Immunization and Vaccine-Preventable Diseases — MCQs

Immunization and Vaccine-Preventable Diseases Indian Medical PG Practice Questions and MCQs

Question 121: What is the efficacy of the BCG vaccine for pulmonary tuberculosis?

A. 0% (Correct Answer)
B. 25%
C. 33%
D. 50%

Explanation: **Explanation:** The efficacy of the BCG (Bacillus Calmette-Guérin) vaccine is one of the most debated topics in preventive medicine due to its wide variability across different geographical regions. In the context of **Pulmonary Tuberculosis (TB)**, particularly in adults and in countries like India, multiple trials (most notably the Chingleput trial) have shown that the protective efficacy of BCG is **0%**. **Why the correct answer is 0%:** While BCG is highly effective against severe childhood forms of TB (disseminated and meningeal TB), it provides little to no protection against the adult pulmonary form of the disease. Factors contributing to this 0% efficacy include interference by environmental mycobacteria, genetic variations in the vaccine strain, and the nature of the immune response required to prevent reactivation of latent TB. **Analysis of Incorrect Options:** * **Options B, C, and D (25%, 33%, 50%):** While some international studies have shown varying degrees of protection (ranging from 0 to 80%), for the purpose of standard medical examinations and based on the landmark Indian trials, the efficacy for pulmonary TB is specifically cited as zero. These percentages do not represent the established consensus for pulmonary protection in endemic regions. **High-Yield Clinical Pearls for NEET-PG:** * **Protective Efficacy:** BCG is **80% effective** against TB Meningitis and Miliary TB in children. * **Strain:** The most common strain used in India is **Danis 1331**. * **Administration:** Given **Intradermally** (0.05 ml for neonates <1 month; 0.1 ml for infants >1 month) using an **Omega syringe**. * **Phenomenon:** BCG is the only vaccine that leaves a permanent scar (at the insertion of the deltoid). * **Direct BCG:** Refers to giving the vaccine without a prior Mantoux test (standard practice in the National Immunization Schedule).

Question 122: Herd immunity plays an important role in the prevention of which of the following diseases EXCEPT?

A. Poliomyelitis
B. Measles
C. Tetanus (Correct Answer)
D. Diphtheria

Explanation: **Explanation:** **1. Why Tetanus is the Correct Answer:** Herd immunity (community immunity) occurs when a large portion of a population is immune to an infectious disease, thereby reducing the likelihood of person-to-person transmission and protecting susceptible individuals. **Tetanus** is the classic exception to this rule because it is **not a communicable disease**. The causative agent, *Clostridium tetani*, is found in the environment (soil, dust, manure) and enters the body through contaminated wounds. Since the disease does not spread from human to human, the immunity of one’s neighbors provides zero protection to an unimmunized individual. Protection against tetanus is strictly dependent on individual active immunization. **2. Why the Other Options are Incorrect:** * **Poliomyelitis:** Spread via the feco-oral route. High vaccine coverage (especially with OPV) reduces the wild virus reservoir in the community, protecting the unvaccinated. * **Measles:** One of the most contagious human-to-human diseases. It requires a very high herd immunity threshold (~95%) to stop outbreaks. * **Diphtheria:** Spread via respiratory droplets. Immunization reduces the carrier state and limits the spread of the toxigenic *Corynebacterium diphtheriae* within a population. **3. NEET-PG High-Yield Pearls:** * **Herd Immunity Threshold:** The proportion of the population that must be immune to stop transmission (calculated as $1 - 1/R_0$). * **Diseases where Herd Immunity does NOT apply:** Tetanus and Rabies (since they are not transmitted person-to-person). * **Environmental Reservoir:** If a disease has an environmental reservoir (like Tetanus) or an animal reservoir, herd immunity is generally ineffective at achieving eradication. * **Key Concept:** Herd immunity protects the "susceptible" (e.g., infants too young for vaccines or immunocompromised individuals) by breaking the chain of transmission.

Question 123: Which strain is used for the BCG vaccine?

