Previously, it was estimated that 70% of cervical cancer is caused by just two strains of the human papillomavirus (HPV) – strains 16 and 18. Those are the same two strains that the HPV vaccine (sold under the name Gardasil in the US) protects against. HPV is a sexually transmitted disease and the vaccine protects women and girls against it if they are vaccinated before sexual contact with a partner carrying the virus.
New research out of Britain suggests the 70% is an underestimate and closer to 80% of cervical cancer may be caused by strains 16 and 18 – meaning closer to 80% of cervical cancer could be prevented with the vaccine.
Despite the promise of this cancer prevention approach, many in the US remain reluctant to vaccinate themselves, their children or members of their community. Estimates are that only 20-30% of eligible girls in the US are vaccinated (compared to nearly 80% in the UK and Australia). In talking about the power of the vaccine, the most common objection I’ve heard is based on the rumor that the vaccine causes more problems than it prevents. As I dug around the internet on a number of websites of questionable validity, it seems that this MISconception largely stems from a misunderstanding about the differences between correlation and causation.
Most public health and prevention research focuses on distinguishing between correlation and causation so it is not a trivial matter and is one that requires a lot of attention in our research. If I take a pill and get a headache, did the pill cause the headache and thus I should no longer take the pill? Perhaps. But based on so little information, it is equally reasonable to say “perhaps not!” The problem with this “study of me” is that there is no comparison – would I have gotten a headache if I hadn’t taken the pill? This is why we look for quality research to have both a group receiving the pill (or intervention in behavior change research) and a group that doesn’t receive the pill (or intervention). In randomized controlled trials, the population is randomly assigned to one group or the other. In observational studies, such as cohort studies, the researchers identify those who took the pill and those who didn’t and then follow them over time to see what happens. By measuring differences in the two groups, the researchers can not only determine if the pill had the beneficial effects desired AND if there were adverse events. Then they analyze the data to see if the number of benefits (or adverse events) differ between the two groups.
Returning to where we started, the researchers who conducted the trials of the HPV vaccine found the number (or rate) of events didn’t significantly differ between the girls getting the vaccine and those who didn’t get the vaccine, but the girls who got the vaccine were significantly less likely to test positive for HPV, which causes cervical cancer (and is likely also related to other cancers including oropharyngeal cancers), over the course of the study follow-up.