The first large study of an Experimental cervical cancer vaccine found it was 100 percent effective, in the short term, at blocking the most common forms of the disease, drugmaker Merck & Co. said Thursday. Its shares rose more than 2 percent in morning trading.

Gardasil, a genetically engineered vaccine, prevents cervical cancer by blocking infection with the two viruses that together cause about 70 percent of cervical cancers. HPV 16 and 18, both sexually transmitted viruses, are two of the 100-plus types of human papilloma virus.

Other types of HPV also can cause cervical cancer and painful genital warts. About 20 million Americans have some form of HPV.

The final-stage study of Gardasil included 10,559 sexually active women ages 16 to 26 in the United States and 12 other countries who were not infected with HPV 16 or 18. Half got three vaccine doses over six months; half got dummy shots.

Among those still virus-free after the six months, none who received the vaccine developed either cervical cancer or precancerous lesions likely to turn cancerous over an average two years of follow-up, compared with 21 who got dummy shots.