A. Edmonston Zagreb strain
B. Oka strain
C. Danish 1331 (Correct Answer)
D. RA 27/3 strain

Explanation: **Explanation:** The **BCG (Bacillus Calmette-Guérin)** vaccine is a live attenuated vaccine derived from *Mycobacterium bovis*. In India and most international programs, the **Danish 1331 strain** is the standard strain used for manufacturing the vaccine. It is preferred due to its consistent immunogenicity and safety profile. **Analysis of Options:** * **Danish 1331 (Correct):** This is the specific sub-strain of *M. bovis* used globally for BCG production. It is administered intradermally (usually over the left deltoid) to provide protection against severe forms of childhood tuberculosis, such as TB meningitis and miliary TB. * **Edmonston Zagreb strain:** This strain is used for the **Measles** vaccine. It is a live attenuated strain commonly used in the Indian Universal Immunization Programme (UIP). * **Oka strain:** This is the live attenuated strain used for the **Varicella (Chickenpox)** and Zoster vaccines. * **RA 27/3 strain:** This is the human diploid cell-adapted strain used for the **Rubella** vaccine (the "R" in MMR). **High-Yield Clinical Pearls for NEET-PG:** * **Diluent:** BCG is a freeze-dried vaccine; it must be reconstituted only with **Normal Saline (0.9% NaCl)**. Using distilled water can cause irritation and inflammation. * **Site & Method:** Left upper arm, **Intradermal** (using a tuberculin/Omega syringe). * **Evolution of the BCG Lesion:** Papule (2-3 weeks) → Glazed ulcer (5-6 weeks) → Permanent **pitted scar** (6-12 weeks). * **Storage:** It is highly light-sensitive. Once reconstituted, it must be used within **4 hours** or discarded.

Question 124: What is the effectiveness of the cholera vaccine for 12 months?

A. 20%
B. 50% (Correct Answer)
C. 20%
D. 50%

Explanation: ### Explanation **Correct Answer: 50%** **1. Understanding the Concept** The effectiveness of the Oral Cholera Vaccine (OCV) is a high-yield topic for NEET-PG. Currently, the WHO recommends killed whole-cell vaccines (like Shanchol and Euvichol). These vaccines provide significant protection in the short term, but the efficacy wanes over time. * **Immediate protection:** In the first 6 months, the efficacy is approximately **85%**. * **12-month protection:** By the end of the first year, the cumulative efficacy drops to approximately **50-60%**. * **Long-term:** Protection persists at about 40% for up to 3 years, after which a booster is generally required. Therefore, for a 12-month timeframe, 50% is the standard accepted value in public health literature. **2. Analysis of Options** * **Options A & C (20%):** This value is too low. While the parenteral (injectable) cholera vaccine—which is no longer recommended—had very poor efficacy and short duration, modern oral vaccines maintain much higher protection than 20% within the first year. * **Options B & D (50%):** This represents the median protective efficacy at the one-year mark. It reflects the rapid decline from the initial 85% post-vaccination. **3. NEET-PG High-Yield Pearls** * **Vaccine Type:** Modern OCVs are **Killed Whole-cell** vaccines. The old parenteral vaccine is obsolete due to low efficacy and high reactogenicity. * **Schedule:** Usually administered in **2 doses**, 14 days apart (minimum interval). * **Age:** Can be given to children as young as **1 year** (Shanchol/Euvichol) or **2 years** (Dukoral). * **Herd Immunity:** OCVs are unique because they provide significant herd protection, reducing transmission even among the unvaccinated in a community. * **Control Strategy:** Vaccination should always be a **supplement** to (not a replacement for) improved water, sanitation, and hygiene (WASH) practices.

Question 125: All of the following are contraindicated during pregnancy except?

