Vaccine Chart

Live Attenuated Vaccines

• Mechanism and Production:

  • Pathogens are cultivated repeatedly in lab media, foreign animal hosts, or altered environmental conditions.

  • This selective process forces adaptation to non-human conditions, causing the microbe to lose key virulence factors while retaining its ability to replicate inside the human host without causing clinical disease.

• Advantages (Positives):

  • Closely mimics a natural infection because the weakened organism actively replicates in the body.

  • Stimulates both humoral (antibody-mediated) and cell-mediated immunity (cytotoxic T cells).

  • Usually confers robust, long-term or lifelong protection, often requiring fewer initial doses or boosters.

• Limitations and Risks (Negatives):

  • Slight chance that weakened microbes can undergo back-mutation to revert to a virulent form.

  • Contraindicated in individuals with compromised immune systems (such as severe immunodeficiencies or active immunosuppressive therapy) and during pregnancy, as uncontrolled replication can lead to disease.

  • Requires strict cold chain maintenance to preserve organism viability.

• Examples:

  • Measles, Mumps, and Rubella (MMR) vaccine

  • Varicella (VAR) vaccine (chickenpox)

  • Inhaled live influenza vaccine

Inactivated ("Killed") Vaccines

• Mechanism and Production:

  • Whole, intact pathogens are cultured and subsequently destroyed using chemical agents (commonly formalin or formaldehyde), heat, or radiation.

  • Inactivation permanently halts replication while preserving microbial surface structures.

• Advantages (Positives):

  • Incapable of replicating or infecting the host, eliminating any possibility of reversion to virulence.

  • Safe: No chance of infectious disease from the vaccine, making it suitable for immunocompromised individuals.

  • Contains many different antigenic surface molecules from the entire pathogen.

• Limitations and Risks (Negatives):

  • Lower immunogenicity compared to live vaccines, primarily driving a humoral antibody response with limited cytotoxic T cell activation.

  • Some antigenic molecules in the vaccine can trigger local or systemic inflammatory side effects.

  • Immunity often wanes over time, necessitating multiple initial priming doses and subsequent booster shots.

• Examples:

  • Inactivated Polio Vaccine (IPV / Salk)

  • Inactivated Influenza Vaccine (IIV)

  • Hepatitis A vaccine

  • Rabies vaccine

Toxoid Vaccines

• Mechanism and Production:

  • Utilized for pathogens where clinical illness is primarily caused by secreted bacterial exotoxins rather than tissue invasion.

  • Exotoxins are harvested and chemically modified (typically using formalin) or heat-treated to destroy their toxic biological activity while maintaining their antigenic structure.

• Advantages (Positives):

  • Safe: Contains no living organisms or active toxins, presenting zero risk of infection.

  • Stimulates the production of highly specific neutralizing antibodies (antitoxins) that neutralize free toxins.

• Limitations and Risks (Negatives):

  • Immunity is relatively short-lived compared to whole-microbe vaccines.

  • Regular booster doses are required throughout life (typically every 10 years) to maintain protective antibody titers.

  • Does not directly prevent bacterial colonization or transmission, only systemic toxicity.

• Examples:

  • Tetanus toxoid vaccine

  • Diphtheria toxoid vaccine

Subunit and Acellular Vaccines

• Mechanism and Production:

  • Composed of isolated, purified microbial antigens (such as surface proteins or capsular fragments) rather than the whole pathogen.

  • Antigens are either chemically extracted from cultured microbes or synthesized recombinantly using cell expression systems.

• Advantages (Positives):

  • Very safe: Entirely free of live agents with zero chance of infection.

  • Exhibits very few non-target antigens, drastically reducing reactogenicity and the chance of severe adverse reactions.

  • Enables precise targeting of the most protective epitopes.

• Limitations and Risks (Negatives):

  • Lacks innate immune-stimulating motifs (PAMPs), generally requiring the addition of immunological adjuvants to stimulate an adequate response.

