Viral vaccines

Viral vaccines constitute a major class of biological medicinal products designed to stimulate active immunity against viral infections. Unlike antiviral medications that treat viral diseases post-infection, viral vaccines prevent infection by priming the host immune system to recognize and neutralize specific viruses. They are classified based on the type of virus they protect against, their composition, and manufacturing method. The development and widespread use of viral vaccines represent one of the most significant achievements in public health, contributing to the eradication (e.g., smallpox), elimination, or control of many once-deadly infectious diseases.

Viral vaccines may contain live attenuated viruses, inactivated viruses, viral subunits, viral vectors, or genetic material (mRNA or DNA). The specific mechanism of action and immunogenic profile varies according to the platform used. They are administered primarily via the intramuscular (IM) or subcutaneous (SC) route, and in certain cases, via oral or intranasal routes.

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Classification of Viral Vaccines by Composition

1. Live Attenuated Viral Vaccines (LAVs)

   • Contain weakened virus capable of replication without causing disease in healthy individuals

   • Induce robust, long-lasting immune response

   • Examples: Measles, Mumps, Rubella (MMR), Varicella, Yellow Fever, Oral Polio Vaccine (OPV), Intranasal Influenza (LAIV)

2. Inactivated Viral Vaccines

   • Contain viruses that have been killed using chemicals or heat

   • Cannot replicate; require multiple doses and adjuvants to elicit strong response

   • Examples: Inactivated Polio Vaccine (IPV), Hepatitis A, Rabies, Japanese Encephalitis

3. Subunit, Recombinant, Polysaccharide, or Conjugate Vaccines

   • Contain purified antigens or viral proteins

   • Safer, with fewer side effects; may require adjuvants

   • Examples: Hepatitis B, Human Papillomavirus (HPV), Influenza (subunit)

4. Viral Vector Vaccines

   • Use a genetically modified virus to deliver genetic material encoding the target antigen

   • Examples: Adenovirus-based COVID-19 vaccines (AstraZeneca, Johnson & Johnson), Ebola vaccine

5. mRNA Vaccines

   • Contain messenger RNA encoding viral antigens encapsulated in lipid nanoparticles

   • Stimulate antigen production in host cells, triggering immune response

   • Examples: COVID-19 vaccines (Pfizer-BioNTech, Moderna)

6. DNA Vaccines (experimental)

   • Plasmid DNA encoding viral proteins delivered via injection

   • Still under development or emergency use in some regions

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Mechanism of Action

1. Antigen Presentation: The viral component is recognized by antigen-presenting cells (APCs), such as dendritic cells and macrophages.

2. Activation of T and B Cells: Helper T cells (CD4+) and cytotoxic T cells (CD8+) are activated, along with B cells that produce antibodies.

3. Memory Formation: Long-lived memory cells remain in circulation, allowing rapid and potent response upon future exposure to the virus.

This adaptive immune response confers protective immunity, which can be:

• Sterilizing: Preventing infection altogether

• Non-sterilizing: Preventing disease severity while allowing mild or asymptomatic infection

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Commonly Used Viral Vaccines and Their Generic Components

1. Measles, Mumps, Rubella (MMR) Vaccine

   • Generic: Live attenuated measles, mumps, and rubella viruses

   • Brand: M-M-R II®, Priorix®

   • Route: SC

   • Doses: 2 doses (age 12–15 months, 4–6 years)

2. Varicella (Chickenpox) Vaccine

   • Generic: Live attenuated varicella-zoster virus

   • Brand: Varivax®

   • Route: SC

   • Doses: 2 doses

3. Hepatitis A Vaccine

   • Generic: Inactivated hepatitis A virus

   • Brand: Havrix®, Vaqta®

   • Route: IM

   • Doses: 2 doses (6 months apart)

4. Hepatitis B Vaccine

   • Generic: Recombinant hepatitis B surface antigen (HBsAg)

   • Brand: Engerix-B®, Recombivax HB®

   • Route: IM

   • Doses: 3-dose series or 2-dose for some formulations

5. Human Papillomavirus (HPV) Vaccine

   • Generic: Recombinant HPV types 6, 11, 16, 18, 31, 33, 45, 52, 58

   • Brand: Gardasil 9®

   • Route: IM

   • Doses: 2–3 doses depending on age

6. Influenza Vaccine (Seasonal)

   • Generic: Inactivated virus or recombinant hemagglutinin proteins (quadrivalent)

   • Brand: Fluzone®, Fluarix®, Flublok® (recombinant), FluMist® (live)

   • Route: IM, SC, or intranasal (live)

   • Doses: Annually

7. Polio Vaccine

   • IPV Generic: Inactivated poliovirus types 1, 2, 3

   • Brand: IPOL®

   • OPV Generic: Live attenuated poliovirus (discontinued in many countries)

   • Route: IM or oral

   • Doses: 4 doses in childhood

8. COVID-19 Vaccines

   • mRNA Vaccines

     • Generic: mRNA encoding SARS-CoV-2 spike protein

     • Brand: Comirnaty® (Pfizer-BioNTech), Spikevax® (Moderna)