A. Measles vaccine
B. Mumps vaccine
C. HPV vaccine
D. Rabies vaccine (Correct Answer)

Explanation: **Explanation:** The core principle in obstetric immunization is that **Live Attenuated Vaccines** are generally contraindicated during pregnancy due to the theoretical risk of the vaccine virus crossing the placenta and causing fetal infection. Conversely, **Inactivated (Killed) Vaccines** and Toxoids are considered safe when the benefits outweigh the risks. **Why Rabies Vaccine is the Correct Answer:** Rabies is a 100% fatal disease. The Rabies vaccine is an **inactivated (killed) virus vaccine**. Because there are no documented adverse fetal effects and the maternal risk of death from rabies is absolute following a category II or III exposure, pregnancy is **not** a contraindication for post-exposure prophylaxis (PEP). It can be safely administered along with Rabies Immunoglobulin (RIG). **Analysis of Incorrect Options:** * **Measles (A) and Mumps (B):** These are components of the MMR vaccine, which is a **Live Attenuated Vaccine**. There is a theoretical risk of congenital infection; therefore, they are strictly contraindicated. Women are advised to avoid pregnancy for at least 28 days after receiving these vaccines. * **HPV Vaccine (C):** While it is a recombinant (non-live) vaccine, it is currently **not recommended** during pregnancy due to a lack of sufficient safety data. If a woman becomes pregnant after starting the series, the remaining doses should be postponed until postpartum. **High-Yield Clinical Pearls for NEET-PG:** * **Safe in Pregnancy:** Tdap (recommended in every pregnancy), Influenza (Inactivated), Hepatitis B, and Rabies. * **Contraindicated:** MMR, Varicella, Yellow Fever, and BCG (all are live). * **Exception:** Yellow Fever vaccine may be given to a pregnant woman if travel to an endemic area is unavoidable and the risk of infection outweighs the risk of vaccination. * **Rule of Thumb:** If the question asks for a vaccine that *must* be given regardless of pregnancy status due to life-threatening risk, the answer is almost always **Rabies**.

Question 126: Which of the following statements about tetanus is incorrect?

A. Tetanus protection requires booster doses every 5 years if previously immunized.
B. Herd immunity is present for tetanus. (Correct Answer)
C. Tetanus cannot be eradicated.
D. Elimination of tetanus is defined as less than 1 case per 1,000 live births.

Explanation: **Explanation:** **1. Why Option B is the Correct Answer (Incorrect Statement):** Herd immunity occurs when a large portion of a population is immune to an infectious disease, thereby providing indirect protection to those who are not immune by reducing the chain of transmission. **Tetanus is the only vaccine-preventable disease for which herd immunity does not exist.** This is because *Clostridium tetani* is an environmental saprophyte found in soil and animal feces; it is not transmitted from person to person. Vaccination protects only the individual immunized and does not reduce the risk of exposure for others. **2. Analysis of Other Options:** * **Option A:** While the primary series provides initial immunity, antitoxin levels decline over time. For sustained protection, booster doses are recommended every 10 years, but in high-risk scenarios or specific national guidelines, a 5-year interval is often cited for maintaining optimal titers. * **Option C:** Tetanus **cannot be eradicated** because the spores are ubiquitous in the environment (soil). Eradication is only possible for diseases with an exclusive human reservoir (e.g., Smallpox, Polio). * **Option D:** The WHO defines the **Elimination of Neonatal Tetanus** as an incidence of **less than 1 case per 1,000 live births** in every district of a country. India achieved this milestone in 2015. **High-Yield Clinical Pearls for NEET-PG:** * **Type of Immunity:** Tetanus toxoid provides **Active Artificial Immunity**. * **Neonatal Tetanus:** Also known as the "8th-day disease." * **Maternal Immunization:** Two doses of Td (Tetanus-diphtheria) are given during pregnancy to prevent neonatal tetanus via transplacental transfer of IgG antibodies. * **Portal of Entry:** Most common in India is through the umbilical cord stump (Neonatal) or injury/otitis media (Adult).

Question 127: Which vaccine is contraindicated in pregnancy?

