Drifting and Shifting: Influenza Virus Evolution

Influenza Virus Adaptation

• To infect humans, influenza viruses evolve to evade the immune system.
• Two main evolutionary processes are involved:
  • Antigenic Drift
  • Antigenic Shift

Antigenic Drift

• Definition: Continual, small changes or mutations in the virus's surface proteins (HA or NA).
• Similar to the concept of small, gradual movement (like clouds drifting).
• Results in new viral strains that are related and may be partially recognized by the immune system (cross-protection).
• Over time, these changes accumulate, making it harder for immune systems to recognize the virus.
• Typically, one or two strains in the influenza vaccine are updated annually due to drift.
• Occurs in influenza viruses A, B, and C.

Antigenic Shift

• Definition: Major, abrupt changes in one or both surface proteins (HA or NA).
• Occurs at irregular intervals, often due to reassortment (exchange of gene segments) between different virus types, usually between humans and birds.
• Results in a completely new influenza A subtype, to which most people have little or no immunity.
• Can lead to worldwide pandemics if the new virus spreads efficiently between people.
• Notable shift example: The 2009 H1N1 pandemic, involving a novel virus from a combination of pig, bird, and human genes.
• Historically, shifts have led to major pandemics.

Comparison: Antigenic Drift vs. Shift

• Drift: Minor, continual changes within the same subtype, may cause epidemics.
• Shift: Major, abrupt changes leading to a new subtype, may cause pandemics.
• Both processes highlight the need to frequently update influenza vaccines to maintain efficacy.

2009 H1N1 Pandemic

• Caused by a novel influenza A (H1N1) virus, first identified in March-April 2009.
• Initially called "swine flu" due to its similarity to pig influenza viruses in North America.
• Classified as a quadruple reassortant virus due to gene combinations from American and Eurasian pigs, birds, and humans.