The vaccine against human papilloma virus (HPV) is safe and effective at preventing infection, according to new findings from one of the longest follow-up industry-funded studies of the four-strain HPV vaccine to date.
The study, published in Pediatrics, also revealed data supporting the rationale behind recommending the vaccine to boys and girls at 9-12 years old. Not only are these ages long before most kids become sexually active, but those who received the vaccine at these younger ages had stronger immunity against HPV than those who received the vaccine more than two years later.
“These findings justify efforts to vaccinate subjects at the earliest opportunity,” wrote Daron Ferris, MD, a professor of obstetrics and gynecology at Augusta University in Augusta, Georgia, and his colleagues. “Moreover, by vaccinating preadolescents before exposure to HPV, the full potential of the vaccine is more likely to be realized.”
The stronger immune response at younger ages is the reason preteens only need two doses of the vaccine while those ages 15-26 still need three doses. The vaccine is not recommended above age 26 because studies have not shown it is effective enough, and most people have been exposed to at least one HPV strain by then.
While older teens and young adults who haven’t received the vaccine should still get it, Ferris’s team writes, they add that “greater efforts must be exerted to ensure that the majority of preadolescents are vaccinated.”
HPV is the most common virus transmitted through intimate contact, including but not limited to sexual contact. The majority of sexually active people have had at least one HPV infection, but most infections show no symptoms and clear on their own with no long-term effects.
A small proportion of strains, however, can develop into any of six types of cancer, including an estimated 3% of cancer cases among women and 2% of cancer cases among men in the US.
Two types, HPV 16 and 18, are responsible for the vast majority of these, and all cancer-causing strains combined account for nearly all cervical cancer, 95% of anal cancers and 70% of all oropharyngeal cancers (head, neck and throat) in the US. At least half of all vaginal and vulvar cancers and about a third of penile cancers are also caused by high-risk HPV strains.
Widespread screening has reduced cervical cancer cases by more than 70%, but no screenings exist to catch most other HPV-caused cancers, including oropharyngeal, whose rates continue to increase especially among men. The quadrivalent, or four-strain, HPV vaccine covers HPV types 6, 11, 16 and 18. A new 9-strain HPV vaccine is now available.
This study began with 1,661 boys and girls, ages 9-15, who were not sexually active and came from 9 countries across four continents. All had been involved in a base study to examine the vaccine’s safety and effectiveness, and this group included those chose to continue in the extended study. The scientists ended up with complete data for 803 participants and partial data for up to 1,134 participants.
Two thirds of the original participants received three doses of the quadrivalent vaccine, with second dose one month after the first and the third dose six months after the first. The other third initially received three placebo injections on the same schedule but then received three doses of the HPV vaccine 2.5 years after the first group. (It is considered unethical to withhold vaccines from placebo groups over the long-term since the vaccine prevents disease.)
Ten years after the first group received the vaccine, and 7.4 years after the second group received it, the researchers measured the participants’ antibodies and level of immunity against each of the strains in the vaccine, HPV 6, 11, 16 and 18. The scientists also looked at how many of the participants had a disease or ongoing infection caused by these HPV types.
Those who received the vaccine between ages 9-12 had approximately 16% to 42% higher levels of antibodies (titers) 10 years later than those who had gotten the vaccine at ages 13-15.
The participants strength of immune response against HPV types 6, 11 and 16 ranged from 89-99%, depending on the strain, the participants’ sex and the test used. Immunity strength against HPV 18 ranged from 77-79%. These numbers do not represent vaccine effectiveness, which is much higher. These numbers indicate seropositivity, which describes the strength of an immune response to a vaccine. Researchers are not sure why the strength of the response was lower for HPV 18 or what, if anything, that means regarding protection.
None of the participants had diseases from any of the four HPV types in the vaccine. Ten of them, however, did have an infection lasting at least 6 months involving one of the four strains, and two participants had an infection lasting at least a year.
None of the participants showed any side effects or other adverse events throughout those 10 years.
This study was funded by Merck & Company, the company that manufactures the Gardasil HPV vaccine. Five of the study authors are employees of Merck, and six have received research funding and/or speakers’ or consulting fees from Merck. The remaining two authors reported no potential financial conflicts of interest.
