Many teens are still not getting the controversial human papillomavirus vaccine, causing concern among public health officials.

New research discovered that vaccination rates have increased slightly for the second year in a row. But despite that increase, four out of 10 adolescent girls and six out of 10 teenage boys haven't started getting the vaccines, according to the Centers for Disease Control and Prevention.

Without the vaccine, persistent HPV infections can cause cancer in the cervix, vagina and vulva in women and in the penis for men, the CDC said Thursday. The agency reported that every year, about 27,000 women and men in the U.S. are diagnosed with a cancer linked to an HPV infection.

There was a 3 percent overall increase in getting the first dose of the HPV vaccine among teen girls and 8 percent for boys in 2013 compared to the year before, according to a 2013 CDC survey. The agency's findings came from more than 20,000 adolescents interviewed by the CDC in 2014.

The agency recommends boys and girls start getting the series of vaccines at 11 or 12 years of age. The vaccines are doled out in a series of three shots over six months.

The vaccine has been at the center of some controversy in recent years, especially in 2011 after then-presidential candidate Michelle Bachmann implied it was linked to mental health problems.

The CDC said on its website that the vaccine is well studied and that there are no "serious safety concerns confirmed in any of these studies."