Hajj‐Associated Outbreak Strain ofNeisseria meningitidisSerogroup W135: Estimates of the Attack Rate in a Defined Population and the Risk of Invasive Disease Developing in Carriers

An outbreak of disease due to Neisseria meningitidis serogroup W135 (W135) occurred in 2000 and 2001 among pilgrims returning from the annual Islamic pilgrimage to Saudi Arabia (the Hajj) and in their contacts. For the Hajj in 2000, the attack rate of W135 disease was 25 cases per 100,000 pilgrims. After the introduction of quadrivalent meningococcal vaccine for the Hajj in 2001, no pilgrim developed W135 disease. The estimated attack rates for household contacts of returning pilgrims were 18 cases and 28 cases per 100,000 contacts for the years 2000 and 2001, respectively. On the basis of rates of transmission of W135 carriage and national epidemiological data, the risk that an unvaccinated household contact who had acquired W135 carriage would develop invasive meningococcal disease was estimated to be 1 case per 70 acquisitions. Public health policies to protect household contacts of Hajj pilgrims need to be implemented.