3.6 The Science of Vaccination

What is the basic theory of vaccination?

Stimulating immunity to a disease by exposing the body to harmless or weakened antigens.

Definition of immunity

Resistance to disease caused by microorganisms or their harmful substances.

Does immunity always prevent infection?

No, immunity prevents disease but not necessarily infection.

What is sterilizing immunity?

Immunity strong enough to prevent infection entirely, not just disease.

Why is sterilizing immunity the goal of vaccination?

It prevents both illness and transmission of the pathogen to others.

Why is sterilizing immunity not always reached?

Some pathogens can still infect cells or partially evade immune responses even after vaccination.

How does vaccination stimulate immunity?

It exposes the body to microbial antigens, triggering a primary immune response.

What forms during the primary immune response after vaccination?

Immunological memory, which prepares the body for faster secondary responses.

What happens if a vaccinated person encounters the real pathogen later?

A rapid and strong secondary immune response usually prevents disease development.

What is immunogenicity?

The ability of an antigen to provoke an immune response.

Why do vaccines differ in effectiveness?

Because different antigens have different levels of immunogenicity.

What is immune system evasion?

The ability of microbes to avoid detection or activation of the immune system.

How does immune system evasion affect vaccine success?

Pathogens that evade immune responses are harder to immunize against effectively.

What are the main components of a vaccine?

Antigens, adjuvants, preservatives/stabilizers, and antibiotics.

Role of vaccine antigen

The molecule that triggers the immune response and creates immunity.

Role of adjuvant

Added substance that boosts the vaccine's ability to activate the immune system.

Role of preservatives or stabilizers

Prevent breakdown of vaccine components during storage.

What usually causes allergic reactions to vaccines?

Sensitivity to preservatives, not the antigens themselves.

Role of antibiotics in vaccines

Prevent contamination by bacteria or fungi during production and storage.

Definition of live attenuated vaccine

Vaccine made from live microbes that can replicate but no longer cause disease.

What does "live" mean in a vaccine?

The microbe can still reproduce in the body.

What does "attenuated" mean in a vaccine?

The microbe's ability to cause disease has been weakened or removed.

Advantages of live attenuated vaccines

Strong activation of both humoral and cellular immunity, long-lasting memory, few doses required.

Disadvantages of live attenuated vaccines

Higher risk of side effects, unsafe for immunocompromised or pregnant people, rare chance of regaining pathogenicity.

What is vaccine reversion?

When an attenuated pathogen mutates and regains the ability to cause disease.

When are live vaccines used?

For diseases with low reversion risk or mild symptoms if reversion occurs.

Definition of inactivated vaccine

Vaccine containing microbes that have been killed and can no longer reproduce.

How are inactivated vaccines made?

By killing microbes using heat or radiation.

Advantages of inactivated vaccines

Safe for most people, no risk of reversion, fewer side effects than live vaccines.

Limitations of inactivated vaccines

Stimulate mainly humoral immunity, weak cellular response, fewer memory cells, often require multiple doses.

Why do inactivated vaccines need boosters?

Because they produce weaker immune memory than live vaccines.

Definition of subunit vaccine

Contains only purified antigenic molecules such as microbial proteins or sugars, not whole microbes.

Do subunit vaccines contain live material?

No, they contain only pieces of the pathogen.

Why do subunit vaccines often include adjuvants?

Because purified antigens alone do not stimulate the immune system strongly.

Advantages of subunit vaccines

Very safe, few side effects, can be given to pregnant or immunocompromised individuals.

Disadvantages of subunit vaccines

Least immunogenic, mainly humoral response, few memory cells, need booster shots for maintenance.

Definition of mRNA vaccine

A vaccine that uses messenger RNA encoding a microbial antigen enclosed in a lipid bubble (liposome).

Role of liposome in mRNA vaccines

Helps the mRNA enter body cells and antigen-presenting cells.

What happens inside the cell after mRNA vaccine entry?

The cell's ribosomes translate mRNA into microbial antigens.

How do mRNA vaccines activate immunity?

The produced antigens are displayed on MHC I and II, triggering activation of T-cytotoxic, T-helper, and B-cells.

How long does the mRNA last inside cells?

Only a few days before being broken down naturally.

Initial effectiveness of mRNA vaccines

Over 90% effective after first doses.

Long-term effectiveness of mRNA vaccines

About 55-60% after two years.

Comparison of mRNA to traditional vaccines

Effectiveness similar to inactivated vaccines; serious side effects are comparable.

Relationship between vaccine safety and effectiveness

Increasing safety usually decreases effectiveness, and vice versa.

Which vaccine type is most effective but riskiest?

Live attenuated vaccines.

Which vaccine type is safest but least effective?

Subunit (acellular) vaccines.

Example of the safety-effectiveness tradeoff

The history of the pertussis (whooping cough) vaccine.

When was the first pertussis vaccine introduced?

In the 1930s as an inactivated vaccine.

What major change occurred in the 1980s pertussis vaccine?

Shift from inactivated to subunit vaccine.

Why was the pertussis vaccine changed?

Concerns about encephalitis (brain inflammation) after the older vaccine.

What is encephalitis?

Inflammation of brain tissue, which can cause neurological complications.

How did switching to the subunit vaccine affect safety?

Greatly reduced risk of encephalitis and side effects.

What is the main drawback of the subunit pertussis vaccine?

Requires booster shots every seven years to maintain immunity.

What are the names of the pertussis combination vaccines?

DTaP (for children) and Tdap (for adults and boosters).

What do DTaP and Tdap protect against?

Diphtheria, tetanus, and pertussis.

Why do many adults lack pertussis immunity?

They don't receive regular Tdap boosters unless exposed to tetanus risk.

What kind of infection does adult pertussis cause?

A mild chronic cough that can last for weeks.

What percent of adults with chronic cough may carry pertussis?

About 20-30%.

What is vaccine hesitancy?

Delay or refusal of recommended vaccinations by parents or individuals.

What has combined to cause modern pertussis outbreaks?

Low booster compliance and rising vaccine hesitancy.

How large have recent pertussis outbreaks been?

Between 15,000 and 20,000 cases per year over the last decade.