By Talía Malagón, postdoctoral fellow in the lab of Dr. Eduardo Franco and the Research Team at the Division of Cancer Epidemiology, McGill University
Some of the most important strides in cancer prevention we have made in the last few decades have come from research on the human papillomavirus (HPV). This March 4th marks the first International HPV Awareness Day, a day to raise awareness of HPV and the knowledge we have gained regarding the prevention of HPV-related diseases.
It had long been suspected by the mid-20th century that cervical cancer was caused by an unknown sexually transmitted agent. The discovery of HPV in cervical tumours in the early 1980s by Harald zur Hausen launched a flurry of scientific activity that enabled to establish beyond a doubt sexually-transmitted HPV as the necessary cause of cervical cancer. HPV was also eventually discovered to be a major cause of anal and oropharyngeal cancers in men and women, as well as other genital cancers (vulvar, vaginal, penile) and anogenital warts. By 1995, there was sufficient evidence for the International Agency for Research on Cancer to declare that HPV types 16 and 18 were human carcinogens, and that several other HPV types were probable or possible carcinogens.
Cervical cancer had long been prevented through the use of Pap tests, which screen for precancerous lesions in order to treat them before they progress to cancer. However, the discovery of HPV provided a promising new target for prevention, that could potentially decrease the incidence of not only cervical cancer but also all other HPV-related cancers and conditions. It took over 10 years of clinical development for pharmaceutical companies to develop HPV vaccines, culminating in massive international clinical trials to prove their efficacy and safety. The results surpassed everyone’s expectations: the vaccines were extremely efficacious at preventing new infections with HPV and associated pre-cancerous lesions in both men and women.
Canada was quick to implement HPV vaccination programs in young preadolescent girls. Despite publically funded screening with Pap smears in Canada, many women still get cervical cancer due to either a failure to screen regularly, the lack of sensitivity of the Pap test, or inadequate follow-up and management. Preadolescence is the best time to get vaccinated against HPV, as the vaccines produce stronger immune responses at this age, and it ensures preadolescents are protected before they begin sexual activity. The growing recognition that HPV causes disease in men as well has led many provinces to include preadolescent boys in their vaccination schedules as well.
HPV research is also revolutionizing cervical cancer screening. Testing for HPV is known to be more sensitive for screening than using Pap tests, and a negative HPV test leads to greater reassurance of the absence of disease for a longer time. Some Canadian provinces are working to replace Pap tests with publically funded HPV tests, and it is likely that in the near future HPV testing will become the norm for screening in most developed countries. The higher accuracy of HPV tests will mean that women can go longer without being rescreened if they are HPV negative, as there is very low risk they will develop precancer or cancer without HPV. Many researchers are also investigating whether HPV biomarkers could be used to screen for other HPV-related cancers, which would represent a radical advancement in the prevention of cancers that until now could not be screened for.
On HPV Awareness Day, the goal is to better inform the public on existing preventive measures they can take to protect themselves and their loved ones against HPV-related diseases. Because 80-90% of people are likely to be infected with at least one strain of oncogenic HPV during their lifetime, the prevention of HPV-related disease involves everyone.
For parents, this means vaccinating their children in preadolescence to provide them with the best protection against HPV infection and future related cancers. While parents sometimes express surprise at having to vaccinate their children at this age, it is the optimal age to be vaccinated both in order to maximize immune response and to ensure children are protected before they enter adolescence. It is for this same reason we have also for a long time vaccinated against Hepatitis B at this age, another cancer-causing virus that can be transmitted sexually. In cases where vaccination of both genders is publically funded, parents should take advantage of the opportunity to protect both their sons and daughters against HPV-related diseases, as vaccination remains for now the only reliable existing preventive measure against HPV-related cancers in men.
For women 25 years and over, this means getting screened for cervical cancer according to recommended guidelines, regardless of whether she has been vaccinated or not. While women at these ages may still get vaccinated, the effectiveness of the vaccine diminishes because it is very likely they may have already been infected with an oncogenic HPV type by this age. HPV is highly prevalent and asymptomatic, so most infected individuals will never know they have been infected. The vaccine does not treat existing HPV infections. The best course of action is therefore to get screened, which will allow detecting whether HPV has caused any abnormal cellular changes, which can either be monitored or treated depending on severity.
For everyone, this means safer sex practices and advocating for more HPV prevention and research. Consistent condom use likely only reduces the risk of HPV transmission by about 70%, but could help reduce the risk of HPV acquisition or transmission to partners. Speaking out against unscientific anti-vaccine sentiments and activism is also important to preserve public trust in HPV vaccination.
The story of HPV is an important achievement in cancer control and prevention, rivaled only by tobacco control. The combination of HPV vaccines and improved screening will likely lead within a few decades to a future where cervical cancer and other HPV-related diseases are extremely rare. In a time where nearly 1 in 2 Canadians will develop cancer in their lifetime, this indeed is good news.
March 1, 2018