ARCHIVED - Recommendations on a Human Papillomavirus Immunization Program

2. Burden of disease

There are approximately 40 genotypes of HPV that affect the human anogenital area, including about 15 that are recognized as carcinogenic. Cervical cancer is the first type of cancer to have been associated with HPV: the virus is present in 99.7% of cervical cancer cases. HPV is also linked with a number of other cancer sites, in particular the anus, vulva, vagina, penis and oropharynx. Types 16 and 18 are present in 70% of cervical cancers in North America, and similar epidemiologic characteristics have been found in many other parts of the world.

The risk of acquiring an HPV infection occurs very shortly after the onset of sexual activity(2). Approximately 20% of 15-year-old Canadians have had a sexual encounter. In North America, the lifetime cumulative incidence of HPV infection is estimated at more than 70% for all types together, which makes HPV the most common sexually transmitted infection. The highest prevalence is found in the 20-24 age range(2,3). In a multi-year study of aboriginal women in Nunavik, northern Quebec, infections with any HPV type and high-risk HPV types were detected in 29.1% and 20.2% of women respectively. The most common HPV type was HPV-16; infections with HPV-16 and HPV-18 accounted for 23.8% of all HPV infections(4). HPV prevalence in this population was found to be similar to that observed among female university students in Montreal(5,6) and health clinic attendees in Winnipeg and Nunavut(7,8).

Most HPV infections are asymptomatic and self-limiting, clearing within 24 months. However, persistent infections with oncogenic types may lead to cancer. This process typically takes a number of years or even decades. Without treatment, most invasive cancers are eventually fatal. Survival rates vary according to treatment and stage at the time of diagnosis.

The age-standardized cervical cancer incidence rate for Canada is estimated to be 7.3 cases per 100,000 for the year 2007, a marked decrease compared with the 1978 rate (14.7 per 100,000). However, in the last 10 years the rate of decline has been slower, the incidence in 1997 being 8.7 cases. At 1,350 new cases estimated for 2007 in Canada, cervical cancer is the 13th most common cancer among Canadian women of all ages but the third most common among those aged 20 to 44. Annually, there are approximately 390 deaths related to cervical cancer in Canada⁽⁹⁾.

There are many avenues for cervical cancer prevention in Canada. Immunization is considered to be part of a primary prevention strategy and cervical cancer screening part of a secondary prevention method. Approximately 5,500,000 cervical cancer screening examinations (Pap tests) are performed each year. Despite this, not all women attend regularly for screening. The results of a meta-analysis indicated that 54% of patients with invasive cervical cancer had inadequate screening histories, and 41.5% had never been screened. An estimated 29.3% of failures to prevent invasive cervical cancer can be attributed to false-negative Pap smears and 11.9% to poor follow-up of abnormal results⁽¹⁰⁾. The psychosocial impacts of an abnormal screening result are significant, and the need for a repeat examination or for treatment creates anxiety and entails substantial inconvenience for women. Screening decreases the risk of progression of a precancerous lesion to cancer but has no role in preventing transmission.

Given that the disease burden involves not only cervical cancer but also cancer precursors detected by screening, the advantage of a primary prevention strategy with immunization is the expected reduction in the financial costs and psychological impact associated with the follow-up of abnormal Pap test results and the early treatment of cancer precursors. When implemented in a school-based program, immunization is likely to reach some groups that may have lower cervical cancer screening rates or poor follow-up.

HPV is also linked with non-cancerous lesions, such as anogenital condylomas. This condition is associated with types 6 and 11 in 90% of cases. While there are no precise epidemiological data on its incidence in Canada, it is a relatively common condition. Recurrent respiratory papillomatosis, a much less common but potentially serious disease, is also associated with HPV.

More detailed information on the burden of HPV-associated diseases can be found in the NACI Statement on Human Papillomavirus Vaccine⁽²⁾.