updated 10/15/2003 6:04:17 PM ET 2003-10-15T22:04:17

New research raises the possibility that an experimental vaccine against cervical cancer might lose some of its effectiveness when a woman ovulates.

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Certain strains of a sexually transmitted virus — the human papillomavirus, or HPV — cause most cervical cancer. Scientists are working to create a vaccine that would fend off the riskiest HPV strains. In its first big trial, one candidate, created by Merck & Co., generated excitement last fall when it appeared highly protective for at least a year after women were inoculated.

But the vast majority of women testing the experimental vaccine also took oral contraceptives, which inhibit ovulation, said Dr. Douglas Lowy of the National Cancer Institute.

Because hormonal changes during the menstrual cycle can alter levels of virus-fighting immune cells in cervical tissue, Lowy and Swiss colleague Denise Nardelli-Haefliger, a specialist in this form of immunity, decided to see if that were true for the experimental HPV vaccine, too.

It was a small study — HPV vaccine was given to seven women who used birth-control pills and 11 who didn’t.

Levels of HPV-fighting antibodies fluctuated substantially in the cervical tissue of the non-pill users, reaching their lowest point around ovulation, the researchers report Tuesday in the Journal of the National Cancer Institute.

Women taking birth-control pills had consistent antibody levels through the month.

Does that mean the vaccine is less protective in women who ovulate? Lowy can’t say — the study didn’t measure that.

But he called for advanced studies of the experimental HPV vaccine that now are under way to directly examine whether the drug prevents viral infection just as well in women who ovulate as those who don’t.

Officials with Merck, the company farthest along in HPV vaccine development, did not return phone calls asking if they’ll study the issue.

© 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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