The vaccine for human papillomavirus has been controversial from the get-go, partly because it protects against a virus that causes cervical cancer and is spread by sexual activity.
The vaccine's safety has also been contested, with media celebrities like Katie Couric publicizing rare reports of people who became ill or died after receiving the vaccine, even though there was no evidence that the vaccine caused the problems.
Scientists were particularly concerned about the risk of potentially deadly blood clots, because two studies had found elevated risk in people who had the HPV vaccine.
But a huge study that involved more than half a million Danish women and girls has found no increased risk of blood clots. The study was published Tuesday in JAMA, the journal of the American Medical Association.
The researchers looked at the medical records of 500,345 women and girls in Denmark, aged 10 through 44, who had had the HPV vaccine. Of them, 4,375 had been diagnosed with a blood clot, or venous thromboembolisms, from October 2006 through July 2013. Twenty percent of those women had gotten the vaccine during that time.
To see if there was an association between the vaccine and the blood clot, the researchers looked at the 44 days after the women got the shot, which is considered the time of highest risk. They didn't have a higher risk of clots then.
The researchers adjusted the data to account for women who were using birth control pills, which does increase the risk of blood clots.
The federal Centers for Disease Control and Prevention recommends that both boys and girls get the HPV vaccine starting at age 11 or 12.