Effective HPV vaccination coverage in Australia by number of doses and two-dose spacing: What if one or two doses are sufficient?

BACKGROUND: Initially, three-dose schedules were recommended for vaccines against human papillomavirus (HPV); subsequently recommendations have been updated to a schedule of two doses delivered at least six (minimum five) months apart for those aged <15 years at dose 1. We aimed to re-estimate effective HPV vaccination coverage in Australia, considering reduced-dose schedules and possible one-dose effectiveness. We also aimed to identify which of the three school visits was most commonly missed amongst two-dose only recipients, to inform optimal timing of visits. METHODS: National vaccination register data were used to estimate: i) vaccination coverage at December 2017, either with a complete course (three or two sufficiently-spaced doses (>151 days apart)), or at least one dose; ii) for each birth cohort offered vaccination, the percentage of the initially targeted cohort with a complete course, or at least one dose (reflecting uptake at the time the vaccine was offered); and iii) among two-dose only recipients, the percentage who missed each of three school visits. RESULTS: Including those with two sufficiently-spaced doses increased end-2017 coverage by 1.3-2.8% points in those vaccinated at school. Including those with at least one dose increased coverage further, by 6.5-9.5% points, mostly due to including those receiving multiple too-closely-spaced doses. One-dose coverage reached 90.9% and 86.9% in females and males respectively born in 2002. Among those vaccinated at school who received only two doses, it was much more common to miss the first (31.0% females; 32.5% males) or the third visit in the school year (54.6% females; 48.6% males) than the second (14.1% females; 18.8% males). CONCLUSIONS: Including those with two sufficiently-spaced doses has a very modest impact on HPV vaccine coverage in Australia. If receiving at least one dose offers substantial protection, these data suggest that the school-based program is now achieving close to 90% coverage on