An experimental vaccine to prevent infection with human papillomavirus type 16, the viral strain responsible for about half of all cases of cervical cancer, may not be effective for women who are ovulating, Reuters Health reports. A study in the Journal of the National Cancer Institute shows that although the vaccine produces HPV-fighting antibodies in all 18 women studied, the antibody levels fell during ovulation. Study participants who were taking oral contraceptives to prevent ovulation showed no fluctuation in antibody levels. The researchers said the findings did not necessarily indicate that the protections offered by the vaccine were compromised during ovulation, because the study was not designed to evaluate overall effectiveness of the vaccine, only the antibody levels. However, they recommend that future studies on the vaccine look specifically at whether the compound offers effective protection in women who are ovulating.