Which formula represents the secondary attack rate?

• A. (# of people exposed to a primary case who develop illness) / (Total # people susceptible after first wave)
• B. (# of people exposed to a primary case who develop illness) / (Total population)
• C. (# of people exposed to a primary case who develop illness) / (Total # people who were exposed)
• D. (# of secondary cases) / (Total population)

Answer: (A)

Secondary attack rate measures the risk to people who were actually exposed to a primary case, among those who are susceptible. It is calculated as the number of secondary cases among exposed contacts divided by the total number of susceptible contacts (those who were at risk after the first case). This matches the idea of counting how many of the people who could be infected after exposure became ill, out of everyone who was susceptible and exposed. The other options either use the whole population or all exposed people (including those not susceptible), which does not reflect the risk among those at risk after exposure.