U.S health authorities recommend getting vaccinated against human papillomavirus (HPV) to prevent infection of certain species of HPV that are known to cause genital warts and certain types of cancer including cervical cancer in women and penile cancer in men.

HPV vaccine is recommended particularly for children between 11 and 12 years old as it is most effective when given before the start of sexual activity, but there are concerns that the vaccine increases risks of serious blood clots. Previous reports from the Centers for Disease Control and Prevention raised concerns on a possible link between getting HPV shots and the formation of a type of blood clot known as venous thromboembolism (VTE).

Because of safety concerns over HPV vaccines, many parents have declined vaccination of their children. A new study that involved half a million Danish women, however, suggests that HPV vaccines do not increase risks of blood clot in women.

For the study "Quadrivalent Human Papillomavirus Vaccine and the Risk of Venous Thromboembolism" published in the Journal of the American Medical Association on July 9, Nikolai Madrid Scheller, from the Department of Epidemiology Research of the Statens Serum Institut in Copenhagen, Denmark, and colleagues examined the data of about 500,000 girls and women in Denmark between 10 and 44 years old who received at least one dose of quadrivalent HPV vaccine between October 1, 2006 and July 1, 2013.

The purpose of the study is to find an association between the HPV vaccine and the risks of VTE. Gardasil, a quadrivalent vaccine produced by Merck & Co. and approved by the Food and Drug Administration in 2006, provides protection from vaginal and vulvar cancers as well as genital warts.

Adjusting for oral contraceptive use, a risk factor for blood clots, the researchers found no evidence of increased risks for VTE within 42 days after the women were vaccinated, the period considered to be the time when the subjects were most vulnerable for clots. Eight hundred sixty of the vaccinated women developed blood clots but outside of the 42-day period and only 29 developed clots within six weeks after they got vaccinated.

"Our results, which were consistent after adjustment for oral contraceptive use and in girls and young women as well as mid-adult women, do not provide support for an increased risk of VTE following quadrivalent HPV vaccination," Scheller and colleagues wrote. "Safety concerns can compromise immunization programs to the detriment of public health, and timely evaluations of such concerns are essential."

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