During this period only 1st generation vaccines were used; priorimmunity in individuals within the population (whether from pre-vious vaccination or exposure) may be underestimated; underlyingimmune issues of patients are unknown; specific descriptions (i.e.Rao’s classification) of the clinical presentation of index patientsand contacts that develop disease are not included; vaccine qualityand administration differences may be present as vaccine qualityprior to 1971 was largely uncontrolled and the bifurcated needlewas not recommended until 1968