VBSC 435 09.30.25

Overview of Influenza Virus Stages and Transmission

  • Influenza viruses have various subtypes tied to their surface proteins, hemagglutinin (HA) and neuraminidase (NA).
  • Not all virus subtypes are relevant for disease in humans.
  • Aquatic birds, such as ducks and geese, are the natural hosts for influenza viruses, where infections are asymptomatic.

Subtypes of Influenza A

  • Important Human Subtypes:

    • H1
    • H2
    • H3

  • Currently co-circulating in human populations:

    • H1 subtype
    • H3 subtype
    • Note: H2 subtype has been present in past human infections.

  • Other Subtypes:

    • H5 and H9 subtypes have sickened humans but did not adapt for sustained human transmission.

Transmission Characteristics

  • Cases of H5 and H9 infections are bird to human, not human to human.
  • Concerns about pandemics arise from potential changes allowing these viruses to adapt for human transmission.
    • New subtypes usually outcompete existing viruses.

Anatomy of Influenza Virus

  • Focus on spikes: hemagglutinin and neuraminidase (NA).
  • Seasonal Influenza Viruses vs. Pandemic Viruses:
    • Seasonal viruses affect mostly during colder months with specific subtypes being more prevalent.
  • Predictive modeling needed for vaccine development (5-20% of population infected seasonally despite immunity).

Clinical Symptoms Caused by Influenza

  • Main symptoms:
    • Fever with onset (characteristic)
    • Chills, muscle pain, fatigue, headache
    • Loss of appetite
  • Differences between influenza and common cold:
    • Influenza: sudden onset, fever present
    • Common cold: gradual onset, fever rarely present, caused by rhinoviruses, coronaviruses, adenoviruses.

Pandemic Potential of Influenza

  • An avian virus adapts to bind differently in human cells to spread effectively.
  • Sialic Acid:
    • Avian cells bind sialic acid via an alpha-2,3 connection (linked to galactose).
    • Human cells use an alpha-2,6 connection.
  • Virus adaptations include overcoming sialic acid binding differences to facilitate transmission.

Historical Context of Influenza Pandemics

  • Notable Pandemics:
    • 1918, H1N1
    • 1957, H2N2
    • 1968, H3N2 (still in circulation).
  • Reassortment of Influenza Viruses:
    • Occurs when two strains infect a single cell, leading to mixed genetic segments, possibly resulting in new, potentially pandemic strains.

Risk Assessment and Further Studies

  • Initial experience with H5N1 in 1997 (Hong Kong): 18 infected, 6 fatalities.
  • H5N1 caused severe disease but struggled for human transmission, all cases linked to contact with infected birds.
  • Ongoing H5N1 outbreak risks are assessed via mutations and adaptations.

Reverse Genetics and Virus Engineering

  • Reverse genetics enables the creation of defined viral mutations for research to verify their role in pathogenicity.
  • Key mutations (ex. q192r) serve as indicators of increased pandemic risk; correlation between specific mutations and binding strengths to human sialic acid assessed.
  • Research requires surveillance of emerging strains for mutation effects.

Mutational Analysis Results

  • Studying individual amino acid changes to assess their effect on binding capabilities to human or avian sialic acid.
  • Some mutations enhance binding, while others reduce it.
    • Example of q192r mutation validating its role in human binding.
    • Research into clade differences reveals variability in response to these mutations by virus strain.

Summary of Key Findings and Precautions

  • Understanding barriers (temperature, sialic acid differences) is critical for pandemic predictions.

  • Future surveillance and research will focus on mutation impacts to preemptively address possible viral adaptations leading to pandemics.

  • Potential implications in scientific research and public health policy regarding gain-of-function research are highlighted, exploring the balance of scientific advancement against pandemic prevention.