Scaling-up the delivery of dog vaccination campaigns against rabies in Tanzania

Abstract

An increasing number of countries are committing to meet the global target to eliminate human deaths from dog-mediated rabies by 2030. Mass dog vaccination is central to this strategy. To interrupt rabies transmission from dogs to humans, the World Health Organization recommends that vaccination campaigns should be carried out every year in all dog-owning communities vaccinating 70% of their susceptible dogs. Monitoring and evaluation of dog vaccination campaigns are needed to measure progress towards elimination. In this study, we measured the delivery performance of large-scale vaccination campaigns implemented in 25 districts in south-east Tanzania from 2010 until 2017. We used regression modelling to infer the factors associated with, and potentially influencing the successful delivery of vaccination campaigns. During 2010-2017, five rounds of vaccination campaigns were carried out, vaccinating in total 349,513 dogs in 2,066 administrative vaccination units (rural villages or urban wards). Progressively more dogs were vaccinated over the successive campaigns. The campaigns did not reach all vaccination units each year, with only 16-28% of districts achieving 100% campaign completeness (where all units were vaccinated). During 2013-2017 when vaccination coverage was monitored, approximately 20% of vaccination units achieved the recommended 70% coverage, with average coverage around 50%. Campaigns were also not completed at annual intervals, with the longest interval between campaigns being 27 months. Our analysis revealed that districts with higher budgets generally achieved higher completeness, with a twofold difference in district budget increasing the odds of a vaccination unit being reached by a campaign by slightly more than twofold (OR: 2.29; 95% CI: 1.69-3.09). However, higher budgets did not necessarily result in higher coverage within vaccination units that were reached. We recommend national programs regularly monitor and evaluate the performance of their vaccination campaigns, so as to identify factors hindering their effective delivery and to guide remedial action.

Fig 1. The study districts in south-east Tanzania where mass dog vaccinations and post-vaccination transects were undertaken.

Shapefiles to make this figure were downloaded from the Diva GIS data portal (https://www.diva-gis.org/).

Fig 2

(a) The number of dogs vaccinated and overall coverage across the study area from 2010 to 2017 and (b) detail on the number of dogs vaccinated in each district. Coverage was calculated from the monthly estimated total number of vaccinated dogs divided by the estimated monthly total dog population estimate (blue line). The number of dogs vaccinated in each district each month ranged from 0 to 13 thousand dogs, indicated by the “heat intensity” of the colours from white to red (maximum).

Fig 3. Coverage achieved in the five rounds of vaccination campaigns at the level of the vaccination unit.

Coverage for rounds 1 and 2 was calculated using projected dog population estimates, while coverage for rounds 3–5 coverage were estimated as explained in the Methods. Darker green shading corresponds to higher coverage while white shading indicates that no vaccination campaign was undertaken. Hatched represents forest reserves or wildlife-protected areas. Shapefiles to make this figure were downloaded from the Diva GIS data portal (https://www.diva-gis.org/).

Fig 4. Coverage achieved during campaigns in each district from 2013 to 2017 (third to fifth round of vaccinations) calculated directly from transects without accounting for the pup:adult ratio (PAR) and excluding villages without transect data.

Black diamonds represent mean coverage across the three rounds. The black dashed line represents the target 70% vaccination coverage threshold.

Fig 5. The campaign timeliness in each of the five regions from 2010 to 2017.

The target coverage (P_target) of 70% was not achieved in all regions (upper red line), and between campaigns coverage declined due to dog population turnover. When annual campaigns achieved high coverage, coverages were sustained above the critical immunity threshold (below the red line labelled P_crit) for approximately 12 months. The time lag between campaigns (up to >20 months), caused coverage to decline below P_crit. Mean coverage across all district in the regional was calculated from the number of vaccinated dogs in each region divided by the number of estimated dogs in each region.

Fig 6. Required vaccination campaign intervals to prevent coverage falling to critical levels.

For the populations in Tanzania, 66% coverage should maintain herd immunity above P_crit of 30% for 12 months, assuming the use of high-quality vaccines (as used in this study) where the average duration of vaccine-induced immunity is 3 years.