Measels vaccine is given to the infant at the age of :

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Accepted Answer

The correct option is 4:
The correct age for the Measles-containing vaccine (MCV), often referred to simply as the Measles vaccine, is typically set at 9 months as per the National Immunization Schedule (NIS) in India and aligned with standard global recommendations, although the window is often specified as 9–12 months. The primary reason for waiting until this age is a concept known as **maternal antibody interference**. A newborn receives passive immunity from the mother through the placenta, specifically Immunoglobulin G (IgG) antibodies. These maternal antibodies protect the infant from measles during the first few months of life, but they also interfere with and neutralize the live attenuated measles vaccine, preventing the baby from mounting a proper immune response. By the age of 9 months, the level of these circulating maternal antibodies is expected to have dropped sufficiently so that the vaccine can successfully stimulate the infant's immune system, leading to long-lasting active immunity. Administering the vaccine at 9 months provides the best balance between timely protection and overcoming maternal interference. A second dose (often given as MMR or MCV) is generally recommended later, usually around 16–24 months, to provide coverage for the small percentage of children who failed to develop immunity after the first dose.

The incorrect option is 1: At birth
The measles vaccine is not given to an infant at birth. This period is reserved for vaccines that are safe and effective even in the presence of maternal antibodies or those needed immediately to prevent early life infections, such as the BCG vaccine (against Tuberculosis), Hepatitis B (HB-1), and the 'zero' dose of Oral Polio Vaccine (OPV-0). Administering the live measles vaccine at birth would be ineffective because the high concentration of maternal antibodies, which were transferred across the placenta, would rapidly bind to and destroy the weakened vaccine virus before it could stimulate the baby's immune cells. Therefore, vaccinating at birth would lead to primary vaccine failure, leaving the infant unprotected once the temporary maternal antibodies naturally wane. This is a critical point of difference in vaccination scheduling principles.

The incorrect option is 2: $1\frac{1}{2}$ months
The age of $1\frac{1}{2}$ months (or 6 weeks) is also too early for the primary dose of the measles vaccine. At this point, the concentration of protective maternal antibodies is still quite high, which would, as explained before, inactivate the live vaccine virus. The immunization schedule at 6 weeks is heavily focused on initiating the primary series of other crucial vaccines, which commonly include the first doses of Pentavalent/DPT (Diphtheria, Pertussis, Tetanus, Hepatitis B, and Haemophilus influenzae type b) and the Oral Polio Vaccine (OPV-1). These early vaccines are typically either killed/inactivated or are sufficiently powerful to bypass or overcome any residual maternal antibody interference, which is not the case for the live attenuated measles vaccine. Delaying the measles vaccine is a necessary public health strategy to ensure its efficacy.

The incorrect option is 3: $2\frac{1}{2}$ months
Similarly, $2\frac{1}{2}$ months (or 10 weeks) is not the recommended age for the measles vaccine. This period continues the primary vaccination series, typically including the second dose of the Pentavalent/DPT vaccine and OPV-2. The fundamental physiological reason for avoiding the measles vaccine at this age remains the risk of maternal antibody neutralization, making the vaccine largely ineffective. The strategy in infant immunization is to time each vaccine perfectly: some are needed immediately at birth, others are started early in the primary series as soon as possible after the maternal antibody threat is low (like DPT/Pentavalent), and the measles vaccine is specifically reserved until the passive immunity has faded, hence the selection of 9 months for the first dose.