Vaccines Practice Questions

Correct answer: Live attenuated vaccine

Live attenuated vaccines contain weakened forms of the pathogen that can replicate without causing disease in immunocompetent individuals. They induce strong cellular and humoral immune responses. Examples include measles and varicella vaccines.

Correct answer: Inactivated vaccine

Inactivated vaccines are produced by killing the pathogen using heat or chemical agents such as formalin. They cannot replicate and are generally safer in immunocompromised patients. However, they often require booster doses.

Correct answer: Bacterial exotoxins

Toxoid vaccines are made from inactivated bacterial exotoxins. They stimulate the immune system to produce neutralizing antibodies against the toxin. Examples include tetanus and diphtheria vaccines.

Correct answer: Adjuvant

Adjuvants are substances added to vaccines to enhance the immune response. They improve antigen presentation and stimulate stronger immunity. Aluminum salts are commonly used adjuvants.

Correct answer: Providing messenger RNA that codes for a viral protein

mRNA vaccines deliver messenger RNA encoding a specific viral antigen, such as a spike protein. Host cells translate the mRNA to produce the antigen, triggering an immune response. They do not integrate into the host genome.

Correct answer: Live attenuated vaccine

Live attenuated vaccines can replicate and may cause disease in immunocompromised individuals. Therefore, they are generally contraindicated in these patients. Inactivated and subunit vaccines are safer alternatives.

Correct answer: IgG

Vaccines typically stimulate production of IgG antibodies, which provide long-term systemic immunity. IgM is produced initially, but IgG predominates during the memory response. IgG is critical for protective immunity.

Correct answer: Vector-based vaccine

Vector-based vaccines use a harmless viral vector to deliver genes encoding the target antigen. The host cells produce the antigen, stimulating immunity. Examples include adenoviral vector vaccines.

Correct answer: Do not stimulate T-cell dependent immunity

Polysaccharide vaccines produce a T-cell independent immune response. Young children have immature immune systems that respond poorly to T-cell independent antigens. Conjugate vaccines overcome this limitation.

Correct answer: Pneumococcal conjugate vaccine

Pneumococcal conjugate vaccine links polysaccharide antigens to a protein carrier. This enhances T-cell dependent immune responses. It is particularly effective in infants and young children.

Correct answer: Enhance the secondary immune response

Booster doses restimulate memory B and T cells, producing a stronger and more sustained immune response. This is known as the secondary immune response. It increases antibody titers and duration of protection.

Correct answer: Active artificial immunity

Vaccination provides active artificial immunity by stimulating the body’s immune system to produce its own antibodies and memory cells. This differs from passive immunity, where antibodies are directly transferred.

Correct answer: Enhance antigen immunogenicity

Adjuvants enhance the immune response to vaccine antigens. They improve antigen uptake and stimulate innate immunity. This leads to stronger and longer-lasting protection.

Correct answer: Injection site pain

Injection site pain is a common local reaction to vaccines. It results from inflammation triggered by immune activation. These reactions are usually mild and self-limiting.

Correct answer: It preserves vaccine potency

Vaccines are sensitive biological products that can lose potency if exposed to inappropriate temperatures. Maintaining the cold chain ensures their effectiveness. Breaks in the cold chain may reduce immunogenicity.

Correct answer: Memory B and T cells

Memory B and T cells are generated after vaccination and provide long-term protection. Upon re-exposure to the antigen, they mount a rapid and robust response. This is the basis of vaccine-induced immunity.

Correct answer: mRNA-based vaccines

mRNA vaccines can be developed rapidly once the genetic sequence of a pathogen is known. They allow quick adaptation to emerging variants. This makes them highly suitable during pandemics.

Correct answer: Transfer of preformed antibodies to an individual

Passive immunization involves administering preformed antibodies, such as immunoglobulins. It provides immediate but temporary protection. Unlike active immunization, it does not generate immunological memory.

Correct answer: The antigen

The antigen is the component that stimulates a specific immune response. It is recognized by immune cells, leading to antibody and memory cell formation. Other components support stability or enhance response but do not determine specificity.

Correct answer: It introduces an isolated protein antigen produced by genetically engineered cells.

Recombinant subunit vaccines are made by inserting the genetic code for a specific antigen (like a viral surface protein) into yeast or bacterial cells, which then mass-produce the protein. This purified protein is used as the vaccine, avoiding the use of entire pathogens. Examples include the Hepatitis B and Human Papillomavirus (HPV) vaccines.

Correct answer: Presenting intact antigens on their surface to B cells within the germinal centers

Follicular dendritic cells reside in the germinal centers of lymph nodes, where they hold intact vaccine antigens on their surfaces for extended periods. This continuous display allows antigen-specific B cells to undergo affinity maturation and clonal selection. FDCs do not actively phagocytize or process antigens through MHC pathways like classical dendritic cells.

Correct answer: To create a localized depot that releases the antigen and stimulates innate immunity

Aluminum salts act as adjuvants by creating a localized 'depot effect' that releases the vaccine antigen slowly over time. They also stimulate local tissue cells to release danger signals, which recruits and activates antigen-presenting cells to the site of injection. This significantly boosts the magnitude and duration of the adaptive immune response.

Correct answer: The shift from undetectable to a measurable, protective specific antibody titer

Seroconversion refers to the phase of an immune response where specific antibodies become detectable in the blood serum at a protective threshold. Before this milestone, diagnostic tests show a negative antibody status for the specific target pathogen. It serves as a primary endpoint clinical trial marker to verify vaccine efficacy.

