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HPV Vaccine Gardasil Does Not Increase Blood Clot Risk in Women

New data refute previous studies

Two human papillomavirus (HPV) vaccines are currently approved by the FDA: quadrivalent Gardasil (Merck) and bivalent Cervarix (GlaxoSmithKline).

A new post-approval study of more than 1.6 million Danish women has shown that quadrivalent HPV vaccination does not increase the risk of venous thromboembolism (VTE) at any time within 42 days of receiving the immunization.

The data were presented in a research letter published July 9 in JAMA.

Researchers obtained information on vaccination, the use of oral contraceptives, the use of anticoagulants, and the outcome of a first hospital diagnosis of VTE not related to pregnancy, surgery, or cancer from Danish registers. Cases were followed from October 2006 to July 2013. The main risk period for VTE was defined as 1 to 42 days after vaccination.

From a source population of 1,613,798 women, including 500,345 (31%) who received the quadrivalent HPV vaccine, the investigators identified 4,375 incident cases of VTE. Of these, 889 women (20%) were vaccinated during the study period.

The researchers found no association between the HPV vaccine and VTE during the 42 days following vaccination. The crude incidence rates were 0.126 and 0.159 events per person-year for the risk and control periods, respectively, for an incidence ratio of 0.77. Moreover, no association between HPV vaccination and VTE was observed in various subgroup analyses, including only anticoagulant-treated cases and only exposed cases, or when adjusting for the use of oral contraceptives.

The authors noted that a potential association between quadrivalent HPV vaccination and VTE had been reported in two previous post-approval safety studies. However, one study was based on data from a passive surveillance system, and the other study included women with known risk factors for VTE.

Source: JAMA; July 9, 2014.

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