U.S. health officials again backed on Thursday the safety of Merck & Co Inc's vaccine to prevent infection by a virus that causes most cases of cervical cancer.

The Gardasil vaccine continues to be safe and effective, and its benefits continue to outweigh its risks, the Food and Drug Administration and the Centers for Disease Control and Prevention said in a statement.

Gardasil protects against infection with four strains of the human papillomavirus (HPV), a common sexually transmitted virus that causes genital warts and most cases of cervical cancer.

Merck distributed more than 23 million doses of Gardasil in the United States since its approval in June 2006 through the end of 2008, the FDA and CDC said. It is one of Merck's top-selling products but sales have been hurt by competition from a rival vaccine from GlaxoSmithKline PLC. Second-quarter Gardasil sales were $268 million.

On Tuesday, CDC and FDA researchers wrote in the Journal of the American Medical Association that problems such as fainting and nausea remained rare among females who received Gardasil and the vaccine did not appear to be causing unusual side effects.

The researchers said 32 deaths were listed in a government database that collects reports of health problems seen in people after vaccination.

The reports show only that a patient became ill or died after receiving a vaccine, not that a vaccine was the cause.

The FDA and CDC statement said concerns have been raised about the reports of deaths of people who received Gardasil.

There was not a common pattern to the deaths that would suggest they were caused by the vaccine, the agencies said.

Also, there is no evidence that Gardasil has increased the rate of Guillain-Barre Syndrome, a rare neurological disorder that causes muscle weakness, the agencies said.

Cervical cancer kills 3,870 women a year in the United States and 280,000 globally.

Merck shares fell less than 1 percent to $31.20 in afternoon trading on the New York Stock Exchange.

The FDA and CDC statement was posted on the FDA website here

Medicine Safety