From Medscape Medical
News
Two Doses of HPV Vaccine May Be Enough
for Protection
Emma Hitt, PhD
September 12, 2011 — Efficacy against HPV strains that can
cause cervical cancer is still seen when fewer than the recommended 3 doses of
bivalent human papillomavirus (HPV) vaccine (Cervarix, GlaxoSmithKline)
are received, according to a randomized vaccine trial conducted in Costa Rica.
Aimée Kreimer, MD, from the National Institutes of Health's
National Cancer Institute in Rockville, Maryland, and colleagues reported their findings online September 9 in the Journal
of the National Cancer Institute.
Conclusions from the current study are based on analysis of
data from a phase 3 clinical investigation designed to evaluate the efficacy of
the standard 3-dose regimen of the HPV vaccine.
About 20% of the enrolled participants received fewer than
the 3 vaccine doses recommended by the Centers for Disease Control and
Prevention's Advisory Committee on Immunization Practices, primarily for
unintentional reasons including pregnancy and colposcopy referrals.
Investigators were therefore able to examine protection against HPV infection
achieved after administering only 1 or 2 doses of the vaccine.
The randomized, double-blinded Costa Rica Vaccine Trial
included 7466 women ranging in age from 18 to 25 years. Participants were
randomly assigned to receive either the HPV vaccine (HPV group) or a control
hepatitis A vaccine (control group) at 0, 1, and 6 months. Women were observed
annually during a median follow-up of 4.2 years.
The full 3 doses of vaccination were administered to 5967
women, 802 women received 2 doses, and 384 women received only 1 dose of
vaccine. No statistically significant difference was observed in the number of
women receiving HPV vaccine vs control in the 3 groups (P = .23).
Vaccine efficacy was determined based on incident HPV16/18
infections that persisted for at least 1 year. Viral presence was detected
through testing for HPV DNA by polymerase chain reaction.
Vaccine efficacy was similar for women who received 2 or 3
doses of the vaccine. Three HPV doses showed an efficacy of 80.9% (95%
confidence interval [CI], 71.1% - 87.7%), whereas vaccine efficacy for 2 doses
was 84.1% (95% CI, 50.2% - 96.3%).
Even 1 dose of HPV vaccine offered protection, with a
vaccine efficacy of 100% (95% CI, 66.5% to 100%). However, the researchers
note that this result, although encouraging, must be taken with caution because
of limited follow-up and the small number of detected infections. They also
suggest that this unexpected result might be a result of previous exposure to
the HPV16/18 virus, as women participating in the trial were predominantly
sexually active (aged 18 - 25 years).
Participants had a similar risk of contracting HPV
infections regardless of the number of vaccine doses they received, confirming
the validity of the vaccine efficacy results. The proportion of new infections
during the 4 years of the study, referred to as the attack rate, was 4.4%,
4.5%, and 5.3%, respectively, for women receiving 3, 2, or 1 dose of the
vaccine.
Bivalent HPV16/18 vaccine administration has previously been
shown to offer some protection against 3 additional types of high-risk HPVs
(HPV31, HPV33, and HPV45) when the standard 3-dose regimen is applied. A similar result for the 3-dose regimen was observed in
this study, but not among those receiving only 2 doses, suggesting that
the full regimen may be needed for added protection against these HPVs.
"If vaccination with fewer than three doses were to
retain the high efficacy of the standard regimen, the ability to vaccinate more
women for the same cost could translate to a greater public health benefit in
underserved areas," Dr. Kreimer and colleagues state.
In a related editorial, Cosette Wheeler, PhD, from the
University of New Mexico Health Sciences Center, in Albuquerque, states that
"it remains unknown...whether HPV vaccine protection, with fewer than
three doses, will be sustainable even for homologous HPV vaccine types 16 and
18. Additional larger studies that are specifically designed to evaluate the
efficacy of one-, two-, and three-dose regimens in young adolescent girls, with
long-term follow-up and more stringent endpoints, could prove critical."
Support for this study was provided by the Ministry of
Health of Costa Rica and GlaxoSmithKline. Vaccine was provided for the trial by
GlaxoSmithKline Biologicals under a Clinical Trials Agreement with the National
Cancer Institute. Dr. Kreimer and Dr. Wheeler have disclosed no relevant
financial relationships.
J Nat Cancer Inst.
Published online September 9, 2011. Abstract
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