Researchers have learned that vaccinating a large portion of a population against a disease can protect even unvaccinated people in the group. The phenomenon is known as herd immunity, and it is showing potential against human papillomavirus, or HPV, a common sexually-transmitted disease that can cause cervical cancer.
Experts say human papillomavirus infects about 90 percent of all sexually-active people. Globally, the virus accounts for about 500,000 cases of cervical cancer each year, and up to 290,000 deaths.
Two strains of the virus, HPV 16 and 19, cause about 70 percent of all cervical cancers, in addition to oral, vaginal and penile cancers. Experts say HPV strains 6 and 11 are responsible for 90 percent of genital warts.
A highly effective vaccine against all four strains of human papillomavirus became widely available within the past two years. Public health officials recommend that girls as well as boys, and both men and women who have not yet had sexual relations, be immunized so they can't become infected with or spread the HPV virus.
"We were really interested in finding out what the real world, public health impact would be of this vaccine. And I think the implications are very encouraging; that we may see a significant decrease in rates of cervical cancer in our community," said Jessica Kahn, an associate professor of pediatrics at Cincinnati Children's Hospital Medical Center in Ohio.
Kahn led a study in the Cincinnati area looking to see whether the vaccine provided herd immunity, a phenomenon in which untreated people in a community - the 'herd' - do not become infected with a communicable disease because vaccinated individuals with whom they have contact do not transmit the illness.
She and her colleagues determined the rate of HPV infections in 2006 and 2007 in a group of 368 sexually active, unvaccinated teen girls. They compared that to the infection rate in 2009 and 2010, in a group of more than 400 sexually young active women, more than half of whom had gotten the vaccine.
Kahn says there was a significant drop in the HPV infection rate among the unimmunized women, as well as those who were vaccinated.
"Among the 13 to 26-year-old young women in this study, all of whom had had degrees of sexual contact, the rates of HPV fell substantially, between 50 and 70 percent," Kahn said.
Kahn says many of the more than 700 women in the study were economically disadvantaged and did not get vaccines or seek medical care, so the findings may not apply in other regions of the US or the world.
Kevin Ault, an associate professor of gynecology and obstetrics at Emory University in Atlanta, Georgia, says he urges his young patients to get immunized with the HPV vaccine. And he's encouraged that data are emerging showing that it has a protective effect in a community setting.
"You know, I try to get a flu shot every year so I won't cough on anybody who doesn't have the vaccine, the flu vaccine, and pass the flu along. And so in my community, that's herd immunity. And in this community they found something somewhat similar," Ault said.