'Herd immunity' effect shows U.S. females now less likely to contract the cancer-causing virus
FRIDAY, Sept. 8, 2017 (HealthDay News) -- Fewer adult women are becoming infected with human papillomavirus (HPV), a trend that includes females who have never received the HPV vaccine, a new study reports.
It appears that enough women have gotten the HPV vaccine to create "herd immunity" that will provide some protection to females who go unvaccinated, said lead researcher Dr. Abbey Berenson.
"While vaccinated women have already had low levels of HPV infections for several years, we are now seeing a reduction in infections among women who have not received the vaccine," said Berenson, director of the Center for Interdisciplinary Research in Women's Health at the University of Texas Medical Branch in Galveston.
"This is further evidence that the vaccine is effective and emphasizes just how important it is for everyone who is eligible to get vaccinated," she added.
This trend should lead to an overall decrease in cervical cancer deaths, Berenson said. The HPV vaccine targets several strains of the virus that are responsible for 9 out of 10 cases of cervical cancer.
"Cervical cancer typically takes multiple decades to develop in a patient and the vaccine has only been available for one decade, so it is too soon to see the full effects," Berenson said. "However, there are already reports of falling rates of precancers -- that is, the number of high-grade cervical lesions in young women in the U.S. is falling."
Analysis of results from a federal health survey revealed a 32 percent decrease in HPV infections among women 18 to 59 years old between 2009 and 2014, the researchers said.
Young women 18 to 26, in particular, benefited most, experiencing a 65 percent decrease in HPV infection during that period, the researchers discovered. In 2009-2010, more than 15 percent had an HPV infection; by 2013-2014, that had declined to about 6 percent.
Even unvaccinated women experienced a decline in HPV infection, as the vaccine worked to prevent sexual transmission of the virus from person to person, the researchers said.
Unvaccinated women 18 to 26 experienced more than a 50 percent decline in HPV infection, from 19.5 percent of the population in 2009-2010 down to 9.7 percent in 2013-2014.
The new study results were published Sept. 7 in the journal Obstetrics & Gynecology.
"There's less HPV 16 and 18 being passed back and forth," said Dr. Linda Eckert of Seattle, referring to two cancer-related strains. "That means the people who are the carriers of it are not catching it as much, so they are not giving it to their female partners who are not vaccinated," said Eckert, a professor of obstetrics and gynecology with the University of Washington. She wrote an editorial accompanying the new study.
The reduction in HPV infection was not the result of a change in sexual practices, such as an increase in abstinence or better use of condoms, Berenson added. She and her colleagues checked infection rates of herpes and gonorrhea, and found those diseases did not become less common during the same period in which HPV infections declined.
Eckert agreed it will take some time for falling HPV infections to translate into fewer cervical cancer deaths, given that the average time between infection and the resulting cancer is 15 to 20 years.
But a 2015 study in the journal Cancer indicated that fewer women are being diagnosed with precancerous cervical lesions, Berenson said.
That study showed a significant decrease in precancer among women 18 to 20 in California, Connecticut, New York and Oregon, and among women 21 to 29 in Connecticut and New York.
Berenson and Eckert predicted that HPV and cervical cancer rates will fall even more as vaccination picks up among boys. By 2014, the last year covered in this study, only 8 percent of young men in the United States had received even one dose of HPV vaccine, Berenson said.
"It will be interesting in the coming years, as HPV vaccination rates in males are expected to climb, to explore whether we see even greater reductions in HPV infections in the U.S.," Berenson said.
Eckert believes "direct immunity and herd immunity will get better as HPV vaccine uptake increases in both males and females."
There also should be a decline in other HPV-related cancers, such as throat cancer and anal cancer, among both men and women, Berenson said.
For more on the HPV vaccine, visit the U.S. Centers for Disease Control and Prevention.