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The study of half a million Danish women who received the HPV vaccine, which protects against cervical cancer and other health problems, found no link between the shots and the formation of blood clots known as venous thromboembolisms. In the United States it's called the Gardasil vaccine. Concerns about such a link had been raised in previous research.
"Our results found no evidence to support that exposure to the HPV vaccine is associated with an increased risk of [venous thromboembolisms] in the period following vaccination," said Nikolai Madrid Scheller, a researcher at Statens Serum Institute in Copenhagen.
HPV vaccines are recommended for 11- and 12-year-olds because they work best before the start of sexual activity.
The researchers conducted the new study, published July 9 in the Journal of the American Medical Association, after earlier research found a risk of blood clots in the weeks after vaccination.
Experts can't explain how the vaccine could elevate the blood clot risk, Scheller said.
For the new study, the researchers looked at data on 1.6 million Danish females ages 10 through 44. There were 4,375 cases of blood clots among more than 500,000 teens and women who got the vaccine. Of these, one-fifth were vaccinated during the study period, from 2006 to 2013.
The researchers analyzed the cases in multiple ways. They first looked at all patients with blood clots -- those who did and didn't get the vaccine.
They also looked only at patients who had received the vaccine. They analyzed different age groups and adjusted for oral contraceptive use, which is known to increase blood clot risk. They zeroed in on the 42-day window after vaccination, considered the most high-risk period for clots, and also looked at a control period, before and after the vulnerable period.
Despite the different analyses, no link was found between the vaccine and a higher risk of getting a blood clot in the vulnerable time period.
Of the vaccinated females who got blood clots, 860 developed them outside that window. Just 29 developed them during the six weeks after receiving the vaccine, the study found.
HPV vaccines are given in a series of three shots over six months. Two vaccines, Gardasil and Cervarix, are available to protect against cervical cancers. Gardasil -- called a quadrivalent vaccine, which was the focus of the study -- also protects from genital warts and anal, vaginal and vulvar cancers. Women can have either vaccine, while only Gardasil is available for males.
While Scheller can't say if this study is the last word on the issue, he said it "expands considerably" on existing evidence.
Dr. Jennifer Wu, an obstetrician-gynecologist at Lenox Hill Hospital in New York City, agrees that the study shows "there is no link between the HPV vaccine and blood clot formation."
Initial concerns about the safety data on the HPV vaccine led many parents to decline vaccination, said Wu, who was not involved in the study.
Her advice? "Health care providers need to discuss with patients and their parents about the benefits of the HPV vaccine and the low risk profile," she said.
The link reported in previous research may have resulted because researchers didn't consider birth control pill use, which boosts clot risk, she said. This study took use of birth control pills into account.
No funding for the study came from vaccine makers, pharmaceutical companies or other sources for the study, Scheller said.
Copyright © 2014 HealthDay. All rights reserved.
SOURCES: Nikolai Madrid Scheller, M.B., researcher, Statens Serum Institute, Copenhagen, Denmark; Jennifer Wu, M.D., obstetrician-gynecologist, Lenox Hill Hospital, New York City; July 9, 2014, Journal of the American Medical Association