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BACKGROUND: During an outbreak of 58 measles cases in New York City (NYC) in 2013, measles, mumps, and rubella vaccine (MMR) or immunoglobulin (IG) was administered to exposed contacts according to Advisory Committee on Immunization Practices (ACIP) guidelines for measles post-exposure prophylaxis (PEP). Current literature on the effectiveness of measles PEP is limited and variable. We examined the effectiveness of measles PEP among susceptible persons exposed to measles during this outbreak.
METHODS: A total of 3,408 contacts were identified as part of routine NYC Department of Health and Mental Hygiene surveillance activities between March 13 and June 30, 2013. Documented immunization records and measles IgG serology results were reviewed. Contacts were considered susceptible at the time of exposure and included in the analysis if they were born during or after 1957, were unvaccinated and, if tested, were measles IgG negative. Non-susceptible contacts and contacts with 1 dose of measles-containing vaccine, unknown vaccination status, or unknown age were excluded. Administration of MMR within 3 days of exposure or IG within 6 days of exposure was considered valid measles PEP. PEP effectiveness was calculated as (1-RR)*100, where RR is the relative risk of developing measles among contacts who received PEP compared to those who did not receive PEP. For the analysis of MMR effectiveness, contacts who received IG were excluded; contacts under 6 months of age were also excluded to avoid bias due to maternal antibody interference with response to vaccination. For the analysis of IG effectiveness, contacts who received MMR were excluded. Effectiveness of any PEP (MMR or IG) among all contacts was also calculated. Analyses were conducted using SAS v9.2 statistical software.
RESULTS: Of the 3,408 contacts identified, 286 (8.4%), 352 (10.3%), and 396 (11.6%), contacts met the inclusion criteria for analysis of MMR, IG, and any (MMR or IG) PEP effectiveness, respectively. Of the contacts included in this analysis, 44 received MMR PEP and 77 received IG PEP. Effectiveness of MMR PEP was 79.2% (95% CI, 17.9, 94.8). No contact who received measles IG developed disease; therefore effectiveness of IG PEP with testing of significance could not be calculated. Effectiveness of receiving any PEP (MMR or IG) was 91.6% (95% CI, 66.0, 97.9).
CONCLUSIONS: Persons exposed to measles who received PEP were less likely to develop disease, supporting current recommendations for measles PEP. These results highlight the importance of intensive public health outbreak response efforts to limit measles transmission following case identification.