Vaccines and Major Infectious Diseases: Microbiology Study Notes

Vaccines

Introduction to Vaccines

Vaccines are biological preparations that provide acquired immunity to specific infectious diseases. They work by stimulating the body's immune system to recognize and combat pathogens, such as viruses or bacteria.

• Variation: The earliest form of smallpox prevention, involving exposure to material from smallpox sores.

• Vaccine: A preparation containing fractions of organisms that induce immunity without causing disease.

• Immunological Principle: Vaccination induces a primary immune response, leading to the formation of antibodies and memory cells, which provide long-term protection.

Types of Vaccines

• Attenuated Vaccines: Contain live, weakened pathogens.

  • Examples: Measles, mumps, rubella, varicella (chickenpox), yellow fever.

  • Provide lifelong immunity by stimulating both humoral and cellular immune responses.

  • Not given to immunocompromised patients due to risk of causing disease.

• Inactivated Vaccines: Contain killed or inactivated microbes.

  • Examples: Polio (Salk), hepatitis A, rabies.

  • Require booster shots to maintain immunity.

  • Induce mostly humoral immunity.

• Subunit Vaccines: Contain only antigenic fragments of the pathogen.

  • Examples: Hepatitis B (recombinant), acellular pertussis.

  • Recombinant subunits produced by genetic modification of yeast or insect cells.

• Toxoid Vaccines: Contain inactivated toxins (toxoids) from bacteria.

  • Examples: Tetanus, diphtheria.

• Virus-like Particle (VLP) Vaccines: Resemble viruses but do not contain genetic material.

  • Example: HPV vaccine.

• Polysaccharide Vaccines: Made from molecules in pathogen capsules; less immunogenic, especially in young children.

• Conjugate Vaccines: Polysaccharides attached to proteins to enhance immune response.

• DNA and mRNA Vaccines: Use genetic material to instruct cells to produce antigen.

  • mRNA vaccines: Encapsulated in lipid nanoparticles, injected into muscle, and direct synthesis of antigen.

  • DNA vaccines: Use plasmids to deliver DNA coding for antigens.

Vaccine Production

• Modern vaccines (DNA, mRNA, recombinant vector) do not require animal hosts for production.

• Use of cell cultures and recombinant systems is common.

• Adjuvants (e.g., alum) are added to improve effectiveness by enhancing immune response.

Vehicle Administration

• Oral vaccines stimulate mucosal immunity, effective against pathogens entering via the GI tract.

• Examples: Oral polio, rotavirus, cholera, typhoid.

• Other routes: Intramuscular, subcutaneous, intranasal.

Vaccine Safety

• Vaccines can rarely cause disease (e.g., oral polio, smallpox).

• Extensive safety testing is required before approval.

Major Infectious Diseases

Dengue

Dengue is a mosquito-borne viral infection caused by the Flavivirus dengue virus, a spherical, enveloped, single-stranded RNA virus.

• Vector: Aedes mosquitoes transmit the virus.

• Signs and Symptoms:

  • Range from asymptomatic and mild fever to severe bleeding and organ impairment.

  • Antibody-dependent enhancement (ADE) can worsen infection if antibodies from a previous infection bind to a different serotype.

  • Dengue is more severe in children previously infected with another serotype.

• Virulence:

  • Enhanced viral replication, infection of immune cells, and immune evasion.

  • Virus suppresses interferon (IFN) production.

• Host Immune Response:

  • Fever, release of interferons, activation of IgM and IgG antibodies, and cytotoxic T cells.

• Control/Prevention:

  • Genetic modification of mosquitoes (e.g., CRISPR gene editing) to prevent virus transmission.

  • Bacterial infection of mosquitoes to block virus spread.

Malaria

Malaria is a common and deadly parasitic infection caused by Plasmodium species, transmitted by Anopheles mosquitoes.

• Signs and Symptoms: Cyclic fever, chills, anemia, and severe organ damage.

• Pathogenesis: Parasites infect red blood cells and capillaries, leading to blockage and tissue damage.

• Treatment: Antimalarial drugs such as Malarone, doxycycline, and artemisinin. Drug resistance is a growing problem.

Tetanus

Tetanus is caused by the bacterium Clostridium tetani, an obligate anaerobe that produces endospores and a potent neurotoxin.

• Pathogenesis: Spores enter wounds, bacteria grow in anaerobic conditions, and release tetanospasmin toxin, which blocks inhibitory neurotransmitters, causing muscle spasms.

• Symptoms: Muscle stiffness, lockjaw, and spasms.

• Prevention: Vaccination with tetanus toxoid (inactivated toxin) induces production of neutralizing antibodies.

• Treatment: Administration of tetanus immune globulin and wound debridement.

Tuberculosis (TB)

Tuberculosis is a chronic bacterial infection caused primarily by Mycobacterium tuberculosis, an acid-fast, obligate aerobe with a slow generation time.

• Pathogenesis: Inhaled bacteria are phagocytosed by alveolar macrophages, survive within phagosomes, and trigger granuloma formation (Ghon's complex).

• Diagnosis: Tuberculin skin test (Mantoux), chest X-ray, acid-fast staining, and nucleic acid amplification tests (NAAT).

• Treatment: Prolonged antibiotic therapy (isoniazid, rifampin, ethambutol, pyrazinamide). Drug resistance is a major concern.

• Vaccine: BCG (live, attenuated strain of M. bovis), not used in the U.S., variable effectiveness.

COVID-19

COVID-19 is caused by the coronavirus SARS-CoV-2, an enveloped, single-stranded RNA virus with high mutation rates.

• Pathogenesis: Virus infects respiratory tract, can cause severe pneumonia, and triggers immune dysregulation (cytokine storm).

• Signs and Symptoms: Fever, cough, shortness of breath, loss of taste/smell, and in severe cases, acute respiratory distress syndrome (ARDS).

• Diagnosis: RT-PCR test for viral RNA, antigen tests.

• Treatment: Supportive care, antiviral drugs in some cases.

• Vaccines: mRNA vaccines (encode spike protein), viral vector vaccines, and subunit vaccines (contain spike protein).

Summary Table: Types of Vaccines

Type	Example	Key Features
Attenuated	MMR, Varicella	Live, weakened pathogen; strong, long-lasting immunity
Inactivated	Polio (Salk), Hepatitis A	Killed pathogen; requires boosters
Subunit	Hepatitis B, HPV	Contains only antigenic fragments; safe, fewer side effects
Toxoid	Tetanus, Diphtheria	Inactivated toxins; induces antitoxin antibodies
mRNA	COVID-19 (Pfizer, Moderna)	Encodes antigen; rapid development

Key Terms

• Antigen: A molecule capable of inducing an immune response.

• Adjuvant: Substance added to vaccines to enhance immune response.

• Serotype: Distinct variation within a species of bacteria or virus, based on antigenic properties.

• Granuloma: A mass of immune cells formed to wall off foreign substances.

• Cytokine Storm: Excessive immune response leading to tissue damage.