Three doses of the human papillomavirus (HPV) vaccine are needed in order for the vaccine to be effective, health officials say, but many people who start the vaccination series don't finish it.
In fact, a 2011 study found that that less than one third of young U.S. women who receive one HPV shot complete the full series.
So what should people do if they start the series, but miss a dose?
Experts recommend that those who have received one or two shots pick up where they left off, even if more time has elapsed than is recommended between shots.
The CDC recommends people receive the second HPV dose one to two months after the first, and receive the third dose six months after the first.
But studies suggest that, even if people don't strictly adhere to this schedule (and end up receiving the second and third doses many months later) the vaccine may still be effective.
The HPV vaccine does not contain live viruses, instead, it is made up of proteins that prompt the body to produce an immune response against virus. This type of vaccine, known as a recombinant vaccine, typically needs to be administered in multiple doses in order to produce an optimal immune response.
Studies in Vietnam suggest that giving a second HPV dose even a year after the first produces an "excellent" immune response, said Dr. Simon Dobson, of the University of British Columbia in Vancouver. "It's never too late," to finish the HPV series, Dobson said.
That being said, it's still best to follow the recommended schedule if you can, said Dr. Jessica Kahn, of the Division of Adolescent Medicine at Cincinnati Children’s Hospital Medical Center. The benefits of this schedule have been studied and proven, Kahn said.
Kahn and colleagues are in the process of studying people in the United States to learn more about the immune response in those who don't strictly follow the vaccination schedule. The researchers hope to find out "just how non-compliant you can be" with the vaccine series and still get a good immune response, Kahn said.
The HPV vaccine is currently licensed for people ages 9 to 26. The CDC recommends girls and boys receive three doses of the vaccine when they are 11 to 12 years old. The vaccine works best if administered before people become sexually active.
Pass it on: People who start the HPV vaccine series but miss a dose should pick up where they left off.
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Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.