Measles vaccine given to contact of measles case exces protective effect within-
Wait, the core concept here is the concept of post-exposure prophylaxis. Normally, vaccines are given before exposure to prevent infection. But in some cases, like after exposure to a contagious disease, giving the vaccine might still be effective. For measles, there's a window of time after exposure where the vaccine can prevent the disease if given promptly. That's probably what the question is testing.
So, the correct answer would be the time frame within which the vaccine is effective after exposure. I remember reading that if someone is exposed to measles, giving the vaccine within 72 hours (3 days) can still prevent the disease. That's a key point. The vaccine works by stimulating the immune system quickly enough to combat the virus before it takes hold.
Now, looking at the options (though they're not listed here), I need to think about common distractors. For example, maybe one option is 24 hours, another 5 days, another 10 days, etc. The correct answer is 72 hours. The incorrect options would be either too short or too long. For instance, 24 hours might be too soon for the vaccine to have an effect, while 14 days would be too late as the virus would have already caused infection.
The clinical pearl here is that post-exposure prophylaxis with the measles vaccine within 72 hours is effective. This is a high-yield fact for exams, as it's a classic example of a vaccine used post-exposure. Also, remember that the vaccine isn't 100% effective, but it can still prevent the disease in a significant number of cases if given in time. Another point is that if the person has already been immunized before, the risk of contracting measles after exposure is much lower, but the question here is about giving the vaccine after exposure to a non-immune contact.
**Core Concept**
Measles vaccine acts as post-exposure prophylaxis (PEP) by inducing rapid immune responses. It is most effective when administered within a specific timeframe after exposure to the measles virus, preventing clinical disease by stimulating cell-mediated and humoral immunity.
**Why the Correct Answer is Right**
The measles vaccine, when given **within 72 hours (3 days)** of exposure, can prevent clinical measles in over 90% of susceptible individuals. The live attenuated vaccine triggers rapid production of IgM and IgG antibodies, along with T-cell activation, which limits viral replication before it establishes infection. This window aligns with the virus’s incubation period (7–14 days), allowing the immune system to mount a response before symptomatic illness develops.
**Why Each Wrong Option is Incorrect**
**Option A:** 24 hours is too short; the immune response requires 3–5 days to develop sufficient protection.
**Option C:** 5 days exceeds the 72-hour window, as viral