The human papillomavirus (HPV)-16/18 vaccine appears to protect against the viral strain even after one or two doses, according to a combined post-hoc analysis published online this week in the journal The Lancet Oncology.

The post-hoc analysis is a combination of the Costa Rica Vaccine Trial and the PATRICIA trial. These two phase 3, double-blind trials evaluated the efficacy of different HPV-16/18 vaccine doses and the researchers randomly assigned women aged 15-25 years to receive three (n=22,327), two (n=1,185), or one (n=543) doses.

After four years of follow-up, the efficacy of the three doses of HPV-16/18 vaccine was 77.0% (95% CI: 74.7, 79.1), two doses was 76.0% (95% CI: 62.0, 85.3), and one dose was 85.7% (95% CI: 70.7, 93.7).

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Furthermore, vaccine efficacy for women who received two-dose regimen was 75% (95% CI: 54.2, 87.5) and 82.6% (95% CI: 42.3, 96.1) when they received the second dose at 1 month and 6 months, respectively.

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However, HPV-31/33/45 vaccine efficacy seems to vary with dosage regimen where three doses was 59.7% (95% CI: 56.0, 63.0), two doses was 37.7% (95% CI: 12.4, 55.9), and one dose was 36.6% (95% CI: –5.4, 62.2).

The findings suggest that the HPV-16/18 vaccine seems to protect against the virus from fewer than three-dose schedule, and there was cross-protection when the administration time for two doses was separated by six months.


  1. Kreimer AR, Struyf F, Del Rosario-Raymundo MR, et al. Efficacy of fewer than three doses of an HPV-1618 AS04-adjuvanted vaccine: combined analysis of data from the Costa Rica Vaccine and PATRICIA trial. Lancet Oncol. 2015. [Epub ahead of print]. doi: 10.1016/S1470-2045(15)00047-9.