A vaccine that protects against four strains of the human papillomavirus, which can lead to cervical cancer, does not increase the risk of blood clots in women, Danish researchers said on Tuesday.
The findings in the Journal of the American Medical Association (JAMA) are based on 500,000 girls and women aged 10 to 44 who received the HPV vaccine between 2006 and 2013.
Using data from Denmark's national health registers, researchers at Statens Serum Institut in Copenhagen found no evidence of an increased risk of venous thromboembolism (VTE) in the 42 days after the shot, which they defined as the main risk period.
Of the 500,000, there were 4,375 cases of blood clots, and of those, 889 had been vaccinated during the study period.
When researchers adjusted for use of oral contraceptives, which can increase the risk of blood clots, they found no association between VTE and the vaccine.
"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," the study said. "Safety concerns can compromise immunization programs to the detriment of public health, and timely evaluations of such concerns are essential."
The JAMA report said a pair of earlier studies had suggested a link between the Gardasil vaccine, made by Merck, and a higher risk of blood clots.
The US Centers for Disease Control and Prevention (CDC) said in 2009 that there was an increase in patient reports of blood clots after the vaccination.
However, on further review, the CDC said 90 percent of those "had a known risk factor for blood clots, such as taking oral contraceptives (birth control pills)."
US health authorities recommend the HPV vaccine for boys and girls before they become sexually active. The Danish Health and Medicine Authority (Sundhedsstyrlsen) also recommends vaccinations and offers it free of charge to all young women aged 12-18.
The vaccine aims to prevent the spread of HPV, which is the most common sexually transmitted infection. Certain HPV strains can cause cancers of the cervix, head, neck and anus.