Herd Immunity and Population Protection — MCQs
You are the MO in charge in a PHC and there has been a plague epidemic in your area. Which of the following measures is the LEAST appropriate for controlling epidemic?
The primary mechanism by which herd immunity protects a population is by:
In a vaccine trial, relative risk is 0.2. What is the vaccine efficacy?
Assertion: VZV vaccine is live attenuated. Reason: It cannot be given to immunocompromised patients.
Which of the following vaccines is classified as a killed vaccine?
To achieve a Net Reproduction Rate (NRR) of 1, the couple protection rate should be
In a population of 5000, there are 19 % eligible couples. To achieve a couple protection rate (CPR) of 60 %, how many of these should be covered for family planning services?
Inactivated microorganisms are used in the manufacture of which of the following vaccines?
Who invented the smallpox vaccine?
Which of the following vaccines is NOT typically given for post-splenectomy infection prophylaxis?
Herd Immunity and Population Protection Indian Medical PG Practice Questions and MCQs
Question 1: You are the MO in charge in a PHC and there has been a plague epidemic in your area. Which of the following measures is the LEAST appropriate for controlling epidemic?
- A. Isolation of contacts
- B. Rapid treatment of cases with streptomycin
- C. Vaccination of all susceptible individuals (Correct Answer)
- D. Early diagnosis and notification
Explanation: ***Vaccination of all susceptible individuals*** - While beneficial for prevention, mass vaccination during an ongoing plague epidemic is **logistically challenging** and **too slow** to effectively control the immediate spread. - The incubation period of plague is short, and an effective vaccine often requires multiple doses and time to induce immunity, making it less suitable for **rapid epidemic containment**. *Isolation of contacts* - **Isolating contacts** helps prevent the disease from spreading further by separating potentially infected individuals who may be asymptomatic or in the incubation period. - This measure is crucial in **breaking chains of transmission** and limiting the epidemic's reach. *Rapid treatment of cases with streptomycin* - **Early and rapid treatment** of confirmed plague cases with effective antibiotics like **streptomycin** is a cornerstone of epidemic control, significantly reducing mortality and infectivity. - This intervention quickly minimizes the source of infection, preventing further transmission to others. *Early diagnosis and notification* - **Early diagnosis** allows for prompt initiation of treatment and isolation, while **notification** triggers public health responses such as contact tracing and surveillance. - These steps are essential for understanding the scope of the epidemic and implementing appropriate control measures efficiently.
Question 2: The primary mechanism by which herd immunity protects a population is by:
- A. Reducing transmission by decreasing susceptible individuals in the population (Correct Answer)
- B. Increases with increased vaccination
- C. Is related to vaccine efficacy
- D. Applies only to infectious diseases
Explanation: ***Reducing transmission by decreasing susceptible individuals in the population*** - **Herd immunity's primary mechanism** is reducing the proportion of susceptible individuals, which decreases the probability of transmission when an infected person encounters others - When a critical threshold of the population is immune (either through vaccination or natural infection), it **breaks the chain of transmission**, making epidemic spread unlikely - This mechanism **indirectly protects unvaccinated individuals** (infants, immunocompromised, vaccine non-responders) by reducing their exposure risk *Increases with increased vaccination* - While increased vaccination coverage **contributes to achieving** herd immunity, this describes a **contributing factor**, not the protective mechanism itself - Vaccination provides individual immunity, which collectively builds toward the herd immunity threshold *Is related to vaccine efficacy* - **Vaccine efficacy** influences the vaccination coverage threshold needed for herd immunity (higher efficacy = lower threshold needed) - This is a **determinant** of herd immunity requirements, not the mechanism by which it protects populations *Applies only to infectious diseases* - This describes the **scope of applicability** of herd immunity, not its mechanism - Herd immunity is specific to communicable diseases because protection depends on preventing **pathogen transmission** between individuals
Question 3: In a vaccine trial, relative risk is 0.2. What is the vaccine efficacy?
