About half of eligible girls were vaccinated against HPV in Ontario last year. (CBC)

Severe side effects of the HPV vaccine Gardasil appear to be rare, according to a study that may help calm the fears of some parents as schools prepare for another round of inoculations against one of the most common sexually transmitted infections.

The Gardasil vaccine protects against four types of human papillomavirus, which causes cervical cancer. All provinces have school-based vaccination programs, but some parents are concerned about the safety and necessity of the inoculations.

Test trials for vaccines generally are not large enough to show any rare or serious side effects that are often found later, when larger studies are done in the general population.

The study is in Monday's online issue of the Canadian Medical Association Journal. Julia Brotherton, a public health physician with Australia's National Centre for Immunization Research and Surveillance in Sydney, and her colleagues followed 260,000 school-aged girls who received the vaccine as part of a campaign in New South Wales in 2007.

They found seven cases of anaphylaxis, a severe allergic reaction with symptoms such as difficulty breathing, nausea and rashes.

That works out to about 2.6 cases of anaphylaxis per 100,000 vaccinations. In comparison, the rate was 0.12 for a similar school program using the C-conjugate meningitis vaccine in the same Australian territory, according to the study.

The elevated rate of anaphylaxis linked to the HPV vaccine was not seen in other parts of Australia. Given the sample size of Brotherton's study, the error range for the rate of anaphylaxis varied from 1.04 to 5.35 per 100,000.

"We do wonder if it's something to do with vaccinating very large numbers in that particular age group," Brotherton said. "And that's why we really are putting this out there to say, 'We've observed this. We need it to be verified in other places'."

The few patients who did react badly recovered completely. There were no cases of anaphylactic shock , the most severe reaction.

The findings should reassure parents, said Dr. Noni McDonald of Halifax, who wrote an editorial accompanying the study.

"I really hope that all of those parents last year who … said, 'Oh, I don't know if I should do this, not enough information, not enough safety data about the vaccine,' that they will look at this study, they will the hear message from this study," McDonald said.

"Looking at serious side effects, [they are] very, very, very, uncommon."

Prepared for rare reactions

Doctors and nurses who give the vaccine are prepared for any serious reactions, said Dr. Shelly McNeil of the Canadian Centre for Vaccinology in Halifax.

"All of the reactions happened within 15 minutes, and we always watch kids for 15 minutes specifically because we want to make sure they don't have an allergic reaction," McNeil said.

"That reassures again that what we're already doing would have picked up these things if they happened in Canada."

As of June 30, more than half a million doses of the vaccine had been distributed in Canada.

From the time the vaccine was approved in July 2006 until Aug. 26, 2008, there had been no confirmed cases of anaphylaxis after HPV vaccination, Philippe Brideau, a spokesperson for the Public Health Agency of Canada, said in an e-mail.

In that same time, 220 adverse events following Gardasil immunization have been reported to the agency. The side effects reported were similar to what was seen in clinical trials, with reactions at the injection site being the most common, Brideau added.

Cost effective?

Critics said while the study's findings quell short-term safety concerns, the study did not address how effective Gardasil is in the long term and why the vaccine has such a large marketing push behind it, including "Charm 4 Life" designer bangles, the proceeds of which go to a cancer prevention campaign.

About 1,300 women will be diagnosed with cervical cancer in Canada this year, and 380 will die from the disease.

Abby Lippman, an epidemiologist at McGill University in Montreal, said that's certainly too many deaths — but it still isn't known if the vaccine will save lives and whether or not it's worth the cost.

Madeline Boscoe, a sexual-health nurse with the Canadian Women's Health Network in Winnipeg, said she also has unanswered questions about the vaccine.

The majority of cervical cancers are slow growing and are well treated, Boscoe said. There is still room for improving access to Pap tests to reduce cervical cancer rates, rather than diverting funds to pay for the HPV vaccine, she added.

Last year, about 50 per cent of eligible girls were vaccinated in Ontario, though uptake was 80 per cent in Nova Scotia, CMAJ said in an editorial.

The Australian study was funded by the New South Wales Department of Health. Two of the study's authors reported receiving grant funding from a pharmaceutical company.

With files from the Canadian Press