18.5 Vaccines

Vaccines revolutionized disease prevention by training the immune system to recognize pathogens before a real infection occurs. Understanding how different vaccine types work, and why some are chosen over others, is a core topic in microbiology that connects directly to immunology, public health, and clinical practice.

Vaccines and Artificial Immunity

Types of Artificial Immunity

There are two ways to acquire immunity artificially, and the key difference is who makes the antibodies.

Active immunity is stimulated by exposing your immune system to an antigen, either from a pathogen or a vaccine. Your body does the work: B cells produce antibodies, and memory cells form so the immune system "remembers" that pathogen. This is why active immunity is durable, often lasting years or even a lifetime. Vaccines for measles, mumps, and rubella (MMR) are classic examples. When enough people in a population have active immunity, you also get herd immunity, which protects those who can't be vaccinated.

Passive immunity skips the immune response entirely. Instead, you receive preformed antibodies from an outside source. Examples include:

• Maternal antibodies transferred across the placenta or through breast milk
• Convalescent sera collected from individuals who recovered from an infection
• Monoclonal antibodies engineered in a lab for specific targets

Passive immunity provides immediate protection, which is useful in emergencies (like a rabies exposure), but it's temporary. The antibodies degrade over weeks to months, and because your own immune system was never activated, no memory cells are formed.

Evolution of Vaccination Techniques

Vaccination didn't start in a modern lab. The concept evolved over centuries, with each step making the process safer and more effective.

Variolation was an ancient technique, most notably used against smallpox. Practitioners would take infectious material from a person with a mild case and deliberately introduce it into a healthy person, usually through a scratch in the skin. The goal was to cause a mild infection that would confer protection. It often worked, but the risks were real: some people developed full-blown disease, and they could spread it to others.

Jenner's cowpox vaccine came next. In the 1790s, Edward Jenner observed that milkmaids who had contracted cowpox (a mild disease) seemed protected against smallpox. In 1796, he tested this by inoculating a boy with material from a cowpox pustule, then later exposing him to smallpox. The boy didn't get sick. This was the first true vaccine, and it eventually led to the complete eradication of smallpox by 1980.

Pasteur's attenuated vaccines introduced the idea of deliberately weakening a pathogen so it could trigger an immune response without causing severe disease. Louis Pasteur applied this principle to develop vaccines for rabies and anthrax. The BCG vaccine for tuberculosis also uses an attenuated strain. This concept of attenuation remains foundational to vaccine design today.

Modern vaccine technologies have expanded the toolkit considerably:

1. Inactivated vaccines: Pathogens killed by heat or chemicals (e.g., the injectable polio vaccine). They can't replicate, so they're safer but often less immunogenic.
2. Subunit vaccines: Only specific antigenic components are used, either extracted from the pathogen or produced via recombinant DNA technology (e.g., the hepatitis B vaccine uses a recombinant surface antigen).
3. Conjugate vaccines: Polysaccharide antigens are chemically linked to a protein carrier. This is necessary because polysaccharides alone are T-independent antigens and produce a weak response, especially in young children. The protein carrier recruits T cell help, boosting immunogenicity (e.g., the Haemophilus influenzae type b vaccine).
4. Nucleic acid vaccines: DNA or mRNA encoding a pathogen antigen is delivered to host cells, which then produce the antigen themselves and trigger an immune response (e.g., the mRNA COVID-19 vaccines from Pfizer-BioNTech and Moderna).

Comparison of Vaccine Types

Each vaccine type involves trade-offs between strength of immune response, safety, storage requirements, and cost.

Live attenuated vaccines

• Advantages:
  • Elicit strong, long-lasting immune responses because the pathogen replicates (at low levels) in the host
  • Often provide immunity with a single dose (e.g., MMR vaccine)
  • Mimic natural infection, activating both humoral and cell-mediated immunity
• Limitations:
  • Small risk of reversion to virulence, meaning the weakened pathogen could mutate back to a disease-causing form
  • Contraindicated in immunocompromised individuals and pregnant women
  • Require cold chain storage, which complicates distribution in resource-limited settings

Inactivated vaccines

• Advantages:
  • Safer than live vaccines since the pathogen cannot replicate
  • More stable and easier to store and transport
  • Can be given to immunocompromised individuals and pregnant women (e.g., inactivated influenza vaccine)
• Limitations:
  • Generally stimulate a weaker immune response, primarily humoral (antibody-based)
  • Usually require multiple doses and periodic boosters to maintain protection

Subunit vaccines

• Advantages:
  • Contain only specific antigenic components, which minimizes the risk of adverse reactions
  • Can be engineered to target particular epitopes or conserved regions of a pathogen
  • Suitable for individuals with allergies or contraindications to other vaccine types
• Limitations:
  • Often require adjuvants (substances like aluminum salts that enhance the immune response) to be effective
  • Typically need multiple doses and boosters
  • More expensive to produce compared to live or inactivated vaccines

Vaccine Implementation and Challenges

Even a highly effective vaccine is only useful if it reaches the people who need it. Three major factors determine real-world success:

Vaccine efficacy is the percentage reduction in disease incidence among vaccinated individuals compared to unvaccinated individuals in controlled clinical trials. For example, a vaccine with 95% efficacy means vaccinated people were 95% less likely to develop the disease than the control group. Efficacy measured in trials can differ from effectiveness measured in real-world conditions, where storage, administration, and population health vary.

Cold chain refers to the temperature-controlled supply chain required to keep vaccines potent from manufacturing through administration. Live attenuated vaccines are especially sensitive to heat. If the cold chain breaks at any point, the vaccine may lose its effectiveness entirely. This is a major logistical challenge in tropical and low-resource regions.

Vaccine hesitancy is the reluctance or refusal to vaccinate despite the availability of vaccines. It can stem from misinformation, distrust of healthcare systems, religious or philosophical objections, or concerns about side effects. Vaccine hesitancy threatens herd immunity because if too few people are vaccinated, outbreaks of preventable diseases (like measles) can resurge even in populations where the disease was previously well controlled.