A good measure of the efficacy of vaccines should be based on an index that standardizes the exposure to infection of those who have been immunized and those who have not. In addition, the measure should not be influenced by indirect effects. In the case of outbreaks directly transmitted agents that cause acute infections diseases, the household secondary rates of attack are the best indicator of vaccine efficacy provided that the data are collected at the household level. If the data are not collected in this manner, the best indicator will be the estimated probability of transmission, even though this estimator and its meaning less obvious than the secondary attack of rate. Other strategies to estimate the efficacy of an immunization are more appropriate for situations that are more complex than outbreaks caused by acute infectious agents.
Longini IM Jr., Haber MJ, Halloran ME. (1990). [Direct and indirect effects of vaccines: an annotation on the estimation of the vaccine efficacy from outbreaks caused by acute infection agents such as measles]. Boletin medico del Hospital Infantil de Mexico, 47(7)