A new study shows that nearly one in four women exposed during routine HPV vaccination is likely to develop genital warts.
The Centers for Disease Control and Prevention said the findings, published Thursday in the New England Journal of Medicine, were among the largest to examine the risks of routine vaccination in the United States.
The study involved more than 9,000 U.S. women and found that nearly 1 in 3 women exposed between December 2009 and April 2010 developed genital wart infection, including more than half who had received the second dose of the vaccine.
They were also more likely to be diagnosed with HPV-related cancers than the general population.
The risk of contracting cervical cancer was similar for the two groups, but the study found a higher incidence of the virus in those who had HPV-preventable infections.
The CDC did not say how the vaccine is made.
The findings were part of the second wave of the HPV vaccine rollout that started in January of this year, and are the first large national study of how many people are vaccinated and how often they are infected.
The number of women infected by HPV is expected to reach 10 million by 2020, the CDC said.
More:Study finds more women than men will get HPV infection, CDC saysThe new findings are not entirely surprising.
The most recent CDC study estimated that one in five people who got the vaccine had genital wars.
The vaccine is given to about one in two women in the U.K. and Australia, and is administered to women who do not have HPV.
The U.N. agency that helps manage global vaccination programs, the World Health Organization, says it is safe and effective.
A similar study published in 2011 found that among 1.4 million men, one in six were infected with HPV and nearly one out of four were at high risk of developing genital wands.
The new study is the first to show that more than 1 in 4 women who get the vaccine may develop genital wart infection, but it’s not yet clear how widespread the infection is.
The HPV vaccine is used for about 4.3 million people in the world.
The vaccine is administered twice a year, once before and twice after sexual intercourse.
The U.V. vaccine is also given to people who have sex with men, including men who have unprotected sex with women, and men who do anal intercourse or oral sex.
The Associated Press contributed to this report.