Descriptive Evaluation of Measles Post-Exposure Prophylaxis Administration and Failure During Two 2011 Outbreaks In Pennsylvania

Background: Measles is a transmissible, vaccine-preventable disease. In Pennsylvania, documented measles immunity includes birth before 1957, physician-diagnosed measles disease, >1 documented dose of MMR vaccine or serologic evidence.  Post-exposure prophylaxis (PEP) is recommended for exposed non-immune persons, using MMR or Immuneglobulin (IG) within 72 hours or 6 days, respectively.

Objectives: To review contact tracing, PEP administration and failures during two Pennsylvania outbreaks.

Methods: Investigators identified locations where infectious measles patients were present and evaluated immunity statuses of identified contacts. PEP timeliness was defined as a binary variable in accordance with current recommendations. PEP failures were defined as persons who received timely PEP and subsequently developed measles. PEP failure rates were calculated by dividing reported failures by total administered doses. Transmission following PEP failure was assessed. Analysis was conducted using SAS 9.2.  

Results: Outbreak A included 6 confirmed cases. Of 290 identified contacts, 48% were considered immune. Fifty-four contacts received PEP (4 MMR, 50 IG); 96% were timely (MMR: 75%, IG: 98%). No MMR failures were identified. One IG failure (2% failure) was reported in a day 6 post-exposure recipient. Outbreak B included 4 confirmed cases. Of 387 identified contacts, 70% were considered immune. Forty-three contacts received PEP (11 MMR, 32 IG); 93% were timely (MMR: 82%, IG: 97%). One MMR failure (9% failure) was reported, in a day 2 post-exposure recipient. No IG failures were identified. Two additional cases occurred among index case household contacts who received MMR on post-exposure day 7.  

Conclusions: Two outbreaks involving many susceptible contacts were investigated. Among 677 exposed persons, 97 (14%) received PEP; 95% of doses were timely. Two PEP failures were reported (2% failure). Further transmission was not identified. These findings demonstrate pockets of under-immunized populations exist in the United States, and emphasize the importance of prompt identification and follow-up of susceptible exposed contacts during measles investigations.