Infections with two strains of the sexually transmitted human papillomavirus (HPV) are showing marked declines among American women, and rising vaccination rates could be driving the trend.
That is the finding from a new study involving thousands of U.S. women who tested positive for precancerous conditions of the cervix.
Infection with HPV is by far the leading cause of cervical cancer, and it has also been tied to genital warts and cancers of the mouth, throat, vulva, vagina, and anus.
But the new study finds rates of infection with HPV 16 or 18 – the two strains most heavily implicated in cervical cancer – have markedly declined between 2008 and 2014.
It is during those years that rates of the use of HPV vaccines such as Gardasil and Cervarix became more widespread. Both vaccines target HPV 16 and 18, among other strains.
The bottom line: "This is clear evidence that the HPV vaccine is working to prevent cervical disease in young women in the United States," said study author Nancy McClung, an epidemiologist researcher at the U.S. Centers for Disease Control and Prevention.
"In the coming years, we should see even greater impact as more women are vaccinated during early adolescence and before exposure to HPV," McClung added.
The CDC currently recommends routine vaccination of girls and boys by ages 11 and 12, because protection is best if done before initiation of sexual activity.
One obstetrician/gynecologist who was not involved in the new study was heartened by the findings.
"I suspect that this initial reduction will continue to increase until we see an approximate 70 percent reduction in HPV-related disease, since HPV 16 and 18 are responsible for 70 percent of cervical cancers worldwide," said Dr. Adi Davidov, interim chair of the department of obstetrics and gynecology at Staten Island University Hospital in New York City.
Certainly, avoiding HPV infection is the key way to avoid cervical cancer, McClung said.
"Almost all sexually active individuals will get HPV at some point in their lifetime, but most HPV infections go away on their own without any treatment," McClung explained in a news release from the American Association for Cancer Research.
"If an HPV infection does not go away, it can cause cell changes that, over time, develop into a lesion on the cervix called a cervical pre-cancer," she said. "Cervical pre-cancers allow us to observe the impact of HPV vaccination earlier than cervical cancer, which can take decades to develop."
So, how effective has vaccination been in curbing the most dangerous HPV infections?
To find out, the CDC team looked at more than 10,000 lab samples of cervical tissue obtained from women diagnosed with pre-cancerous cervical conditions between 2008 and 2014. The women ranged in age from 18-39.
The investigators tracked the presence of 37 strains of HPV, but were most interested in HPV 16 or HPV 18.
The result: In 2008, either of these two strains most often tied to cervical cancer were detected in 55 percent of samples taken from vaccinated women, but by 2014 that number had dropped to just 33 percent, the study showed.
McClung explained the vaccinated women had probably contracted HPV before being inoculated. Most of the vaccinated women in the study got the shots in their 20s – typically past the age where they had begun sexual activity.
In any case, the steep, recent decline in HPV 16 and 18 infection is encouraging, the researchers said.
Less dramatic – but still significant – declines were observed in samples taken from unvaccinated women. In this group, HPV 16/18 infection fell from 51 percent in 2008 to just over 47 percent by 2014.
Even unvaccinated women have benefited by the widespread uptake of the HPV vaccine, McClung explained, due to what is known as "herd immunity." Herd immunity occurs when widespread inoculation against a germ means that it simply ceases to circulate as widely as it used to in a population.
According to the latest CDC statistics, 49.5 percent of girls and 37.5 percent of boys aged 13 to 17 are up-to-date on all recommended doses of the HPV vaccine.
Dr. Jennifer Wu is an obstetrician/gynecologist at Lenox Hill Hospital in New York City. She called the new findings "very exciting."
"Rates of cervical pre-cancers are declining significantly," she said, and "as vaccinated patients get older, decreasing rates of cervical pre-cancerous lesions show the continued effectiveness of the vaccine."
Wu pointed out the study showed certain populations – Hispanic and Asian Americans, specifically – did not seem to benefit as much as white or black women. But that could change, she said.
"Rates of HPV 16 and 18 did not fall as much on Asian and Hispanic populations, which typically had lower vaccination rates," Wu said. "But current data shows that these populations have reversed trends and now have very good vaccination rates."
The new findings were published Feb. 21 in the journal Cancer Epidemiology, Biomarkers & Prevention.