   • Viral Vector Vaccines

     • Generic: Chimpanzee adenovirus vector encoding spike protein

     • Brand: Vaxzevria® (AstraZeneca), Jcovden® (Johnson & Johnson)

   • Route: IM

   • Doses: 2-dose primary series + booster(s)

9. Rabies Vaccine

   • Generic: Inactivated rabies virus

   • Brand: Imovax®, RabAvert®

   • Route: IM

   • Doses: 4-dose post-exposure; 2–3-dose pre-exposure

10. Yellow Fever Vaccine

• Generic: Live attenuated yellow fever virus (17D strain)

• Brand: YF-VAX®, Stamaril®

• Route: SC

• Doses: Single dose with lifelong immunity

11. Japanese Encephalitis Vaccine

• Generic: Inactivated Japanese encephalitis virus

• Brand: Ixiaro®

• Route: IM

• Doses: 2 doses

12. Rotavirus Vaccine

• Generic: Live attenuated rotavirus

• Brand: RotaTeq®, Rotarix®

• Route: Oral

• Doses: 2–3 doses in infancy

13. Zoster (Shingles) Vaccine

• Generic: Recombinant zoster antigen + adjuvant (Shingrix®) OR Live attenuated zoster virus (Zostavax®, discontinued in many regions)

• Route: IM (Shingrix)

• Doses: 2 doses

14. Ebola Vaccine

• Generic: Recombinant vesicular stomatitis virus (rVSV) expressing Ebola glycoprotein

• Brand: Ervebo®

• Route: IM

• Doses: Single dose

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Indications by Population

Vaccine	Target Groups
MMR	All children, healthcare workers, outbreak control
Varicella	All children, non-immune adults
Hepatitis A & B	Infants, healthcare workers, travelers, chronic liver disease
HPV	Adolescents, unvaccinated adults up to age 45
Influenza	Annual vaccination for all >6 months old
Polio	Universal childhood vaccination
COVID-19	Universal adult, elderly, high-risk individuals
Rabies	Pre-exposure (vets, travelers), post-exposure prophylaxis
Yellow Fever	Travelers to endemic areas, required for some visa applications
JE	Travelers to endemic areas, military
Rotavirus	All infants <6 months
Zoster (Shingrix)	Adults ≥50 years or immunocompromised
Ebola	Healthcare workers in endemic zones

Contraindications

• Severe allergic reaction (anaphylaxis) to a previous dose or vaccine component

• Immunosuppression (for live vaccines): HIV/AIDS, chemotherapy, organ transplant

• Pregnancy (for live vaccines)

• Moderate/severe acute illness (postpone vaccination)

• Infants <6 months (influenza, MMR, etc.)

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Precautions and Warnings

• Live vaccines should not be given simultaneously with blood products (interference with immunity)

• Multidose vials may contain preservatives like thimerosal

• Vaccine-associated adverse events (e.g., Guillain-Barré syndrome with influenza vaccine—rare)

• Delayed hypersensitivity reactions, especially with gelatin-containing formulations

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Adverse Effects

1. Common (Mild)

   • Injection site pain, redness, swelling

   • Fever, fatigue, headache

   • Myalgia or malaise

2. Moderate

   • Rash (e.g., after MMR or varicella)

   • Temporary joint pain (especially rubella in adults)

3. Serious (Rare)

   • Anaphylaxis

   • Febrile seizures

   • Vaccine-associated paralytic poliomyelitis (VAPP) with OPV

   • Intussusception (rare risk with early rotavirus vaccines)

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Storage and Handling

• Most require cold-chain storage (2–8°C)

• mRNA vaccines (e.g., Pfizer) require ultra-cold temperatures (-70°C) pre-dilution

• Live vaccines (e.g., MMR, Varicella) may require reconstitution prior to use

• Must avoid freezing unless specifically indicated

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Drug Interactions

• Immunosuppressants (e.g., corticosteroids, biologics): Reduce vaccine efficacy, contraindicated with live vaccines

• Antiviral drugs: May reduce efficacy of live virus vaccines (e.g., acyclovir + varicella vaccine)

• Blood products: Delay live vaccine administration (e.g., MMR, varicella) by 3–11 months

• Tuberculin testing (PPD): Should be administered separately to avoid false-negative results post-live vaccination

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Vaccine Schedules and Boosters

• Routine schedules follow national immunization programs (e.g., CDC, NHS, WHO EPI)

• Booster doses are required for:

  • Tetanus-containing vaccines every 10 years

  • COVID-19 for new variants

  • Hepatitis B in certain populations (dialysis, immunocompromised)

  • Yellow fever: generally no longer requires boosters unless specified

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Global Impact and Public Health Importance

• Smallpox eradication (1980) was achieved through viral vaccination

• Polio is near-eradicated globally due to IPV/OPV campaigns

• HPV vaccination reduces risk of cervical and other anogenital cancers

• COVID-19 vaccination reduced hospitalizations and death rates worldwide

• Herd immunity: Achieved when a critical portion of the population is immunized, protecting unvaccinated individuals