Adverse events like fainting or nausea remain rare among young women and girls given Merck and Co's Gardasil vaccine against cervical cancer, U.S. researchers reported on Tuesday.

The vaccine, which protects against the human papillomavirus, or HPV, that causes genital warts and cervical, head and neck, and penile cancer, does not appear to be causing any unusual side-effects, Dr. Barbara Slade of the U.S. Centers for Disease Control and Prevention and colleagues reported.

About 23 million doses of the vaccine have been given in the United States. Fewer than 1 percent of patients - or about 54 people out of every 100,000 - experienced adverse events like fainting or feelings of nausea or dizziness, Slade's team reported in the Journal of the American Medical Association.

They found that 6.2 percent of the adverse events reported were serious, including 32 deaths.

Such reports do not show the vaccine caused sickness or death, only that a patient became ill or died after receiving the vaccine.

The U.S. Food and Drug Administration licensed Gardasil in 2006. GlaxoSmithKline makes a rival vaccine called Cervarix, which is more commonly used in Europe and is approved for use in developing countries.

Federal officials recommend the three-dose vaccine for girls aged 9 to 12, but other groups suggest young women as old as 26 should get it. It is controversial because Pap smears can catch cervical cancer before it even develops.

The FDA and CDC are conducting careful studies of adverse events for the vaccine as it is used in more young women and girls. The FDA has scheduled an advisory panel review of both Gardasil and Cervarix for Sept. 9, including a discussion of whether it might be given to boys and men.

"Whether a risk is worth taking depends not only on the absolute risk, but on the relationship between the potential risk and the potential benefit," Dr. Charlotte Haug of the Journal of the Norwegian Medical Association wrote in a commentary.

"If the potential benefits are substantial, most individuals would be willing to accept the risks. But the net benefit of the HPV vaccine to a woman is uncertain."

"Even if persistently infected with HPV, a woman most likely will not develop cancer if she is regularly screened. So rationally she should be willing to accept only a small risk of harmful effects from the vaccine."

Merck welcomed the findings.

"We welcome continued study and discussion about the safety of this important vaccine," said Dr. Richard Haupt, head of the company's clinical program for Gardasil.

The company said that since its launch in 2006 more than 50 million doses of Gardasil have been distributed worldwide.

HPV is the most common sexually transmitted disease in the world. About 20 million Americans are infected, according to the CDC. It is the main cause of cervical cancer, which kills 3,870 women a year in the United States and 280,000 globally.