- A. 90%
- B. 80% (Correct Answer)
- C. 95%
- D. 20%
Explanation: ***80%*** - Vaccine efficacy is calculated as **(1 - Relative Risk) x 100%**. Given a relative risk of 0.2, the efficacy is (1 - 0.2) x 100% = **80%**. - This value represents the **proportionate reduction** in disease incidence in the vaccinated group compared to an unvaccinated group. *90%* - This would imply a relative risk of 0.1, as **(1 - 0.1) x 100% = 90%**. - The given relative risk of **0.2** does not correspond to 90% efficacy. *95%* - This would imply a relative risk of 0.05, as **(1 - 0.05) x 100% = 95%**. - The given relative risk of **0.2** does not correspond to 95% efficacy. *20%* - This value directly represents the **Relative Risk (RR)** itself, or an efficacy calculated incorrectly as RR x 100%. - Vaccine efficacy is a measure of reduction from the unvaccinated state, hence it is **1 - RR**.
Question 4: Assertion: VZV vaccine is live attenuated. Reason: It cannot be given to immunocompromised patients.
- A. Both true, reason doesn't explain assertion
- B. Assertion true, reason false
- C. Assertion false, reason true
- D. Both true, reason explains assertion (Correct Answer)
Explanation: ***Both true, reason explains assertion*** - The **VZV (varicella-zoster virus) vaccine** is indeed a **live attenuated vaccine** containing weakened virus - the assertion is **TRUE** - It **cannot be given to immunocompromised patients** due to risk of vaccine-strain disease - the reason is **TRUE** - The reason **directly explains the assertion**: BECAUSE the vaccine is live attenuated, it poses infection risk and therefore cannot be used in immunocompromised individuals - The **causal relationship** is clear: live attenuated nature → contraindication in immunocompromised patients *Both true, reason doesn't explain assertion* - While both statements are factually true, this option would only be correct if the reason was unrelated to the assertion - However, the reason **directly explains WHY** the live attenuated nature is clinically significant - The contraindication is a **direct consequence** of the vaccine being live attenuated, so the reason does explain the assertion *Assertion true, reason false* - The assertion is true (VZV vaccine is live attenuated) - However, the reason is also **TRUE** - live attenuated vaccines are indeed contraindicated in immunocompromised patients due to risk of disseminated vaccine-strain infection - Since both statements are true, this option is incorrect *Assertion false, reason true* - The assertion is **TRUE**, not false - VZV vaccine (Varivax, Zostavax) is a **live attenuated vaccine** containing the Oka strain - This option incorrectly claims the assertion is false - Since the assertion is factually correct, this option cannot be right
Question 5: Which of the following vaccines is classified as a killed vaccine?
- A. Varicella
- B. BCG
- C. OPV
- D. Meningococcal vaccine (Correct Answer)
Explanation: ***Meningococcal vaccine*** - The meningococcal conjugate and polysaccharide vaccines are **killed vaccines**, containing inactivated bacterial components (polysaccharides) that stimulate an immune response. - They provide protection against *Neisseria meningitidis* and are considered safe for most populations due to their non-live nature. *Varicella* - The varicella vaccine is a **live-attenuated vaccine**, meaning it contains a weakened form of the **varicella-zoster virus**. - This attenuated virus can replicate in the recipient, eliciting a strong and long-lasting immune response, similar to natural infection. *BCG* - The **Bacillus Calmette-Guérin (BCG)** vaccine is a **live-attenuated vaccine** used to prevent tuberculosis. - It contains a weakened strain of **_Mycobacterium bovis_**, which is closely related to *Mycobacterium tuberculosis* but has lost its virulence. *OPV* - The **Oral Polio Vaccine (OPV)** is a **live-attenuated vaccine** that contains weakened but live strains of all three poliovirus serotypes. - It induces strong mucosal immunity in the gut, which is crucial for preventing the wild poliovirus from replicating and spreading.
