The drop is even bigger than researchers expected, either because just one or two doses of the vaccine is more effective than previously thought, or because vaccinated people slow the spread of the virus among non-vaccinated people, called herd immunity.
The bottom line of the study is that we can protect the next generation of girls from cancer caused by HPV, we just need to start vaccinating more people — only a third of girls are getting the full vaccine now.
"We owe it to the next generation, our sisters our daughters and our patients, to protect them against these cancers," CDC Director Tom Frieden said.
A deadly virus
HPV made the news lately after actor Michael Douglas claimed that the virus, contracted from oral sex, was the cause of his oral cancer. It was likely a factor, in addition to his smoking and drinking habits.
The virus causes about 27,000 cases of cancer every year in the U.S. — 19,000 in women and 8,000 in men.
The new evidence, published in the June issue of the Journal Of Infectious Disease, shows that the vaccines (Gardasil and Cervarix) against cancer-causing strains of the virus have lowered the rates of the virus in female teenagers.
Specifically, the prevalence of strains that the vaccines protect against has decreased by more than half.
An effective vaccine
Among females aged 14–19 years, the vaccine-type HPV prevalence decreased from 11.5% in 2003–2006 to 5.1% in 2007–2010.
These numbers may seem low, but before the vaccine was introduced the majority of sexually active people were exposed to at least one strain of the virus at some time in their lives — not necessarily during their teenage years.
Stopping the virus from infecting women early in their life stops the virus from being passed around the sexually active community later and decreases cancer risk for the whole population in the future.
The vaccine was first introduced in 2006, and is recommended for all 11-year-old girls and was recommended for boys in 2009. There's no data yet on how many boys have gotten the vaccine, or how effective it is at the population level for preventing HPV-included genital warts and precancerous lesions (it was effective in pilot studies).
Only about a third of the girls are being vaccinated, sometimes because parents fear that vaccinating against a sexually transmitted disease would send the girls a signal that they can then have sex. The study showed that this isn't true — sexual behavior didn't change in the two groups.
Because of low vaccination rates, Frieden says that 50,000 girls who should have been vaccinated since 2006 will get cervical cancer just because they haven't been vaccinated.
Every year that more girls aren't vaccinated means 4,400 more cases.
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