**Core Concept**
Second attack rate is a measure used in epidemiology to describe the number of new cases of a disease that occur among a group of people who have been exposed to a specific risk factor or intervention, relative to the number of people who were not exposed. In the context of vaccine trials, it is used to assess the efficacy of a vaccine in preventing disease.

**Why the Correct Answer is Right**
The second attack rate is a measure of vaccine efficacy and is calculated as the number of cases in the vaccinated group divided by the number of cases in the unvaccinated group. A lower second attack rate in the vaccinated group indicates a higher vaccine efficacy. In a study, if the vaccine efficacy is 90%, the second attack rate would be lower in the vaccinated group compared to the unvaccinated group.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the concept of second attack rate.
* **Option B:** This option is incorrect because second attack rate is not a measure of vaccine efficacy in the context of a single dose.
* **Option C:** This option is incorrect because second attack rate is not a measure of the number of cases in a population.

**Clinical Pearl / High-Yield Fact**
A 90% vaccine efficacy means that 90% of vaccinated individuals will be protected against the disease, but 10% may still develop the disease despite being vaccinated.

**Correct Answer:** B.