicella significantly increased with the time since vaccination, from 1.6 cases per 1000 person-years within 1 year after vaccination to 9.0 per 1000 person-years at 5 years and 58.2 per 1000 person-years at 9 years," the report states.

But the study's findings don't suggest the vaccine isn't working. Of the 11,356 cases of varicella in the surveillance period, only about 10 per cent were in people who had been vaccinated, suggesting the vaccine provides good -- but not absolute -- protection against the disease.

Dr. Jane Seward, one of the authors of the study and an official at the Centers of Disease Control, Atlanta, told CTV News that researchers decided to embark on the study after noticing outbreaks were happening despite vaccination programs.

Seward said the chickenpox vaccine provides 85 per cent protection, but of the remaining 15 per cent, most will get mild cases of chicken pox -- with a small number getting more severe illness.

However, Seward pointed out that the kids in the study -- which was based on data from Antelope Valley, Calif. -- had received just one vaccination shot. Now, children in the U.S. get two shots -- one at 12 to 15 months, the other at four to six years old.

Currently in Canada, children aged one to 12 years old get the shot once. Teens who are 13 and older, and adults who have never had chickenpox, get two shots, four weeks apart.

It has been demonstrated that immunity from varicella vaccination wanes among health care workers in terms of measurable antibodies, but this is the first study to demonstrate that vaccination-induced immunity breaks down over time in children.

"Waning of immunity is of particular public health interest because it may result in increased susceptibility later in life, when the risk of severe complications may be greater than that in childhood," the study says.

With files from CTV medical producer Elizabeth St. Philip in Toronto