Two years ago, Merck & Co. got approval for its Gardasil, the first cervical cancer vaccine to block four types of human papilloma virus, a sexually transmitted virus, which can cause cervical cancer and genital herpes. It was the first time a company succeeded to come up with such a vaccine.

The vaccine was approved for use in girls and women aged 9 to 26 in order to prevent future infections.

Its approval was based on studies involving about 11,000 people who only experienced mild to moderate reactions, such as pain or tenderness at the injection site.

One year later, the Centers for Disease Control and Prevention added Gardasil to its childhood immunization schedule, which was also good news.

Since its approval, more than 26 million doses of Gardasil have been distributed globally, including nearly 16 million in the U.S., Merck reported.

However, good news appears to head to an ending, as Gardasil’s success is presently shadowed by some 7,802 reports of adverse events in people who received it. The Vaccine Adverse Events Reporting System (VAERS), a system of monitoring vaccines’ side effects belonging to the CDC and the Food and Drug Administration, registered the reports between June 8, 2006 and April 30, 2008.

These reports include 15 deaths and 31 cases of Guillain-Barre syndrome, a life-threatening condition in which the body’s immune system attacks part of the nervous system, eventually leading to paralysis.

Merck defended the vaccine in a statement reading that a report of an adverse reaction “does not mean that a causal relationship between and event and vaccination has been established – just that the event occurred after vaccination,” CNN reported.

The reports are currently studied by the company, the CDC and the FDA to establish if there was a direct link between the signaled adverse effects and the vaccine.

Cervical cancer is the second most common cause of cancer death in women worldwide, resulting in nearly a half-million diagnoses and 280,000 deaths each year. In 2007, there were an estimated 11,000 new cases of cervical cancer and 3,600 deaths in the US.

Currently Gardasil is the only cervical cancer vaccine on the market. It has racked up about $1 billion in sales since its June 2006 U.S. launch.

After the reports were made public, Merck’s shares dropped 4.8 percent.