Question 6: To achieve a Net Reproduction Rate (NRR) of 1, the couple protection rate should be
- A. 60% (Correct Answer)
- B. 55%
- C. <50%
- D. 50%
Explanation: ***60%*** - A **Net Reproduction Rate (NRR)** of 1 indicates that each generation of women is exactly replacing itself, leading to zero population growth. - To achieve an NRR of 1, a **couple protection rate** of approximately **60%** is generally required, meaning a significant majority of couples use contraception. *55%* - While 55% is closer, it is typically considered slightly **below the threshold** needed to achieve an NRR of 1. - An NRR of 1 requires a higher proportion of couples to be actively using contraception. *50%* - A **50% couple protection rate** would generally lead to an NRR greater than 1, implying continued population growth. - This rate does not provide enough protection to balance births and deaths to reach replacement-level fertility. *<50%* - A couple protection rate of less than **50%** would likely result in an **NRR greater than 1**, indicating population growth. - This level of contraceptive use is insufficient to achieve replacement-level fertility.
Question 7: In a population of 5000, there are 19 % eligible couples. To achieve a couple protection rate (CPR) of 60 %, how many of these should be covered for family planning services?
- A. 550
- B. 530
- C. 590
- D. 570 (Correct Answer)
Explanation: ***570*** - First, calculate the total number of **eligible couples**: 19% of 5000 = (19/100) * 5000 = **950 couples**. - To achieve a **Couple Protection Rate (CPR) of 60%**, calculate 60% of the eligible couples: 60% of 950 = (60/100) * 950 = **570 couples**. *550* - This option indicates a protection rate of approximately **57.9%** (550/950 * 100), which is less than the target of 60%. - It does not meet the specified target for **Couple Protection Rate**. *530* - This option would result in a protection rate of approximately **55.8%** (530/950 * 100), which is significantly lower than the desired 60%. - This value is an underestimation of the number of couples needed to achieve the target CPR. *590* - This option indicates a protection rate of approximately **62.1%** (590/950 * 100), which exceeds the target of 60%. - While protecting more couples is generally good, the question asks for how many *should* be covered to achieve *60%* specifically, making 570 the exact answer.
Question 8: Inactivated microorganisms are used in the manufacture of which of the following vaccines?
- A. Salk vaccine (Correct Answer)
- B. Tetanus toxoid
- C. Sabin's oral vaccine
- D. All of the above
Explanation: **Explanation:** The core concept tested here is the classification of vaccines based on the state of the immunizing agent. **1. Why Salk Vaccine is Correct:** The **Salk vaccine (IPV - Inactivated Poliovirus Vaccine)** is a classic example of a **killed/inactivated vaccine**. In these vaccines, the microorganism (in this case, Poliovirus types 1, 2, and 3) is grown in culture and then killed using heat or chemicals (usually formaldehyde). While the virus can no longer replicate, its structural proteins remain intact to trigger an immune response, primarily inducing humoral immunity (IgG). **2. Why the other options are incorrect:** * **Tetanus Toxoid:** This is a **toxoid vaccine**, not an inactivated whole microorganism. It is prepared by detoxifying the exotoxin produced by *Clostridium tetani* using formalin. It induces immunity against the toxin rather than the bacteria itself. * **Sabin’s Oral Vaccine (OPV):** This is a **Live Attenuated Vaccine**. It contains weakened but live viruses that replicate in the gut to induce both mucosal (IgA) and systemic (IgG) immunity. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Killed Vaccines:** "**K**illed **P**olice **R**elieve **A**ll **I**nfluenza **B**y **T**yping" (**K**illed: **P**ertussis, **R**abies, **A**-Hepatitis A, **I**nfluenza, **B**-Hepatitis B [Recombinant], **T**yphoid [injectable]). * **Salk vs. Sabin:** Salk (IPV) is safer for immunocompromised individuals as there is zero risk of Vaccine-Associated Paralytic Poliomyelitis (VAPP), a rare complication seen with Sabin (OPV). * **Current Schedule:** Under India’s Universal Immunization Programme (UIP), a combination of bOPV and fractional doses of IPV (fIPV) is used.
