The human papillomavirus (HPV) is the most common viral infection of the reproductive tract as well as the leading cause of cervical cancer. 70% of cervical cancers are specifically due to HPV types 16 and 18. Fortunately, researchers have developed an HPV vaccine to target and prevent HPV 16 and 18 infections.
Scientists have shown this vaccine can prevent many invasive cervical cancers, since preventing HPV lessens the chances of developing these cancers. The HPV vaccine has been available to the public since 2006 and has been successful in the short term, but the long-term effects of such a recent vaccine are still unknown.
A team of scientists from the USA and Costa Rica recently investigated the long-term effects of the HPV vaccine. The researchers followed up on a clinical vaccine trial in Costa Rica in 2004 and 2005.
The team tracked if participants in the Costa Rica trial developed two types of HPV-associated abnormal cell growth in the cervix, or CIN disease, denoted as CIN2+ and CIN3+. These are precursors to more invasive cervical cancers, although the CIN3+ type involves more abnormal cells than the CIN2+ type. Since their study examined the health of the participants 11 years post-vaccination, it was the longest HPV vaccine follow-up study to date.
The researchers trialed 7,466 women aged 18-25 who were randomly assigned to receive either the HPV vaccine or a hepatitis A control vaccine. Neither the participants nor the researchers knew which vaccine each participant received, since this was a double-blind trial. The researchers gave the patients three separate doses of the vaccine, each in the arm: an initial dose, a second dose one month later, and a third dose six months after the first dose.
To compare data from the vaccinated and control individuals, the scientists conducted annual pelvic exams of each participant. They collected cervical cell samples and identified any differences in the genome of each individual using a procedure called HPV genotyping. They also conducted a cervical biopsy procedure if the patients’ exam results showed atypical cell growth. This double-blinded part of the study lasted for four years. At the end of the four years, all of the participants were given an HPV vaccine as they were released from the study.
After the first four years, the scientists conducted an additional seven-year unblinded observation period. For these seven years, the researchers conducted the same tests on the same HPV vaccinated experimental group as before. In contrast, a separate population of participants was included as a new control group. The women were of similar age and health, meaning the results would be unbiased between the experimental and control groups.
The researchers used the pelvic exam and cervical biopsy results from all 11 years of the study to see if any of the participants had developed CIN disease. They discovered major differences between CIN found in vaccinated individuals and the control individuals. The HPV vaccine worked effectively to prevent CIN2+ 97% of the time and to prevent CIN3+ 95% of the time. The researchers later discovered that the 5% of cases where they observed CIN3+ in vaccinated individuals could have been due to preexisting infections, so the vaccine could be even more effective.
The scientists concluded the HPV vaccine almost perfectly protected individuals from HPV-associated CIN disease. However, this was only the case for individuals who were HPV-negative when they received the vaccine. The researchers also reported high amounts of HPV antibodies circulating in the vaccinated individual’s body after 11 years. They suggested the vaccine produced long-term protection over this period of time, since high antibody levels lead to a higher immunity against infection.
The scientists also noted the vaccine showed no serious adverse health effects in the individuals they studied. The most common health changes reported were pregnancy and reproductive or breast disorders, but these effects were similar between the experimental and control groups. The team therefore suggested there are no serious health consequences to getting vaccinated against HPV.
These scientists have shown invasive cervical cancers can be prevented by long-term HPV prevention. The authors indicated there are limitations to the study, because they replaced some of the participants during the last seven years of the long-term follow-up. Even so, their results still compare to previous studies which have shown that the HPV vaccine provides a high level of protection against HPV 16 and 18 infections, and also against CIN disease. They also suggested researchers should continue to collect data from other HPV vaccine trials to increase sample size and further understand long-term effects.