September 14, 2009
By Claire McCarthy, M.D.
Boston Children's Hospital
None of us wants a child of ours to get cancer. It's a terrible thought unthinkable, even. So a vaccine that prevents cervical cancer sounds wonderful. We give it to our daughters, and there's one less thing for us to worry about as they grow up.
The problem is, it's not quite that simple.
Most cases of cervical cancer are caused by the human papillomavirus, or HPV. HPV is the most common sexually transmitted illness; over a lifetime, the risk of catching any kind of genital HPV is 75-90%.
Cervical cancer rates aren't nearly that high, though. That's because our immune systems are generally pretty good at getting rid of HPV. There are about a hundred different strains of the virus; 15 of them are known to cause cancer. Four strains 16, 18, 31, and 45 cause 80% of the cancer we see. Gardasil, the most common HPV vaccine, protects against only two of the cancer-causing strains.
While I am a proponent of the HPV vaccine (my eldest daughter has received all three doses), I think it's important that parents know all the facts as they decide whether or not to vaccinate their daughters.
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How Common Is Cervical Cancer?
More than 11,000 women are diagnosed every year with cervical cancer. It used to be one of the most common causes of cancer deaths in U.S. women.
But between 1955 and 1992, the death rate went down by 74%. That's because of the Papanicolau test, or Pap smear as it's generally called. This test, in which some cells are removed from the surface of the cervix and checked for any signs of cancer, allows doctors to catch the cancer early. When caught early, cervical cancer has a 5-year survival rate of 92%.
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How Good Is the Vaccine?
The vaccine seems to do a really good job at protecting against strains 16 and 18 of the virus. In a study published in 2007, precancerous lesions on the cervix caused by 16 and 18 decreased significantly in girls who had received the vaccine. This is great, of course, but there is some fine print to the study that is worth knowing:
- While the vaccine significantly decreased precancerous lesions caused by these two strains, overall the vaccine only decreased the total number of precancerous lesions by 17% that's because cancer is caused by other types than 16 and 18.
- The vaccine did a good job of preventing grade 2 precancerous lesions. But it didn't appear to have an effect on the more serious grade 3 lesions, and 40% to 50% of grade 2 lesions get better by themselves.
- While it wasn't enough to be statistically significant, scientists noted in the study that they were seeing more lesions caused by other types of HPV than they would normally see. This raises the concern that if we protect against strains 16 and 18, other types will increase in numbers to take their place.
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Promising Results, But We Need More Studies
There has only been one big study published that looked at the vaccine's effectiveness. (Interestingly, it was a year after the vaccine was licensed.) It only followed young women for three years. Ninety-three percent of the women were already sexually active. Scientists believe that to be most effective, the vaccine should be given before any possible exposure to HPV namely, before becoming sexually active. (While the manufacturer says that the vaccine can be given through age 26, the American Cancer Society doesn't see any benefit to giving it after age 18.)
Studies in younger girls (9- to 12-year-olds), for whom the vaccine is recommended, have measured the vaccine's effectiveness using blood tests to check for antibodies to 16 and 18. Antibodies are special proteins that can weaken or kill the viruses. (Girls in this age group don't usually have pelvic exams.) We think that the antibodies are a sign of protection against cancer, but we don't know for sure. And we don't know if the vaccine will still protect them at mid-life, when cervical cancer is usually diagnosed.
It's particularly important that parents realize that their daughters will still need regular Pap smears throughout their lives. Other strains of HPV can cause cancer, and because we don't know how long the vaccine's immunity will last, getting tested regularly is crucial.
The whole issue of whether or not to vaccinate has been further clouded by revelations that Merck, the company that makes the vaccine, funded vaccine education provided by some professional organizations. This doesn't mean that it wasn't good, accurate education but it does make many people uncomfortable.
But despite all of this, I still had my eldest daughter vaccinated. Why?
- It offers her some protection against cervical cancer, and I'll take whatever protection my daughter can get. While I've told her time and time again that she'll need regular Pap smears in her lifetime, I have no real control on whether she gets them; she'll be a grown-up then, in charge of her own life and health.
- There wasn't a whole lot of time to wait for more data on the vaccine, given that it's most effective when given before girls become sexually active. My daughter was starting high school when she got the vaccine; by senior year, two-thirds of American teens are sexually active, and many start before then. I talk to her lots and lots about that, too, but as a parent one needs to be a little realistic sometimes.
- It appears safe, or at least as safe as any other vaccine. Sure, we need years probably to be certain. But what we know as of now is reassuring. The most common side effect appears to be fainting, which is why it's a good idea to have girls sit and be observed for awhile after getting the vaccine.
It's taking a chance, yes. But the reality is that medicine isn't always clear. It's often more art than science. Science, too, has a way of evolving; we learn new things every day that change how we think. As patients and as parents of patients, all we can ever do is learn as much as we can, weigh the risks and benefits, and with the help of our doctor, make our best choice.
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Claire McCarthy, M.D., is an assistant professor in pediatrics at Harvard Medical School, an attending physician at Children's Hospital of Boston, and medical director of the Martha Eliot Health Center, a neighborhood health service of Children's Hospital. She is a senior medical editor for Harvard Health Publications.