HealthDay News — Women who receive only one dose of a human papillomavirus (HPV) vaccine have readily detectable antibody levels that remain stable for four years, suggesting that one dose of the vaccine may be adequate to confer protection.
Receiving one dose of HPV vaccine induced geometric mean titers (GMTs) to HPV serotypes 18 and 16 that were five times and nine times higher, respectively, than those seen in non-immunized seropositive women, Mahboobeh Safaeian, PhD, from the National Cancer Institute in Bethesda, Md., and colleagues reported in Cancer Prevention Research.
Receiving two doses in the three dose HPV vaccine schedule induced even greater GMTs — 14 to 24 times higher than GMTs in nonimmunized women, they added.
“Our study is the first to show that even a single HPV 16/18 vaccine dose induces an antibody response that was readily detected in all vaccinated young women at the end of the 4-year follow-up, although the titers were lower than after two or three doses and the number of one- and two-dose recipients was limited,” Safaeian and colleagues wrote.
The findings have important implications because many patients fail to complete the full three-dose HPV vaccine series, as well as for developing nations with limited resources.
To better understand the relationship between HPV vaccine doses and vaccine efficacy, Safaeian and colleagues examined the magnitude and durability of antibodies to a bivalent HPV16/18 L1 virus-like particle vaccine (Cervarix, GlaxoSmithKline) in four groups: 78 women who received one dose, 140 women who received two doses one month apart, 52 women who received two doses six months apart and 120 women who received all three scheduled doses.
Antibody titers were also measured in 113 unvaccinated women who were HPV 16- or HPV 18-seropositive before vaccination.
All patients were participants in the Costa Rica Vaccine Trial, which involved 7,466 women randomly assigned to receive HPV vaccine or hepatitis A vaccine. The researchers determined HPV seropositivity by analyzing serum samples with enzyme-linked immunosorbent assay (ELISA), and quantified HPV DNA using polymerase chain reaction (PCR).
At one month, nearly all vaccinated patients were seropositive to the first dose of vaccines, with similar GMTs observed in all vaccine groups.
After four years of follow-up, all women in all four vaccinated groups remained seropositive for HPV16/18. The ratios of HPV 16 and HPV 18 GMTs for the one-dose group versus the three-dose group were 0.18 (95% CI 0.14-0.24) and 0.21 (95% CI 0.16-0.28).
Compared with the natural infection group, HPV16/18 titers were at least 14 to 24 times higher for the two-dose groups and five to nine times higher for the one-dose group. GMTs were similar among women who received two doses six months apart and who received three doses.
“The high efficacy after single dose suggests that long-term protection may not require the 5-fold higher titers induced by three doses of the vaccine. Fewer doses would be less expensive and logistically easier to deliver, therefore increasing vaccine accessibility worldwide,” the researchers wrote.
They acknowledged that alternate HPV vaccine schedules could limit additional protection against related HPV types due to cross-neutralizing antibodies observed with the standard three-dose vaccine schedule, but noted this may be less important as second generation HPV vaccines that target more oncogenic HPV types are developed.