Contribution of Contact Sampling in Increasing Sensitivity of Poliovirus Detection During A Polio Outbreak—Somalia, 2013

Background.  In May 2013, a wild poliovirus type 1 (WPV1) outbreak reported in Somalia provided an opportunity to examine the contribution of testing contacts to WPV detection. Methods.  We reviewed acute flaccid paralysis (AFP) case-patients and linked contacts reported in the Somalia Surveillance Database from May 9 to December 31, 2013. We restricted our analysis to AFP case-patients that had ≥3 contacts and calculated the contribution of each contact to case detection. Results.  Among 546 AFP cases identified, 328 AFP cases had ≥3 contacts. Among the 328 AFP cases with ≥3 contacts, 93 WPV1 cases were detected: 58 cases (62%; 95% confidence interval [CI], 52%-72%) were detected through testing stool specimens from AFP case-patients; and 35 cases (38%; 95% CI, 28%-48%) were detected through testing stool specimens from contacts, including 19 cases (20%; 95% CI, 14%-30%) from the first contact, 11 cases (12%; 95% CI, 7%-20%) from the second contact, and 5 cases (5%; 95% CI, 2%-12%) from the third contact. Among the 103 AFP cases with ≥4 contacts, 3 (6%; 95% CI, 2%-16%) of 52 WPV1 cases were detected by testing the fourth contact. No additional WPV1 cases were detected by testing >4 contacts. Conclusions.  Stool specimens from 3 to 4 contacts of persons with AFP during polio outbreaks are needed to maximize detection of WPV cases.