COS 192-2 - Evaluating Outbreak Response Immunization Campaigns with In-Outbreak Metrics

Background/Question/Methods

The primary goal of an outbreak response immunization (ORI) campaign is to limit the spread of the outbreak by decreasing the number of susceptible individuals. A secondary goal is to recognize potential gaps in the existing vaccination coverage to facilitate better targeted ORI campaigns. Evaluating the effectiveness of an ORI is challenging in the case of resurgent outbreaks in high vaccination settings. Without recent non-intervention outbreaks for comparison, quantitative metrics of ORI effectiveness become difficult to assess. Here we consider two metrics that can be used in the absence of an independent comparator outbreak. We calculate the ratio of age-specific attack rates before and after an ORI, or relative risk of infection (RR), and change in effective reproductive number (Rt) to evaluate the success of two distinct ORI campaigns during two-year measles outbreak in Mongolia between 2015 and 2016.

Results/Conclusions

The first ORI targeted children between 6-month-6-year-olds. The RR within the targeted age groups were significantly < 1 after the ORI; 95% CI [0.33,0.55], [0.22, 0.50], [0.27,0.50] for 6-month-1-year, 1-3-year, and 3-6-year-olds, respectively. The decrease in infection risk for targeted age-classes ranged between 50.9-64.1% after the ORI. The second ORI targeted adults between 18-years and 35-years old. The RR was significantly < 1 for adults between 18-21, 21-24, 24-27, and 27-30 (95% CI [0.03,0.61], [0.36,0.87], [0.19,0.77], [0.24, 0.89], respectively), but not for 30-35-year-old adults (95% CI [0.58, 1.04]). The reduction in risk of infection ranges between 18.7-67.9% for all targeted ages. Despite the success of the ORIs within targeted cohorts, there was no meaningful decrease in Rt associated with either ORI campaign since Rt was trending downward or below one at the time. Our results indicated that the child campaign was effective in reducing susceptibles within each cohort targeted, but not sufficient to stop a second outbreak. The adult campaign was not successful at reducing RR across all targeted age-classes. We find that there was a direct reduction in RR in target cohorts after both ORI campaigns, but no evidence that they provided a meaningful impact to non-target cohorts.