A. Tuberculin (Correct Answer)
B. Typhoid
C. Influenza
D. HBV

Explanation: **Explanation:** The correct answer is **Tuberculin (Option A)**. However, it is important to clarify a technical distinction: Tuberculin (PPD) is a **diagnostic antigen**, not a vaccine. In the context of this question, it is contraindicated not because it is teratogenic, but because pregnancy can cause a transient suppression of cell-mediated immunity, leading to **false-negative results**. Furthermore, the BCG vaccine (the actual vaccine for TB) is a **Live Attenuated Vaccine**, and all live vaccines are generally contraindicated in pregnancy due to the theoretical risk of fetal infection. **Analysis of Options:** * **Typhoid (Option B):** The injectable Vi polysaccharide vaccine is an inactivated vaccine and can be administered if the risk of infection is high. (Note: The oral Ty21a live vaccine is contraindicated). * **Influenza (Option C):** The Inactivated Influenza Vaccine (IIV) is not only safe but **strongly recommended** for all pregnant women during the flu season to prevent maternal complications and provide passive immunity to the newborn. * **HBV (Option D):** Hepatitis B is a subunit (inactivated) vaccine. It is safe and indicated for pregnant women at high risk of infection. **NEET-PG High-Yield Pearls:** 1. **General Rule:** Live vaccines (e.g., MMR, Varicella, Yellow Fever, BCG) are **contraindicated** in pregnancy. 2. **The Exception:** Tetanus Toxoid (TT) or Tdap is **mandatory** during pregnancy to prevent Neonatal Tetanus. 3. **Killed/Inactivated Vaccines:** Generally considered safe (HBV, IPV, Rabies) if the benefit outweighs the risk. 4. **Post-exposure:** Rabies vaccine is never contraindicated if a pregnant woman is bitten by a suspected animal.

Question 128: To eradicate measles, what percentage of the population needs to be vaccinated at a minimum?

A. 70%
B. 80%
C. 85%
D. 95% (Correct Answer)

Explanation: ### Explanation **1. Why 95% is the Correct Answer:** The core concept here is **Herd Immunity Threshold (HIT)**. Measles is one of the most highly infectious diseases known, with a Basic Reproduction Number ($R_0$) typically ranging between **12 and 18**. This means a single infected individual can spread the virus to 12–18 susceptible people. To achieve "herd immunity" and effectively halt transmission (leading to eradication), the proportion of the population that must be immune is calculated by the formula: $HIT = 1 - (1/R_0)$. Given the high $R_0$ of measles, the mathematical threshold is approximately **92–95%**. In public health practice, a minimum of **95% coverage** with two doses of the Measles-Containing Vaccine (MCV) is required to create a "firebreak" that prevents outbreaks. **2. Why Other Options are Incorrect:** * **70% (Option A):** This is the threshold for diseases with much lower $R_0$ values (e.g., Diphtheria or Polio, where $R_0$ is 4–7). * **80% & 85% (Options B & C):** While these levels of coverage provide significant individual protection and reduce the burden of disease, they are insufficient to stop the circulation of the measles virus. At these levels, "pockets" of susceptible individuals remain large enough to sustain epidemic transmission. **3. High-Yield Clinical Pearls for NEET-PG:** * **Target Age:** Under the National Immunization Schedule (NIS) in India, MR vaccine is given at **9 completed months** (1st dose) and **16–24 months** (2nd dose). * **Vaccine Type:** Live attenuated (Edmonston-Zagreb strain). * **Outbreak Definition:** Five or more suspected cases in a cluster or one confirmed case. * **Vitamin A:** Always administered with the measles vaccine (1 lakh IU at 9 months; 2 lakh IU for subsequent doses) to reduce morbidity and mortality. * **Eradication vs. Elimination:** While India aims for **elimination** (zero transmission in a specific geographic area), **eradication** refers to global extinction of the pathogen. Only Smallpox has been eradicated to date.

Question 129: Which of the following statements is true about BCG vaccination?

