Whenever a new medical intervention comes onto the scene, doctors have hard choices to make. Do we really know enough about its potential benefits and risks to recommend using it? If in doubt, should we err on the side of caution or on the side of hope?
These are difficult questions because medical knowledge is usually incomplete and ambiguous. They are especially difficult to answer for drugs that may prevent disease in the future, particularly when those drugs are given to otherwise healthy people. Vaccines, particularly the human papillomavirus (HPV) vaccine, are a case in point.
The HPV vaccine is designed to prevent cervical cancer, which currently kills around 250,000 women a year worldwide. It works by inducing immunity to HPV, the leading cause of the disease and the world’s most common sexually transmitted infection: around 79 per cent of people catch it at some point in their lives. Almost all infections are quickly cleared by the immune system, but in a few women infection persists and can progress to precancerous lesions and eventually cervical cancer.
The first HPV vaccine came on the market in 2006 and public health officials in the US, Canada, Australia and several European countries now recommend vaccination of preadolescent girls.
But despite claims that the vaccine will cut cancer deaths by two-thirds or more, its overall effectiveness in the prevention of cervical cancer remains unknown and will not be known for decades.
The theory behind the vaccine is sound: if HPV infection can be prevented, cervical cancer will not occur. But in practice the issue is more complex.
Why? First, there are more than 100 different types of HPV, at least 15 of which cause cancer. But the vaccine protects only against the two most important cancer-causing strains, HPV-16 and HPV-18, though it may also offer partial protection against some closely related strains (The Lancet, vol 374, p 301). But the remaining strains still cause cancer.
Second, though clinical trials have shown that the vaccine reduces the incidence of precancerous lesions, we cannot say for sure that this will translate into cutting cervical cancer and deaths 20 to 40 years in the future.
There are many other gaps in our knowledge. How long does the vaccine provide protection without a booster? Does it affect natural immunity against HPV, and with what consequences? Can we really be sure that the vaccine protects preadolescent girls when proper clinical trials have been carried out only in women aged 16 to 24?