  • Typically demands multiple doses and periodic boosters to produce sustained immunity.

• Examples:

  • Acellular pertussis (aP) vaccine (component of DTaP / Tdap)

  • Human papillomavirus 9-valent (HPV9) vaccine

  • Recombinant Hepatitis B vaccine

Conjugate Vaccines

• Mechanism and Production:

  • Designed to target encapsulated bacteria whose outer polysaccharide coating is poorly recognized by immature immune systems.

  • Polysaccharide sugars from the bacterial surface are chemically linked (conjugated) to a foreign, immunogenic protein carrier (such as diphtheria or tetanus toxoid).

  • The carrier protein recruits helper T cells, converting a T-independent antigen response into a robust T-dependent immune response.

• Advantages (Positives):

  • Safe: No chance of infectious disease.

  • Significantly improves immunogenicity in infants and children under 18 to 24 months by inducing B cell memory and class switching.

  • Induces protective mucosal immunity and reduces nasopharyngeal carriage.

• Limitations and Risks (Negatives):

  • Complex and costly biochemical manufacturing processes.

  • Provides protection only against the specific polysaccharide serotypes included in the vaccine formulation.

• Examples:

  • Haemophilus influenzae type b (Hib) conjugate vaccine

  • Pneumococcal conjugate vaccine (e.g., PCV13, PCV15, PCV20)

  • Meningococcal conjugate vaccine (MenACWY)

mRNA Vaccines

• Mechanism and Production:

  • Synthetic messenger RNA (mRNA) containing instructions to build a specific pathogen antigen (such as a viral surface glycoprotein) is encapsulated within lipid nanoparticles (LNPs).

  • The lipid nanoparticles protect the mRNA from enzymatic degradation and facilitate entry into host cells.

  • Once inside the cytoplasm, host ribosomes translate the mRNA into the target antigen, which is presented on cell surfaces to stimulate an immune response.

• Advantages (Positives):

  • Safe and non-infectious: Contains no pathogen, does not enter the host nucleus, and cannot integrate into host genomic DNA.

  • Triggers balanced immune activation, generating both neutralizing antibodies and strong cytotoxic T cell responses.

  • Rapid and adaptable development: Reformulation to match new variants or strains can be quickly accomplished by modifying the nucleotide sequence without redesigning the production platform.

• Limitations and Risks (Negatives):

  • Longevity of protection can be uncertain, often requiring booster doses as antibody titers decline or new variants emerge.

  • Lipid nanoparticles and mRNA require strict cold or ultra-cold chain storage.

  • Common, transient reactogenic symptoms (fever, local injection site pain, fatigue) after vaccination.

• Examples:

  • Pfizer-BioNTech COVID-19 vaccine (Comirnaty)

  • Moderna COVID-19 vaccine (Spikevax)

Viral Vector Vaccines

• Mechanism and Production:

  • The genetic sequence encoding a specific pathogen antigen is inserted into the genome of a harmless, replication-deficient viral carrier (such as an engineered adenovirus).

  • The vector infects human cells and delivers the genetic payload into the nucleus, where it is transcribed into mRNA and subsequently translated into foreign antigen by host machinery.

• Advantages (Positives):

  • Robust intracellular antigen presentation: Effectively engages both MHC Class I and Class II pathways, generating strong antibody and T cell responses.

  • Generally more temperature-stable than mRNA vaccines, often remaining stable under standard refrigeration temperatures.

• Limitations and Risks (Negatives):

  • Vector immunity: Pre-existing immunity or antibodies generated against the viral carrier virus during initial doses can neutralize the vector, potentially reducing booster effectiveness.

  • Rare, idiosyncratic adverse reactions associated with specific adenovirus platforms.

• Examples:

  • Johnson & Johnson (Janssen) COVID-19 vaccine

  • Oxford-AstraZeneca COVID-19 vaccine

  • Ebola viral vector vaccine (rVSV-ZEBOV)