A vaccine that blocks infection by the four virus types that cause most cervical cancers and genital warts appears safe and effective, but may actually enhance disease in some patients, according to Food and Drug Administration documents released Wednesday.

Merck & Co. seeks FDA approval for its Gardasil vaccine against four types of human papilloma virus, or HPV. Two of those four types are believed responsible for about 70 percent of cervical cancer cases. The cancer kills roughly 3,500 U.S. women a year; the other two types cause 90 percent of genital wart cases.

An FDA panel of outside experts is to meet Thursday and discuss whether to recommend that the agency approve the vaccine. The FDA isn't required to follow the advice of its expert panels but usually does.

According to briefing documents released ahead of the meeting, the panel should discuss various items of concern only if its members first agree that studies show the vaccine is safe and effective. That suggests the FDA favors its approval.

Merck said the vaccine has the potential to reduce the annual number of new cervical cancers worldwide to roughly 150,000, from the current 500,000, and cut global deaths from the cancer by more than two-thirds, to an estimated 90,000.

An FDA review of the results of studies on the vaccine found two important concerns, according to the documents released ahead of Thursday's meeting of the Vaccines and Related Biological Products advisory committee.

The first is that the vaccine may lead to an increased number of cases of a cancer precursor among patients already infected by any of the four virus types at the time they receive the vaccine, and whose immune systems have not cleared the virus from their bodies.

The second concern is that any advantage the vaccine provides in protecting against the four virus types could be offset by infection by any of the multiple other types of HPV that the vaccine does not cover, according to the FDA documents.

FDA staff also asked that the committee examine five cases where children with birth defects were born to women who had received the vaccine around the time of conception.

Merck developed the vaccine and tested it in both women and men, but Thursday's discussion is expected to focus on its use in preventing HPV-related disease in women.

An FDA decision is expected by June 8. Should it approve it, the national Advisory Committee on Immunization Practices will decide later that month whether to endorse routine vaccination with the vaccine. The committee's HPV vaccine workgroup is recommending the vaccine be given to girls 11 and 12, and the committee will consider recommendations for females 13 to 26.