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‘It is an excellent vaccine, but it is also a very expensive vaccine,’ said lead researcher Dr. Simon Dobson.HARRY CABLUCK/The Associated Press

Medical experts generally agree that Gardasil is an effective vaccine against the human papillomavirus, or HPV, which is the principal cause of cervical cancer and genital warts.

But it doesn't come cheap. It is usually administered in three shots, separated by a period of several months. The manufacturer, Merck Frosst Canada Inc., charges $135 per dose – although provincial governments get a discount when they buy in bulk.

Now, however, a new study suggests that two doses may be enough to produce an immune response. If future studies confirm this finding, it could lead to cost savings for the health-care system and individuals paying out of their own pockets for the vaccine.

"It is an excellent vaccine, but it is also a very expensive vaccine," said the lead researcher, Dr. Simon Dobson of the University of British Columbia.

For the study, researchers analyzed the immune responses – or antibody levels – in 830 girls and young women who received either two or three shots of the vaccine, which guards against four common strains of HPV – 6, 11, 16 and 18. (In addition to cervical cancer, HPV is also linked to oral, penile, anal and vaginal cancers.)

"Girls given two doses versus three doses had a non-inferior antibody response for all four vaccine genotypes," the researchers write in their study published Tuesday in the Journal of the American Medical Association. sBy "non-inferior" the researchers mean that two shots are "no worse than" three shots. They carefully avoided saying that two shots are as good as or equal to three because they lack the evidence to make such a claim.

One important factor to consider is age. The younger the first shot is administered, the stronger the immune response, Dobson said. Those getting two shots were between 9 and 13 years of age. Those receiving three shots were older.

Dobson noted that most of the original research on Gardasil was carried out on females between the ages of 16 to 26. These studies indicated that three shots were needed to guard against the four HPV strains contained in the vaccine. When clinical trials were started in preadolescent females, "they noticed that 9-to-13-year-olds actually produced a much better immune response than those 16 to 26," he said.

This promising work in younger girls prompted Dobson and other researchers to consider the possibility that just two doses could do the trick – provided the vaccine recipient was under 13.

"You could get the same protection across your population for a lower price if you went to two doses instead of three," Dobson said.

Or, for the same money, the vaccine program could be expanded to protect more people against HPV, which is spread through sexual contact with an infected person.

Prince Edward Island, for instance, recently launched a program to inoculate young males. "By offering the vaccine to males, we can hopefully decrease the spread of HPV infection," Dr. Lamont Sweet, deputy chief public health officer for PEI, said in a press release.

Each province runs its own vaccine program, although the federal Government provided start-up funding. Last year, for instance, British Columbia spent $3-million on the vaccine. Based on preliminary data produced by Dobson, B.C. moved towards a two-shot program in September 2010, two years after starting the program with 3 shots in grade 6 girls. "We hedged our bets, and basically extended the dose schedule," said Dr. Monika Naus with the BC Centre for Disease Control. The province is giving girls in grade six two doses within six months. The plan was to give a dose in grade 11 if it is deemed necessary.

"We are hedging our bets, and we're basically extending the dose schedule," said Dr. Monika Naus of the BC Centre for Disease Control. The province is giving girls in Grade 6 two doses within six months. They will get a third dose in Grade 11 if it is deemed necessary. Originally, all three shots were given within six months.

"Young girls have sky-high responses – they have a far more robust antibody response to the vaccine," Naus said.

Measuring antibody levels, however, is an indirect method of determining the effectiveness of the vaccine. It will take time to tell what level of antibodies are necessary to block infection.

Even so, shifting the vaccine to a younger age group might help remove some of the stigma associated with a shot that provides protection against a sexually transmitted disease. Some groups, such as the Catholic school boards in Alberta, were opposed to school-based vaccination programs, arguing that immunizing teen girls may entice them to become sexually active because they wouldn't have to worry about becoming infected with HPV.

Dobson hopes that giving the vaccine to children might "divorce it from the whole sexual-acquisition aspect of HPV," essentially turning it into just another childhood vaccine.

He noted that hepatitis B is also a sexually transmitted virus and "the hepatitis B vaccine is well accepted in the population because it is given in early childhood."