HPV Vaccines

VACCINES PREVENT HPV INFECTIONS AND ASSOCIATED DISEASES

CLINICAL TRIALS DEMONSTRATE HPV VACCINES' POTENTIAL

HPV VACCINES ALREADY ARE BENEFITING MANY

HPV VACCINES ARE SAFE

VACCINES PREVENT HPV INFECTIONS AND ASSOCIATED DISEASES

Two vaccines—Cervarix® and Gardasil®—are approved by the U.S. Food and Drug Administration (FDA) for prevention of several HPV-associated diseases (Table 3). Cervarix® protects against the two types of HPV most commonly found in cancers: HPV16 and HPV18. In addition to protecting against these two cancer-causing HPV types, Gardasil® protects against HPV6 and HPV11. While these two forms of the virus do not cause cancers, they are responsible for most cases of genital warts and RRP. Both vaccines are highly effective at preventing HPV infections and the diseases caused by the HPV types they target. HPV16 and HPV18 cause about 75 percent of cervical cancers in the U.S. and even higher proportions of noncervical cancers associated with HPV, which implies that most HPV-associated cancers potentially could be prevented by these vaccines (Table 2).[1]

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CLINICAL TRIALS DEMONSTRATE HPV VACCINES' POTENTIAL

Clinical trials have shown that HPV vaccines could prevent the majority of cervical cancers if used optimally. Of 10,000 young women who were vaccinated as part of clinical trials before they were exposed to cancer-causing forms of HPV, none developed high-grade HPV16/18-associated cervical lesions, which are precursors to invasive cancer.[2,3] (See How HPV Infections Cause Cancers.) Since these lesions are on the pathway to cervical cancer, the striking results led experts to estimate that universal uptake of available HPV vaccines likely would prevent more than two-thirds of cervical cancers worldwide (virtually all of those caused by HPV16/18).[4] Gardasil® prevents other HPV16/18-associated anogenital precancers and HPV6/11-associated genital warts with similar efficacy.[3,5] Neither currently approved HPV vaccine is licensed for prevention of oropharyngeal cancers. However, based on what is known about the biology of this disease, it is highly likely that the vaccines will provide protection against oropharyngeal cancers.[6] Women who received Cervarix® as part of a clinical trial had much lower prevalence of oral HPV infection than participants in the trial who had not received the HPV vaccine.[7]

How HPV Infections Cause Cancers

HPV infection of cervical cells initiates a series of molecular events that sometimes, but not always, results in invasive cancer. Although less research has been done on other HPV-associated cancers, the progression from infection to cancer appears to occur through analagous processes. Many HPV infections are cleared by the immune system. Some of these transient infections result in low-grade precancerous lesions consisting of a few abnormally sized and oddly shaped cells. Many of these lesions disappear within a few months with no treatment. In some cases, persistent HPV infection causes high-grade precancerous lesions. These precancerous lesions sometimes regress spontaneously or can be treated if detected. However, some will progress to invasive cancer. The interval between cervical HPV infection and invasive cancer is usually at least 20 years; however, precancerous lesions usually develop much earlier. Detection and treatment of precancerous lesions through screening has reduced cervical cancer incidence and mortality in the United States and other high-income countries over the past several decades. HPV vaccines have potential to further reduce the burden of this disease by preventing both precancerous lesions and invasive cancers.

Sources: Woodman CB, Collins SI, Young LS. The natural history of cervical HPV infection: unresolved issues. Nat Rev Cancer. 2007;7(1):11-22. Available from: http://www.ncbi.nlm.nih.gov/pubmed/17186016; Crow JM. HPV: the global burden. Nature Outlook. 2012;488(7413):S2-3. Available from: http://www.ncbi.nlm.nih.gov/pubmed/22932437

Modified from: Crow JM. HPV: the global burden. Nature Outlook. 2012;488(7413):S2-3. Available from: http://www.ncbi.nlm.nih.gov/pubmed/22932437

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HPV VACCINES ALREADY ARE BENEFITING MANY

While clinical trial results are important, the value of the HPV vaccines depends on their effectiveness in preventing diseases in real-world settings, as is true of the impact of hepatitis B vaccines on liver cancer rates. It may take decades for the full impact of HPV vaccination on cancer rates to emerge, given the long interval between initial infection and development of invasive cancer (see How HPV Infections Cause Cancers). However, earlier endpoints, including HPV infection prevalence and genital warts incidence (for Gardasil®), can provide critical insight into effectiveness of the vaccines. Measuring these endpoints poses challenges. Monitoring HPV infection requires sampling from the site of infection, DNA extraction, and identification of specific HPV types. In addition, genital warts cases are not consistently reported in many countries. However, investments in these types of surveillance activities are critically important.

Data collected through surveillance have revealed dramatic declines in HPV infection rates and genital warts among vaccinated age groups in the United States, and other countries with HPV vaccine programs provide early indicators of vaccine efficacy.[8-11] In the United States, rates of cervical infection with HPV types covered by the vaccines fell by more than 50 percent among girls 14 to 19 years of age in the four years following vaccine introduction (Figure 3). This study included both vaccinated and unvaccinated girls.[8] The decline was even more dramatic in Australia, where the national HPV program has achieved higher levels of vaccination than in the United States (see Part 4). In that country, the prevalence of infections with HPV types covered by the vaccines fell by more than 75 percent among women 18 to 24 years of age.[9] The prevalence of genital warts in Australia also plummeted; a 92 percent reduction was observed among females under 21 years of age.[10]

Figure 3

Source: Markowitz LE, Hariri S, Lin C, Dunne EF, Steinau M, McQuillan G, et al. Reduction in human papillomavirus (HPV) prevalence among young women following HPV vaccine introduction in the United States, National Health and Nutrition Examination Surveys, 2003-2010. J Infect Dis. 2013;208(3):385-93. Available from: http://www.ncbi.nlm.nih.gov/pubmed/23785124

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HPV VACCINES ARE SAFE

By early 2013, more than 56 million HPV vaccine doses had been administered in the United States.[13] HPV vaccines have excellent safety profiles, similar to those of other licensed adolescent vaccines. Safety data on the vaccines are drawn from both prelicensure clinical trial data and postlicensure safety monitoring conducted by CDC, FDA, and vaccine manufacturers. While there is always the possibility for an individual to have a serious reaction to an HPV vaccine, as is the case with any medical procedure, the risk of such a reaction is extremely small. To date, three population-based safety studies have been conducted in the United States.[14-16] These studies have identified no serious safety concerns, although one observed increased risk of syncope (fainting) on the day of vaccination and skin infections in the two weeks following vaccination.[16]* The risk of syncope is not unique to HPV vaccination; immunizations in general have been linked to syncope, particularly among adolescents.[17,18] FDA and CDC continue to monitor the safety of the HPV vaccines and follow up on individual reports of serious adverse events. Additional information on safety monitoring of these vaccines can be found on the CDC website.[19]

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