Influenza vaccination

Influenza Virus - The Shifty Shape-shifter

• Virus: Enveloped, single-stranded RNA Orthomyxovirus with a segmented genome (8 segments).

• Key Antigens: Hemagglutinin (HA) for entry, Neuraminidase (NA) for release.

• Antigenic Drift:

  • Minor point mutations in HA/NA genes.
  • Allows evasion of host immunity, causing seasonal epidemics.
  • Occurs in both Influenza A and B.

• Antigenic Shift:

  • Major, abrupt antigen change via genetic reassortment of RNA segments.
  • Creates new viral subtypes.
  • Leads to devastating pandemics.
  • Occurs in Influenza A only.

⭐ The segmented genome is the linchpin for antigenic shift. It allows Influenza A viruses from different species (e.g., avian, human) to exchange genes, creating a novel virus to which there is little population immunity.

Vaccine Types - The Annual Armor

• Inactivated Influenza Vaccine (IIV - "Flu Shot")
  • Killed virus, administered intramuscularly (IM).
  • Safe for most, including infants >6 months, pregnant women, and the immunocompromised.
  • Includes standard-dose, high-dose (for ≥65y), and adjuvanted (for ≥65y) formulations to enhance immune response in the elderly.
• Live Attenuated Influenza Vaccine (LAIV - "Nasal Spray")
  • Live, temperature-sensitive weakened virus; administered intranasally.
  • ⚠️ Contraindicated in pregnancy, immunocompromised individuals, and adults ≥50 years.
• Recombinant Influenza Vaccine (RIV)
  • Produced via recombinant DNA technology; completely egg-free.
  • Indicated for individuals ≥18 years, especially those with severe egg allergies.

⭐ Most modern flu vaccines are quadrivalent, designed to protect against two influenza A viruses (an H1N1 and an H3N2) and two influenza B viruses from different lineages (Victoria and Yamagata).

ACIP Recommendations - The Rulebook

• Annual vaccination is recommended for all persons aged ≥6 months who do not have contraindications.
• Timing: Vaccinate by the end of October, before the peak of the influenza season.
• Adults ≥65 years: Preferential use of higher-dose or adjuvanted influenza vaccines (e.g., Fluzone High-Dose Quadrivalent, Flublok Quadrivalent, or Fluad Quadrivalent).
• Children (6 months-8 years): Require 2 doses (administered ≥4 weeks apart) during their first season of vaccination.

⭐ For individuals with a history of severe allergic reaction to eggs (e.g., anaphylaxis), any age-appropriate inactivated or recombinant influenza vaccine may be administered in a medical setting, supervised by a provider able to manage severe allergic reactions.

Contraindications - Red Flags & Risks

• Absolute Contraindication:
  • History of severe allergic reaction (e.g., anaphylaxis) to a previous dose or any vaccine component.
• Precautions (Proceed with caution):
  • History of Guillain-Barré Syndrome (GBS) within 6 weeks of a prior influenza vaccine dose.
  • Moderate-to-severe acute illness (defer until recovered).
• Live Attenuated Vaccine (LAIV) Specific Contraindications:
  • Immunocompromised individuals & pregnant women.
  • Children aged 2-4 years with asthma.
  • Close contacts of severely immunosuppressed persons.

⭐ A history of egg allergy is no longer a contraindication. Patients with a history of severe allergic reaction to eggs (e.g., anaphylaxis) can receive any licensed influenza vaccine in a medical setting.

• Annual vaccination is crucial due to antigenic drift (minor point mutations) in hemagglutinin (HA) and neuraminidase (NA) genes.
• The inactivated (killed) vaccine is the most common type; the live attenuated intranasal vaccine is contraindicated in pregnancy and immunocompromised individuals.
• Vaccine-induced immunity primarily targets the hemagglutinin (HA) antigen, which blocks viral entry into cells.
• Universal vaccination is recommended for everyone ≥6 months of age annually.
• Antigenic shift, a major genetic reassortment, can cause pandemics.