The effective contact rate is the product of which two quantities?

• A. The product of contact rate and probability of transmission
• B. The product of incubation period and contact rate
• C. The product of reproduction number and immunity
• D. The product of case fatality rate and duration of illness

Answer: (A)

The speed at which an infection spreads depends on two things: how often infectious people have meaningful contacts with others, and how likely each of those contacts is to transmit the pathogen. The effective contact rate is the product of the contact rate (how many potential transmission contacts occur per unit time) and the probability of transmission per contact.

For example, if an infectious person has 8 relevant contacts per day and each contact has a 0.02 chance of transmitting, the effective contact rate is 0.16 new infections per day per infectious person (assuming those contacts are with susceptible individuals). This multiplication captures how both more frequent contacts and higher transmission probability amplify spread.

The other options don’t describe transmission dynamics: the incubation period is time to symptom onset and doesn’t quantify transmission; the reproduction number times immunity is a broader metric that reflects population-level spread but isn’t the direct product describing how transmission per unit time occurs; the case fatality rate times duration relates to mortality, not transmission.