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167 - MODELLING THE LONG-TERM PNEUMOCOCCAL TRANSMISSION DYNAMICS IN A CRCT OF THREE- AND REDUCED-DOSE PCV SCHEDULES IN A NAÏVE POPULATION IN VIETNAM (ID 883)

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Abstract

Background

A cRCT in 24 communes in Nha Trang, Vietnam investigated non-inferiority of reduced-dose PCV schedules (1p+1 and 0p+1) to 3-dose schedules (2p+1 and 3p+0). At the start of vaccination in Jan 2017, children <3y received catch-up doses of PCV. We investigated the role of the catch-up in trial outcomes measured in Oct 2020.

Methods

We used a dynamic meta-population transmission model to simulate pneumococcal transmission of VT and NVT in all arms simultaneously. Mixing rates between age-groups and trial arms were informed by population-specific data, and the model was fit to carriage prevalence estimates from annual cross-sectional carriage surveys in 1440 infants and 1440 toddlers, and their carers.

Results

In Oct 2020 VT carriage was substantially reduced in all intervention arms, meeting the primary endpoint (reported elsewhere). Continued PCV use in allocated schedules was projected to sustain reduction in VT prevalence of 83% (95% CrI: 51 - 100) in the 1p+1 and 90% (42 - 100) in the 0p+1 arms compared to pre-vaccination prevalence, and 96% (55 - 100) in the 2p+1 and 98% (62 - 100) in the 3p+0 arms. The model estimated low (<15%) probability of resurgence (return to trial year 3 levels) of VT carriage in the 1p+1 arm as protection from the catch-up campaign waned. The estimated duration of booster dose protection in 1p+1 was 0.5 (0.1 - 3.9) that of the booster in the 2p+1.

Conclusions

The similarity in impact of the schedules in year 3 and 4 of the trial were unlikely attributable to the catch-up campaign.

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