One of the study's authors told Reuters Health she was aware the number of people completing the vaccine series was low to begin with, but she did not expect to see it getting even lower.

"We thought that that would be increasing over time as more people became aware of the vaccine and how it was to be administered," said Dr. Abbey Berenson, a professor at the University of Texas Medical Branch at Galveston.

The HPV vaccine, which goes by the brand names Gardasil and Cervarix, protects girls and women from the sexually-transmitted virus that's linked to cervical cancer.

Girls as young as age nine can start the vaccine series, and catch-up vaccination is recommended for women up to age 26 who have not yet gotten their shots. The vaccine is given in three doses over a six-month period.

One earlier study found that 48 percent of teenage girls had received at least one dose in the vaccine series (see Reuters Health report of October 18, 2011).

Berenson said she wanted to know how many of those who start the shot series end up completing it.

She and her colleagues looked at the health insurance records of more than 271,000 girls and women, age nine and up, who had gotten a first Gardasil vaccine.

They found just 38 out of every 100 of them received the second and third shots in the next year.

Berenson's group also found that since the vaccine became available in 2006, the number of people completing all three doses declined.

In pre-teens, for instance, 57 percent of girls in 2006 completed the vaccine series, compared to 21 percent in 2009.

The numbers were similar for teenagers.

Among women 19 to 26 years old who got their first Gardasil shot, the number of those completing the series dropped from 44 percent in 2006 to 23 percent in 2009.

Berenson pointed out that her study, published in the journal Cancer, included all shots given within a 12-month window, even though the recommendation is to complete the series in six months.

"We have a window beyond the current recommendations, which makes the data even more surprising," she said.

As to why so few girls and women are completing the series of shots, "the honest answer is, we don't know," said Chun Chao, a researcher at Southern California Permanente Medical Group in Pasadena, who was not involved in this study.

Chao said even among patients in her health organization, who don't have to pay an extra cost to get the HPV vaccine, the rate of people who get all three shots is only about 40 percent.

"It's always more difficult to get a patient to come to the doctor for multiple appointments," said Berenson. "It requires quite an initiative from the patient's perspective."

"The scheduling might not be that easy to remember, and physicians might not do a good job explaining" the need for three shots, Chao told Reuters Health.

It's unclear whether getting one or two shots offers women any immunity from HPV.

There is some evidence that two shots of Cervarix can be effective, but that doesn't necessarily hold true for the women in this study who received Gardasil.

According to the Centers for Disease Control and Prevention, at least half of sexually-active people will get HPV at some point in their lives -- but most strains aren't cancer-causing and will be cleared on their own.

Still, of the 12,000 women in the U.S. who get cervical cancer each year, almost all cases are linked to HPV.

Berenson said if people have not completed the three vaccine doses, and are concerned that it's too late, "just go ahead and complete it now. It's a much better insurance against contracting HPV than forgetting about the last two doses."

SOURCE: bit.ly/KxkxF3 Cancer, online April 27, 2012.

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