Correct answer: Local host cells, including myocytes and dendritic cells

Viral-vector vaccines utilize a modified, harmless virus (like an un-replicating adenovirus) to enter the host's cells, such as muscle cells or local antigen-presenting cells. Once inside, the host cell's machinery uses the delivered genetic blueprint to synthesize the target antigen. These antigens are then displayed on the cell surface to alert the immune system.

Correct answer: Replacing standard uridine bases with modified nucleosides like pseudouridine

Incorporating modified nucleosides like N1-methylpseudouridine suppresses the activation of toll-like receptors that would otherwise recognize foreign RNA and destroy it immediately. This modification reduces innate inflammatory toxicity while dramatically increasing the stability and translation efficiency of the mRNA template inside host cells.

Correct answer: Long-lived plasma cells and neutralizing antibodies

Long-lived plasma cells reside in the bone marrow for decades, continuously secreting steady levels of high-affinity neutralizing antibodies into the bloodstream. These pre-existing antibodies bind and neutralize invading pathogens before they can establish an infection. Memory B and T cells respond rapidly if an infection takes root, but antibodies provide the immediate shield.

Correct answer: To convert a T-independent antigen into a T-dependent one, inducing memory

Pure polysaccharide antigens stimulate B cells directly in a T-independent manner, which fails to produce memory cells and works poorly in children under two. Linking the polysaccharide to a protein carrier lets helper T cells recognise the protein component and supply the costimulatory signals required for class switching and long-term memory formation.

Correct answer: Live-attenuated viral vaccines

Live-attenuated vaccines can enter and replicate inside host cells, meaning the antigens are processed internally via the endogenous pathway and presented on MHC Class I molecules. This specific presentation is mandatory for activating CD8+ cytotoxic T cells, which destroy infected cells. Inactivated or subunit vaccines are processed primarily via exogenous pathways, which strongly favor CD4+ T-cell and B-cell antibody responses.

Correct answer: The immune system favors antibodies against the original strain rather than new variants

Original antigenic sin occurs when the immune system relies on its existing memory cells from an encounter with a historical strain of a virus, rather than mounting a fresh response to altered epitopes on a new vaccine variant. This can blunt the effectiveness of updated booster shots, as the body expends energy making mismatched antibodies from old templates.

Correct answer: Formaldehyde

Formaldehyde is a cross-linking agent that alters the nucleic acids and proteins of a pathogen, rendering it entirely incapable of replication while leaving its overall surface structure intact for immune recognition. This allows the immune system to safely study the physical features of the threat without any risk of active infection.

Correct answer: To optimize the active transplacental transfer of IgG antibodies from mother to fetus

During the third trimester, maternal IgG antibodies are actively transported across the placenta via neonatal Fc receptors (FcRn). Vaccinating the mother during this window elevates her antibody titers, maximizing the volume of protective antibodies passed to the baby. This provides crucial passive protection to the newborn during the first few vulnerable months of life.

Correct answer: Herd immunity threshold

The herd immunity threshold is the mathematically calculated percentage of immune individuals required in a community to protect unvaccinated or vulnerable members. When this threshold is met, an infectious individual is highly likely to encounter an immune person rather than a susceptible one, causing the transmission chain to sputter out. This value is directly calculated from the pathogen's basic reproduction number (R0).

Correct answer: The virus can mutate and revert to a virulent form called vaccine-derived poliovirus.

Live-attenuated viruses replicate within the human gut to stimulate mucosal immunity and are excreted in stool. In areas with poor sanitation and low overall vaccine coverage, the weakened virus can circulate from person to person for months, giving it time to accumulate mutations that restore its neurovirulence and cause paralytic polio.

Correct answer: Toxoid vaccine

Diseases like tetanus and diphtheria are caused by harmful toxins released by bacteria, rather than the bacteria themselves. Toxoid vaccines use chemically neutralized versions of these toxins to train the immune system to make antitoxin antibodies, neutralizing the danger before it can bind to nerve or tissue receptors.

Correct answer: The weakened pathogen may replicate unchecked, causing severe systemic infection.

Live-attenuated vaccines rely on a intact, functional immune system to keep the weakened pathogen's replication tightly contained. If a patient's immune defenses are suppressed by drugs or disease, the weakened virus or bacteria can behave like a wild-type pathogen, expanding uncontrollably and causing clinical disease.

Correct answer: IgA

Intranasal or oral routes mimic the natural entry point of many pathogens, stimulating localized mucosal-associated lymphoid tissue (MALT). This localized activation results in the production of secretory IgA dimers, which are actively secreted across mucosal surfaces to block pathogens from attaching to epithelial cells. Intramuscular injections favor systemic IgG production instead.

Correct answer: Efficacy is performance under ideal trial conditions; effectiveness, in the real world.

Vaccine efficacy is calculated from randomized controlled trials where conditions are strictly managed, and participants are highly select. Vaccine effectiveness measures how well the vaccine protects the general, messy population in real-world field conditions, accounting for variables like improper cold chain storage, missed doses, and diverse underlying health profiles.

Correct answer: A Type III hypersensitivity reaction from local antigen-antibody immune complexes

An Arthus reaction is a localized Type III hypersensitivity reaction that can occur when an individual with high levels of pre-existing circulating IgG antibodies receives a booster dose. The injected antigen quickly complexes with the concentrated local antibodies, depositing immune complexes in the vessel walls, activating complement pathways, and triggering localized vascular inflammation.

Correct answer: They can be made rapidly from the genetic sequence alone, without growing live organisms.

Traditional vaccines require cultivating massive batches of live viruses or bacteria in eggs or bioreactors, which takes months to optimize and scale safely. Genetic platforms only require the digital sequence of the antigen, allowing synthetic production to start within days. This speed makes them exceptionally suited for responding to fast-moving viral outbreaks and shifting pandemics.