Measles vaccine given to contact of measles case exerts protective effect within –
**Core Concept**
The measles vaccine works by inducing **active immunity** against the measles virus, providing protection against the disease. This is achieved through the production of **antibodies** and **cell-mediated immunity**. The vaccine's effectiveness in preventing measles depends on the immune response it elicits.
**Why the Correct Answer is Right**
The measles vaccine can exert a protective effect within a certain timeframe when given to contacts of a measles case, typically within 72 hours. This is because the vaccine can still prevent the disease or modify its severity if administered shortly after exposure, before the onset of symptoms. The exact timeframe may vary, but the key principle is that early vaccination can intercept the virus's replication cycle, aided by **interferon** and other immune mechanisms.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately reflect the timeframe within which the measles vaccine can exert a protective effect.
**Option B:** Similarly, this option is incorrect as it does not align with the known immune response timeframe.
**Option D:** This option is also incorrect as it suggests a timeframe that is too long for the vaccine to effectively prevent measles after exposure.
**Clinical Pearl / High-Yield Fact**
A crucial point to remember is that the measles vaccine is most effective when administered before exposure or shortly after, ideally within 72 hours, to contacts of a measles case. This knowledge is vital for **outbreak control** and **public health** measures.
**Correct Answer:** C. 72 hours