I had a really, really good Epidemiological Methods class today. It was really good because we put all of the concepts we learned over this term (an eight-week term) and looked at the epidemiological evidence for HPV causing cervical cancer and for the HPV vaccine preventing HPV infection and, thus, cervical cancer. I found myself really interested in the class and following along and participating.
Basically, the evidence for HPV causing cervical cancer is this:
In the 1800s, observant scientists noticed that breast cancer (and other cancers) did not afflict virgins and nuns like they would afflict other women. So they theorized that sexual behavior had something to do with this. Then, in the 1960s, there was more evidence that sexual activity had something to do with cervical cancer. Around the same time, the HPV virus was being identified and studied. Soon after that, DNA from the HPV virus was found in cervical cancer tumors.
That discovery alone was not enough. There were questions about which came first, the HPV or the cancer. You could make the argument that something else was causing the cancer and the HPV just came along afterward. So we needed more evidence. Then, in 1987, there was a cross-sectional study of women in Germany. Basically, they took cervical swabs from over 9,000 women and tested them for HPV and looked at slides for cancer. They saw that women who had HPV had a higher prevalence of cervical abnormalities and cancer. Still, the question of “which came first?” was out there.
In 1992, a large, international case-control study was done. That study found that women who had cervical cancer had greater odds of being exposed to HPV. That study was well-designed, with cases (women with cervical cancer) and controls (women without cervical cancer). So these women were matched on everything except their cancer status. So there was little chance for being confused by something.
Finally, in 2001, a large cohort study would settle the issue of causality. Researchers took over 3,000 women and assessed whether or not they had cervical cancer and HPV exposure. Then they followed these women for a couple of years. From that study, they found that women who were exposed to several strains of HPV were more likely to develop abnormal pap smears and pre-cancerous lesions. (Why not wait and see if they develop cancer? Because it’s unethical to see abnormal pap smears and precancerous lesions and not do anything about it. Remember the Tuskegee syphilis experiments?) In essence, women who had normal cervical specimens but were exposed to HPV went on to develop these abnormal tests at a significantly higher rate than women who were not exposed to HPV.
That last cohort study proved beyond a reasonable doubt that some strains HPV leads to cervical cancer. Even the non-carcinogenic strains, the ones that cause just warts, can lead to abnormal pap smears that need to be followed-up, a costly and scary proposition to most reasonably-minded women.
Finally, we looked at the evidence for the HPV vaccine and how women who were vaccinated were less likely to be infected and even less likely to develop cervical cancer. At the end of the class, I wanted nothing more than to drag every anti-vaccine person who opposes the HPV vaccine for their daughters into that room and show them the evidence. If only one of them would have changed their mind, it would have been worth it.
Still, seeing how all the concepts we learned (and some of us re-learned) over the previous eight weeks can come together and change the world was pretty neat.
René F. Najera, DrPH
I'm a Doctor of Public Health, having studied at the Johns Hopkins University Bloomberg School of Public Health.
All opinions are my own and in no way represent anyone else or any of the organizations for which I work.
About History of Vaccines: I am the editor of the History of Vaccines site, a project of the College of Physicians of Philadelphia. Please read the About page on the site for more information.
About Epidemiological: I am the sole contributor to Epidemiological, my personal blog to discuss all sorts of issues. It also has an About page you should check out.