The Virus/Cancer Connection (Part 4): Vaccines, Cervical Cancer, and a Recap

Everyone is familiar with the idea that getting vaccinated can keep viruses away. Every year millions of people get the flu shot for just that purpose. If viruses can cause cancer, can vaccines be used to prevent cancer? Of course they can! The best example of this is a pair of new vaccines called Gardasil and Cervarix which prevent cervical cancer.

Cervical cancer? The cancer women get Pap Smears to detect?

Yes, cervical cancer. It turns out that cervical cancer is caused by a virus called human papilloma virus, or HPV. There are more than 100 different strains of HPV, and at least 30 of them are transmitted through sexual intercourse. HPV infection is common, and by age 50 at least 80% of women will have acquired a genital HPV infection.

If 80% of women develop an HPV infection, why don’t 80% of women get cervical cancer? Most of these infections are cleared by the woman’s immune system with no long term consequences. Fortunately, only a small fraction of HPV strains are capable of causing cancer, and these only in the case of a persistent infection. So once again, it appears that a viral cause of cancer requires more than just exposure to the virus. For cancer to develop, it requires a complex interaction between the virus and the patient’s immune system.

Where do the vaccines fit in?

The vaccines cause the body to develop antibodies against 4 of the most common strains of HPV that are associated with persistent infection and with the development of genital warts and cervical cancer. These antibodies help the patient clear the viral infection before it becomes persistent, and thus prevent the development of cancer.

Why is this controversial?

Well, these vaccines are preventive, rather than therapeutic. So they have to be given before exposure to the virus. If the virus is spread through sexual intercourse, that means prior to the initiation of sexual activity. The Advisory Committee on Immunization Practices recommends the vaccine for girls as young as 11, and some school districts have begun to require HPV vaccination just as they require vaccination against chicken pox and whooping cough. One possibly troublesome issue about requiring an HPV vaccination is that some parents may have an issue with vaccinating their little girls against a sexually transmitted infection.

What about men?

Don’t sexually transmitted infections go both ways? Certainly they do. But because cervical cancer is a disease limited to women, all of the clinical testing was done on women, so the vaccines are currently only approved for women (and girls). Clinical trials are ongoing in men, but because they are not completed, there are no data regarding the safety or efficacy of the vaccine for men and no data about whether vaccinating men impacts the rate of cervical cancer in their partners. Also, HPV has been associated with penile and anal cancers in men, so there may be a direct benefit to vaccinating men, but this has not yet been studied either.

Recap: So what have we learned?

Although we don’t usually think about it this way, cancer can be caused by a viral infection. Many cancer-associated viruses, like EBV, are very common, and yet they rarely cause cancer. This is because the interaction between the virus and the patient is quite complex, and only in very rare circumstances does EBV (or HPV, for that matter) cause cancer. However, the fact that viruses can cause cancer teaches us a lot about the role of the immune system in controlling cancer, the impact of the environment on cancer risk, and a host of other lessons that are important in thinking about all forms of cancer.