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Vaccinations began this week for many Grade 8 girls across the country against HPV, the virus that is linked to cervical cancer. But there remain lingering questions about the vaccine's safety.

The vaccine, called Gardasil, is designed to protect against four strains of HPV linked to genital warts and cervical cancer. At least four provinces are rolling out mass innoculations.

Health authorities in the U.S. and Canada say the vaccine is safe, though there have been some adverse reactions. About 95 per cent of the reactions reported have been minor: pain in the arm, nausea, fever and dizziness.

"The safety data we are looking at in the U.S. so far, I think it's reassuring that it is acceptably safe," says Dr. John Iskander of the Immunization Safety Office with the U.S. Centers for Disease Control.

He says altogether there have 100 cases of reactions.

"Through the end of June, approximately 7 million doses have been distributed in the U.S.," he says. "So the number of events, say over 100, is quite small compared to 7 million doses distributed."

But Shannon Nelson, an 18-year-old athlete from outside of Chicago, wonders about the vaccine. She got the HPV shot at the same time as two other vaccines. Within a week, she developed Guillian-Barre Syndrome (GBS), a mysterious autoimmune disorder that causes muscle weakness and paralysis. In Nelson's case, she developed paralysis that lasted for over two months.

"I couldn't sit up in bed, I could not lift up my arms," she recalls. "I appear recovered but I'm not. I can't feel my hands and feet that good."

Nelson's parents reported what happened to their daughter to U.S. health authorities. So did more than a dozen other people who suspected their cases of GBS were linked to the HPV vaccine after they received it either on its own or with other vaccines.

The CDC says they cannot prove a link.

"In most of the cases we reviewed, it appeared as though there were other factors more likely to be the cause of the Guillain-Barr�," says Iskander. "It's a pretty rare event. But you know, in a population of 7 million people, some cases are going to happen."

Iskander says a vaccine used in the U.S. against meningococcal disease, called Menactra, has been linked to GBS, in rare occasions.

"In fact, about half of the reports involving Gardasil also involved this other vaccine. So it may be there is a bit of sort of innocent bystander effect, if you will, which is part of why those reports are showing up," he says.

There also have been seven deaths reported in the U.S after an HPV injection. But four have been attributed to other causes. In three cases, officials say there's insufficient data to conclude the HPV vaccine was the cause.

So far, Canadian officials haven't seen any serious problems. But they and U.S authorities continue to monitor side effects reported to public health authorities.

"The vaccine does appear to be safe," says Dr. Theresa Tam, the Public Health Agency of Canada's associate director of immunization. "And the rare reports of serious events are quite in line with what we would expect to regularly see in the population that hasn't received the vaccine."

"The risk versus the benefit is such that you're looking at a very rare side effect versus the benefit of getting immunized, and the benefit still outweighs the risk.

According to Health Canada, HPV is one of the most common sexually transmitted diseases in Canada. Here are some facts:

• It's estimated up to 75 per cent of sexually active Canadians will have at least one HPV infection in their lifetime. While there is no cure, most people with healthy immune systems will eventually rid their bodies of the disease.
• There are various types of HPV. The type that affects the genital area can be spread by intimate skin-to-skin contact with an infected person.
• One of the symptoms is genital warts. In men, they may appear on the penis, scrotum, thigh, anus, rectum or in the urethra; in women, they are usually found on the cervix, but also on the vulva, thigh, anus, rectum or in the vagina or urethra.
• Many people infected with HPV have no obvious signs of infection.
• Experts do not believe HPV affects a woman's ability to become pregnant, but it's uncertain what effect the disease can have on the baby.