Public-health officials may not have to worry so much about the low percentage of girls who don’t get all three doses of the human papillomavirus (HPV) vaccine.
The HPV vaccine is one of the best ways to prevent genital warts and infection with the virus that can cause cervical cancer, so since 2007, the Centers for Disease Control and Prevention recommended girls get three doses of the shot when they are 11 or 12 years old, and since 2011, advised that boys of the same age get routinely immunized as well. But about a third of eligible young people in the U.S. start the three-shot regimen, which takes six months to complete, and never complete the schedule; around half of eligible children are fully immunized against HPV.
Cost — all three doses of the immunization cost about $400 — and the lack of follow-up over six months contribute to the low compliance rate, so scientists have been investigating whether fewer doses could protect against infection just as well. A 2011 study reported that people had similar levels of protection at two doses, and now researchers believe even one shot may be enough.
The researchers who published their findings this week in Cancer Prevention Research, studied Cervarix, which, like the other HPV vaccine approved by the Food and Drug Administration, Gardasil, protects against HPV subtypes 16 and 18. The researchers measured the immune response to the vaccine in groups of 78, 192 and 120 women who received one, two and three doses of the vaccine, respectively. They then compared the amount of antibodies these women produced against those of a group of 113 women who did not get immunized but had been infected with HPV at some point in their lives.
The vaccinated women in all three groups continued to show HPV antibodies in their blood for up to four years. These levels were similar among women who received two doses and those who were vaccinated with all three doses. But the women who just got one dose also had stable antibody levels, even though they were lower than those found in the other groups. These levels, however, were still 24 times higher than those among women who never received the vaccine but had been infected with the virus. So a single dose of the vaccine, the researchers believe, may be sufficient to provide some protection against infection with HPV.
They still aren’t sure, however, whether the antibodies generated by the vaccine are sufficient for long-term protection, such as that provided by immunizations against hepatitis A and polio, or whether people will need to boost waning levels of antibodies with additional shots over time.
There are hints that three doses may not be necessary, however. In Chile and British Columbia, public-health officials recommend just two doses of the HPV vaccine. But before the current recommendation in the U.S. is changed, more research is needed to clarify what type of protection the vaccines provide, and whether there are differences between the two currently approved shots. So for now, it’s not likely that the needle-phobic will get a break when it comes to getting up to date on their HPV shots.