HPV Vaccine More Effective Than Thought

Sept. 20, 2007 — -- A study by the company that makes Gardasil, a vaccine that protects against cervical cancer, suggests it may be even more effective than previously thought.

The vaccine, manufactured by Merck, had previously been thought to only protect against 70 percent of cervical cancer caused by viruses. However, the new research indicates that it may also have at least some effect against viral strains that cause the other 30 percent.

The new data was presented this week by Merck at the Interscience Conference on Antimicrobial Agents and Chemotherapy.

The finding was an unexpected one, says study investigator Dr. Darron Brown, professor of medicine, microbiology and immunology at Indiana University School of Medicine in Indianapolis.

"I was surprised at the high degree of cross protection — pleasantly surprised," he said.

And doctors unaffiliated with Merck agree that the research is promising.

"The data are very encouraging because they show cross protection against infection and disease caused by other HPV types that are not included in the vaccine," said Dr. Jessica Kahn, associate professor of pediatrics at Cincinnati Children's Hospital Medical Center in Ohio.

The vaccine protects against cervical pre-cancer by priming the body's defenses against a sexually transmitted virus called the human papilloma virus (HPV). It specifically protects against four strains of the virus — strains 16 and 18, which lead to cervical pre-cancer, and strains 6 and 11, which cause genital warts.

What researchers found in the new study was that the vaccine may also offer partial protection against pre-cancerous lesions caused by 10 additional strains of HPV.

This could be especially important in preventing long-lasting infections, which are more likely to eventually lead to cancer.

"Seventy-five to 90 percent of infections go away," said Dr. Diane Harper, study investigator and professor of obstetrics and gynecology at Dartmouth Medical School in Lebanon, N.H. "What we really want to prevent are persistent infections — those infections that stay and don't clear on their own."

Global Implications

At the heart of the new finding is the concept of "cross protection," which means that although the vaccine is designed to protect against lesions caused by a few specific types of HPV, it can also have some effect, albeit a smaller one, on other strains that are closely related.

"This high degree of cross protection … provides an extra measure of protection for young women," said Brown.

The new findings may be even more important in other parts of the world where cervical cancer is more prevalent.

"In many developing regions, cervical cancer is the No. 1 cancer," said Kahn. "About half a million women around the world get cervical cancer, so it has tremendous public health impact."

The additional strains studied include HPV strain 52, which is more prevalent in some parts of Asia, notes Dr. Kevin Ault, study investigator and associate professor of gynecology and obstetrics at Emory University in Atlanta.

More Bang for Your Buck

Ault says the new findings may make it easier for many to justify the cost of the vaccine, which runs at around $150 per dose.

"You really get a little more bang for your buck; you get some modest protection against other [strains] based on this data," said Ault. "Seventy percent [of cervical cancer] is caused by strains 16 or 18, so that leaves a 30 percent window that we've been worried about … so you might be able to get at some of those other cases through this cross-protection effect."

Despite the new findings that show increased protection, doctors point to an important limitation of the vaccine — it is only 39 percent effective against lesions caused by these other strains of HPV. The majority of that efficacy was seen in just two of the other 10 strains, notes Harper.

However, physicians are still optimistic about the new findings and hope additional coverage will eventually help reduce cervical cancer rates.

"The vaccine is very safe and effective in preventing [cervical pre-cancer] caused by the two most common cancer-causing HPV types," said Brown. "[Women] should also know that they can have this extra measure of protection against other types."

But Harper notes that there is no substitute for other means of prevention and early detection.

"Even with the coverage of more types … people that have Gardasil can still get cancer," she said. "Pap screening is still important."