Community intervention of a single-dose or 2-dose regimen of bivalent human papillomavirus vaccine in schoolgirls in Thailand: vaccine effectiveness 2 years and 4 years after vaccination.

Background: With accumulating evidence of single-dose human papillomavirus (HPV) vaccine efficacy in young women, we conducted a community vaccine effectiveness study comparing HPV single-dose and 2-dose regimens (0 and 6 months) of a bivalent HPV vaccine among grade 8 schoolgirls (aged 13-14 years) in Thailand.

Methods: In 2018, eligible grade 8 schoolgirls in Udon Thani (single dose) and Buri Ram (2 doses) provinces were offered HPV vaccine per assigned dose regimen. Concurrently, a cross-sectional survey for measuring baseline HPV prevalence was conducted in grade 10 (n = 2600) and grade 12 unvaccinated schoolgirls (n = 2000) in each province. HPV infection was assessed in first-void urine samples, tested by DNA polymerase chain reaction on the cobas 4800 system (Roche Molecular Diagnostics, Pleasanton, CA). All samples positive on the cobas system and an equal number of negative samples were also tested by Anyplex II HPV28 Detection (Seegene, Seoul, South Korea). The surveys were repeated in 2020 and 2022, when vaccinated grade 8 schoolgirls reached grade 10, and then subsequently grade 12, respectively. Vaccine effectiveness was estimated by comparing the weighted prevalence of HPV-16 or HPV-18 between grade-matched unvaccinated schoolgirls on the baseline survey (2018) and vaccinated schoolgirls in the year-2 (2020) and year-4 (2022) surveys. Adjustment methods were used in the analysis to account for potential differences in sexual behavior due to the noncontemporaneous comparison.

Results: The prevalence of HPV-16 and HPV-18 on the baseline survey among unvaccinated grade 10/grade 12 schoolgirls was 2.90% (95% confidence interval [CI] = 2.54% to 3.31%)/3.98% (95% CI = 3.52% to 4.49%) for Udon Thani and 3.87% (95% CI = 3.46% to 4.34%)/6.13% (95% CI = 5.56% to 6.75%) for Buri Ram. On the year-2 survey, the prevalence among vaccinated grade 10 schoolgirls was 0.57% (95% CI = 0.42% to 0.77%) for Udon Thani and 0.31% (95% CI = 0.21% to 0.47%) for Buri Ram. The 2-year postvaccination crude vaccine effectiveness for the single-dose regimen was estimated at 80.4% (95% CI = 73.9% to 86.9%), and for the 2-dose regimen at 91.9% (95% CI = 88.5% to 95.4%). On the year-4 survey, the prevalence among vaccinated grade 12 schoolgirls was 0.37% (95% CI = 0.25% to 0.56%) for Udon Thani and 0.28% (95% CI = 0.18% to 0.45%) for Buri Ram. Four-year postvaccination crude vaccine effectiveness for the single-dose regimen was estimated at 90.6% (95% CI = 86.6% to 94.6%) and for the 2-dose regimen was estimated at 95.4% (95% CI = 93.2% to 97.6%). All adjustment methods minimally affected vaccine effectiveness for the single-dose and 2-dose regimens. At 4 years after vaccination, the difference in crude vaccine effectiveness between the single-dose and 2-dose regimens was ‒4.79% (95% CI = ‒9.32% to ‒0.25%), meeting the study's noninferiority criteria.

Conclusions: Our study demonstrated that both single-dose and 2-dose HPV vaccination significantly decreased HPV-16/18 point prevalence 2 years and 4 years after vaccination. Crude vaccine effectiveness at 4 years after vaccination was greater than 90% for both the single-dose and 2-dose regimens; the single-dose regimen was not inferior to the 2-dose regimen. These data show that a single dose of HPV vaccine provides high levels of protection when administered to schoolgirls younger than 15 years of age.