The human papilloma virus (HPV) vaccine is widely viewed as an effective way to protect women from both cervical cancer and genital warts.

But doctors suspect the high cost and inconvenience associated with the vaccine, which consist of three shots given over the course of six months, may be preventing women and young girls from receiving it.

“It is one of the most expensive routinely used childhood vaccines,” Dr. Simon R.M. Dobson, of the of the University of British Columbia, Vancouver, told

This dilemma spurred Dobson and his colleagues to examine whether the vaccine could still be effective if doctors skipped the second shot, administering only the first and third doses. 

“We knew the vaccine was great, but also that it was going to be expensive, and in order to use it effectively, we wanted to come up with research questions,” Dobson said. “High up on the list was the question about two doses.”

The HPV vaccine is considered a protein vaccine – much like the vaccines for hepatitis A and hepatitis B.

“Interestingly, both of those vaccines (for hepatitis A and B) in teens and above are being used now in two-dose schedules,” Dobson added. “They started off as three-dose and became two-dose schedules because similar studies to this one were done.”

These types of vaccines are given in multiple shots in order to teach the immune system how to build up its own response to the virus. The first dose signals to the immune system that a foreign protein has entered the body –  and the immune system begins to create antibodies to fight it. The second dose of the vaccine is meant to reinforce this immune system response.

By the third dose, the memory of the protein has been established in the immune system, so the body produces even more antibodies when it is exposed to the protein again.

“The question we’re asking is, does that second dose given right after, does it really make that much difference? Or does the first dose tell the immune memory what to do enough?” Dobson said.

‘This vaccine is too good’

To address this question, Dobson and his colleagues studied a group of 830 Canadian females between August 2007 and February 2011. Researchers first compared a group of girls, ages nine to 13, who received two doses of the HPV vaccine to a group of women, ages 16 to 26, who received three doses of the HPV vaccine.

“Sure enough, for all of the four genotypes in the vaccine, the four types of HPV virus being protected against, the two doses in girls was no worse than the three doses in young women, out to 36 months,” Dobson said.

Researchers then compared girls between the ages of nine and 13, who received two doses of the vaccine, with a group of girls in the same age group, who received three doses. In this comparison, the antibody responses for HPV-18, which is linked to cervical cancer, and HPV-6, which is linked to genital warts, were not as strong among the girls who received only two doses of the vaccine.

Researchers said this doesn’t necessarily mean the girls aren’t protected against those strands of HPV – because researchers don’t know exactly how many antibodies are needed to protect against the virus.

“It’s a strange thing to say, but this vaccine is too good; so in efficacy trials in 16 to 26 year olds, there weren’t enough failures (and) there wasn’t a magic antibody number below which you weren’t protected,” Dobson said.

Dobson said while the results of this study were promising, more research still needs to be done before universally lowering the number of shots given for the HPV vaccine.

The study will appear in the May 1 issue of JAMA.