MALMO, SWEDEN – Robust human papillomavirus antibody responses persist through 36 months in boys and girls who received two doses of the 9-valent HPV vaccine known as Gardasil 9 at age 9-14 years, according to an open-label randomized immunogenicity study conducted in 15 countries.

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This is reassuring news supportive of regulatory decisions made in 2016/2017 to license the more convenient two-dose schedule in the United States, European Union, Canada, and other countries, Rosybel Drury, PhD, observed in reporting the results at the annual meeting of the European Society for Paediatric Infectious Diseases.

Moreover, HPV type-specific antibody levels at 36 months post vaccination in boys and girls who received two doses were similar to or greater than in the 36-month follow-up of adolescent and young adult females who received three doses at ages 16-26 years.

“This result supports the bridging of efficacy findings in young women receiving three doses to girls and boys receiving two doses,” according to Dr. Drury, a vaccine scientist at Merck Sharp & Dohme in Lyon, France.

She presented an update of a five-cohort study including roughly 1,500 recipients of either two doses of the 9-valent HPV vaccine given 6 or 12 months apart or three doses administered at 0, 2, and 6 months. Four cohorts were composed of 9- to 14-year-olds. The fifth consisted of adolescent girls and young women who got three doses of Gardasil 9 over the course of 6 months at ages 16-26 years. The previous report from this major study provided only short-term data based upon measurements of immunogenicity obtained 1 month after the last dose of vaccine (JAMA. 2016 Dec. 13;316(22):2411-21).

Anti-HPV geometric mean titers were highest 1 month after completing a two- or three-dose series, dropped off sharply during the next 6-12 months, then declined more slowly through 36 months.

While the demonstration of persistent immunogenicity over 3 years of follow-up was reassuring overall, there was a potential hitch: Significantly lower antibody levels for some HPV types were observed in girls who received two doses of the vaccine than in those who got three. Specifically, levels of anti-HPV antibodies to HPV types 18, 31, 45, and 52 were significantly lower in the girls who got two doses of the 9-valent HPV vaccine than in those who got three doses at the same age. Antibody levels directed against HPV types 6, 11, 16, 33, and 58 were similar in the two patient populations.

“The clinical significance of this finding remains unknown,” Dr. Drury said.

Challenged as to why the study didn’t include a single-dose vaccine arm, which would be the preferred preventive strategy in low-income countries where the burden of anogenital cancers and warts due to HPV is greatest, she replied that while there is great interest in this approach, and some modeling studies suggest it would be beneficial, the study she presented was designed to evaluate immunogenicity over time, not clinical efficacy, which needs to be assessed before single-dose public health programs are implemented.

The study was funded by Merck and presented by a company employee.

bjancin@mdedge.com

Immunogenicity of two-dose Gardasil 9 persists at 36 months