Timing is Everything

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Science  31 Aug 2007:
Vol. 317, Issue 5842, pp. 1147
DOI: 10.1126/science.317.5842.1147b

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Cervical cancer is the second leading cause of cancer deaths in women, with more than 80% of these occurring in developing countries that have limited access to screening programs. Some strains of a sexually transmitted virus, human papillomavirus (HPV), play an essential role in the pathogenesis of this cancer. Newly developed vaccines directed against these oncogenic strains have shown promising results in clinical trials aimed at assessing their prophylactic activity—that is, their ability to prevent high-grade precancerous lesions or cervical cancer in women who had not been exposed to HPV before vaccination.

Hildesheim et al. have examined whether HPV vaccination can promote an immune response to HPV in women who are already infected with the virus. Such therapeutic activity had not been observed in animal studies of the HPV vaccines, but data addressing this question in humans are important for ongoing discussions of when and to whom the vaccines should be administered to maximize their benefits. In a study involving about 2000 HPV-positive women in Costa Rica who were monitored for 12 months, the authors found that HPV clearance rates—measured as cell-mediated immunity to the virus—were comparable in subjects receiving the HPV vaccine (specifically, the bivalent HPV-16/18 cervical cancer candidate vaccine) and those who had received a control vaccine directed against an unrelated virus. Although the long-term effects of the current HPV vaccines are not yet known, the apparent absence of therapeutic efficacy noted in this study reinforces the view that the optimal time to vaccinate is before the onset of sexual activity. — PAK

J. Am. Med. Assoc. 298, 743 (2007).

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