Just a single dose of a 3-dose vaccine against 2 strains of the human papillomavirus (HPV) may protect a woman for years, according to surprising results from a clinical trial in Costa Rica. The findings raise the possibility that fewer doses may be a feasible approach to protecting women, particularly in the developing world where costs and logistics can make multidose vaccinations hard for women to complete.
The vaccine is an important public health tool to protect women against infection with strains of the virus that may lead to cervical cancer. But the cost and the need for 3 doses of the vaccine have contributed to poor uptake of the vaccination in the United States. These barriers to vaccination are greater in the developing world, where women may be even more sensitive to costs or may face logistical challenges to getting repeated doses. With less access to screening and treatment for cervical cancer, women in the developing world are more at risk of death from cervical cancer, which is the most common cause of cancer-related death among women in the developing world.
But the findings by Mahboobeh Safaeian, PhD, an investigator at the National Cancer Institute, and her colleagues offer preliminary evidence that a less arduous HPV vaccination regimen may be possible. The researchers analyzed the HPV-16– and HPV-18–specific antibody levels in women who had received 1 dose of the HPV-16/18 vaccine (78 women), 2 doses one month apart (140 women), 2 doses 6 months apart (52), or 3 doses (120 women) over 4 years*. They also analyzed antibody levels in blood samples from 113 women who tested positive for HPV infection at enrollment. All of the vaccinated women remained seropositive for HPV-16/18 at 4 years after vaccination, with the group that received 2 doses 6 months apart having levels comparable with the 3-dose group. Even the 1-dose group had antibody levels to HPV-16 and HPV-18 that were 9 and 5 times higher, respectively, than women who were infected at enrollment, and the 2-dose group had antibody levels 24 and 14 times higher, respectively, than those who were presumably infected naturally. The antibody levels in the 1-dose group stayed steady from 6 months to 4 years after vaccination.
Further studies will be necessary to confirm the findings and prove that the 1-dose antibody levels are sufficient to protect against cervical cancer. In the meantime, however, the findings may have important implications for vaccine developers. Safaeian and her colleagues noted that the durable antibody response they documented is more similar to the response to a live-attenuated vaccine rather than a protein-based vaccine, which requires frequent boosting.
“Our findings challenge previous dogma that protein subunit vaccines require multiple doses to generate long-lived responses,” Safaeian said in a statement.
*This blog has been updated to include information omitted in the original draft.