Question 9: Who invented the smallpox vaccine?
- A. Louis Pasteur
- B. Edward Jenner (Correct Answer)
- C. Paul Eugene
- D. John Snow
Explanation: **Explanation:** **Correct Answer: B. Edward Jenner** Edward Jenner is known as the **"Father of Immunology"** for his pioneering work in 1796. He observed that milkmaids who contracted cowpox (a milder disease) were immune to smallpox. He tested this by inoculating an 8-year-old boy, James Phipps, with material from a cowpox lesion and later challenging him with smallpox matter. The boy did not develop the disease. This was the first scientific demonstration of vaccination (from the Latin *vacca*, meaning cow). **Incorrect Options:** * **A. Louis Pasteur:** Known as the "Father of Microbiology," he developed vaccines for **Rabies, Anthrax, and Fowl Cholera**. He also proposed the Germ Theory of Disease and invented pasteurization. * **C. Paul Ehrlich:** (Often confused with Paul Eugene) He is the "Father of Chemotherapy" and developed the **Side-Chain Theory** of antibody formation. He also discovered the first effective treatment for syphilis (Salvarsan). * **D. John Snow:** Known as the "Father of Modern Epidemiology" for his work in tracing the source of a **Cholera** outbreak in London (the Broad Street pump). **High-Yield Facts for NEET-PG:** * **Smallpox Eradication:** Smallpox is the only human infectious disease to be globally eradicated. The last naturally occurring case was in **Somalia (1977)**. * **Official Declaration:** The WHO declared the world free of smallpox on **May 8, 1980**. * **Vaccine Type:** The smallpox vaccine used the **Vaccinia virus** (a live virus), not the Variola virus. * **Bifurcated Needle:** The specific tool used for the "multiple puncture" vaccination technique during the eradication campaign.
Question 10: Which of the following vaccines is NOT typically given for post-splenectomy infection prophylaxis?
- A. Streptococcus pneumoniae
- B. Haemophilus influenzae
- C. Neisseria meningitidis
- D. Escherichia coli (Correct Answer)
Explanation: ### Explanation The spleen plays a critical role in filtering the blood and contains specialized macrophages and B-cells that are essential for clearing **encapsulated organisms**. Following a splenectomy, patients are at a lifelong increased risk of **Overwhelming Post-Splenectomy Infection (OPSI)**, which is characterized by rapid-onset sepsis with high mortality. **Why Escherichia coli is the correct answer:** While *E. coli* can cause sepsis, it is not a primary target for post-splenectomy prophylaxis. The risk in asplenic patients is specifically linked to organisms that require **splenic opsonization** for clearance. There is currently no routine vaccine for *E. coli* used in this clinical context, as it is not one of the "Big Three" encapsulated pathogens that dominate OPSI cases. **Why the other options are incorrect:** * **Streptococcus pneumoniae (Option A):** The most common cause of OPSI (responsible for ~50-90% of cases). Vaccination with both PCV13 and PPSV23 is mandatory. * **Haemophilus influenzae type b (Option B):** A major encapsulated pathogen that causes severe respiratory and systemic infections in asplenic individuals. * **Neisseria meningitidis (Option C):** Asplenic patients have a significantly higher risk of meningococcemia; therefore, the quadrivalent (MenACWY) and Serogroup B vaccines are indicated. **NEET-PG High-Yield Pearls:** * **The "Big Three":** Remember the mnemonic **"SHiN"** (*S. pneumoniae, H. influenzae, N. meningitidis*) for encapsulated organisms requiring vaccination. * **Timing of Vaccination:** * **Elective Splenectomy:** Administer vaccines at least **14 days before** surgery. * **Emergency Splenectomy:** Administer vaccines **14 days after** surgery (to avoid the period of post-surgical "immunological stun"). * **Other Risks:** Asplenic patients are also at increased risk for intraerythrocytic parasites like *Babesia* and *Plasmodium* (Malaria).
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