Secondary Attack Rate Calculation

• Total under-five children = 100
• Vaccinated with 2 doses (60% coverage) = 60 children
• Assuming 100% protection rate → all 60 are immune
• Susceptible (unprotected) children = 100 - 60 = 40

SAR = (Number of exposed/susceptible persons developing disease within the incubation period / Total number of exposed "susceptible" contacts) × 100 The primary case is excluded from both the numerator and the denominator.

• The primary (index) measles case is one of the 40 susceptible children, so it must be removed from the denominator: 40 - 1 = 39 susceptible contacts
• Secondary cases (those developing measles following the primary case) = 26

Comment

• Vaccine coverage of 60% is far below the herd immunity threshold for measles (~93-95%), because measles is one of the most contagious diseases known (R0 approximately 12-18). Even assuming perfect (100%) vaccine efficacy, leaving 40% of children susceptible is more than enough to sustain an outbreak once an index case is introduced.
• The near two-thirds attack rate among the 39 susceptible contacts is consistent with what is expected when a highly infectious pathogen spreads through a largely unprotected pool of close contacts (household/playmate-level exposure in under-fives), similar in magnitude to the textbook illustrative example (2/3 = 66.6% SAR among susceptible family contacts).
• This outbreak illustrates why measles elimination programs target >95% two-dose coverage: partial coverage (60%) still permits explosive secondary transmission.
• Public health response should include: identifying and isolating cases, conducting a mopping-up/catch-up immunization campaign in the village for all susceptible under-fives (and any older unimmunized children), case-based surveillance, and vitamin A supplementation for cases per national guidelines, along with root-cause analysis of why routine coverage stalled at 60% (access, hesitancy, cold-chain issues, missed second dose).