If 3 or more cases of meningococcal disease have occurred in either an organization or community-based outbreak in a period <= 3 months starting from the time of the first case, a primary disease attack rate should be calculated.
attack rate per 100,000 =
= ((number of definite meningococcal disease cases during a 3 month period) + number of probable meningococcal disease cases during a 3 month period)) / (number of people in population at risk) * (100,000)
where:
• Secondary cases should be excluded.
• Co-primary sets should be counted as one case.
• All cases with culture isolates should be serogrouped.
Interpretation:
• If the attack rate is > 10 cases per 100,000 persons, then vaccination of the population at risk should be considered.
• For many organizations, the occurrence of 3 cases is sufficient to consider vaccination. In some situations vaccination may be started after only 2 cases have been identified.
• Vaccination may be of little use if serogroup B is the pathogen being spread.
• If all of the cases occurred in a short period and a period of several weeks have passed since without additional cases, then vaccination may not be warranted.