A. Distilled water is used as diluent for BCG
B. The site of injection should be cleaned with spirit
C. Mantoux test becomes positive after 48 hours of vaccination
D. WHO recommends Danish 1331 strain for vaccine production (Correct Answer)

Explanation: **Explanation:** **Correct Answer: D. WHO recommends Danish 1331 strain for vaccine production** The BCG (Bacillus Calmette-Guérin) vaccine is a live attenuated vaccine derived from *Mycobacterium bovis*. To ensure global uniformity in potency and efficacy, the WHO recommends the **Danish 1331 strain** for vaccine production. This strain is known for providing a consistent immunological response. **Analysis of Incorrect Options:** * **Option A:** **Normal Saline (0.9% NaCl)** is the recommended diluent for BCG, not distilled water. Distilled water is hypotonic and can cause irritation or damage the live bacilli, leading to decreased vaccine effectiveness. * **Option B:** The injection site should **not** be cleaned with spirit or any antiseptic. Residual antiseptic can kill the live attenuated bacilli in the vaccine, rendering it ineffective. If the site is dirty, it should be cleaned with plain water and dried. * **Option C:** Post-vaccination tuberculin sensitivity (Mantoux conversion) does not happen immediately. It typically takes **8 to 12 weeks** for the Mantoux test to become positive, reflecting the development of cell-mediated immunity. **High-Yield NEET-PG Pearls for BCG:** * **Type:** Live attenuated vaccine (Freeze-dried form). * **Dose:** 0.05 ml for neonates (<1 month); 0.1 ml for infants (>1 month). * **Route:** Strictly **Intradermal** (Left deltoid) using an Omega/Tuberculin syringe. * **Storage:** 2°C to 8°C; must be used within 4–6 hours of reconstitution. * **Phenomenon:** It leaves a permanent scar. The sequence is: Papule (2-3 weeks) → Vesicle → Pustule → Shallow Ulcer → Scar (6-12 weeks). * **Protection:** Primarily protects against severe childhood forms of TB (Miliary TB and TB Meningitis), but has variable efficacy against adult pulmonary TB.

Question 130: Which of the following statements regarding tetanus is true?

A. Tetanus has no period of communicability. (Correct Answer)
B. Tetanus was eliminated from India in 2010.
C. Tetanus is also known as 'second day disease'.
D. A single dose of tetanus toxoid provides lifelong immunity.

Explanation: ### Explanation **1. Why Option A is Correct:** Tetanus is caused by the neurotoxin of *Clostridium tetani*. Unlike many infectious diseases, it is **not transmitted from person to person**. The bacteria enter the body through contaminated wounds (soil, street dust, or animal feces). Since there is no horizontal transmission between humans, it is said to have **no period of communicability**. **2. Why the Other Options are Incorrect:** * **Option B:** India was declared to have achieved **Maternal and Neonatal Tetanus Elimination (MNTE)** by the WHO in **May 2015** (officially July 2016), not 2010. Note that "elimination" in this context refers to <1 case per 1,000 live births in every district; tetanus can never be "eradicated" because the spores persist in the soil. * **Option C:** Neonatal tetanus is traditionally known as the **"8th-day disease"** because symptoms typically appear about a week after birth due to umbilical cord contamination. * **Option D:** Tetanus toxoid is not a live vaccine and does not provide lifelong immunity with a single dose. The primary schedule followed by periodic boosters (every 10 years) is required to maintain protective antibody levels. **3. High-Yield NEET-PG Pearls:** * **Incubation Period:** Typically 6–10 days. The shorter the incubation period, the worse the prognosis. * **Clinical Hallmark:** *Trismus* (lockjaw) is the most common presenting symptom, followed by *risus sardonicus* (facial spasms) and *opisthotonus* (backward arching). * **Herd Immunity:** Tetanus is the only vaccine-preventable disease where **herd immunity does not exist**, as the protection is purely individual. * **Type of Immunity:** Tetanus toxoid provides **active immunity**, while Tetanus Immunoglobulin (TIG) provides **passive immunity**.

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Principles of Immunization

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Types of Vaccines

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Universal Immunization Program

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Cold Chain System

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Vaccine Storage and Handling

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Adverse Events Following Immunization

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National Immunization Schedule

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Polio Eradication

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Measles Elimination

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Tetanus Control

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New and Underutilized Vaccines

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Vaccination Coverage Assessment

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