On: 1918 Spanish flu pandemic—historical account, parallels to today, and lessons

Episode: 107

Date: 17 April 2020

Historian and author of “The Great Influenza: The Story of the Deadliest Pandemic in History”.

Key Subjects

• 1918 Spanish Flu pandemic killed between 50-100 million people (out of 1.8 billion people).
  • Three waves (spring 2018, sep-dec 2018, spring 2019): second wave 2/3 of fatalities.
• Different from common flue in the way it afflicted young people with strong immune systems.
  • Common flu: mortality is U-shaped -> affects the young and the old / immunocompromised.
  • Spanish flu: mortality is W-shaped -> also affects people in their 20s and 30s (peak age 28 years old) with strong immune systems.
  • Strong immune system response = more damage to the lungs.
• High mortality (case mortality of about 2% in the Western world), as virus spread rapidly.
  • Virus bound to upper respiratory tract cells, making it easy to spread.
  • Virus bound to lower respiratory tract cells, causing viral pneumonia.
• Some immunity across waves of infection, but virus may have changed over time:
  • Exposure to first wave infection: largely immune against second wave.
  • Exposure to first / second wave infection: not necessarily immune against third wave.
  • May be due to antigenic drift in the virus (genetic variation).
• Sub-optimal government reaction may have made things worse.
  • Only thing we learn from history is that we learn nothing from history.
  • Government spread misinformation, eroding societal level of trust.
• 1918 Spanish Flu and COVID-19 – similarities:
  • Both are respiratory viruses that jumped from animal to human.
  • Both bind to upper and lower respiratory tract cells.
• 1918 Spanish Flu and COVID-19 – differences:
  • Spanish Flu – H1N1 virus; COVID-19 – corona virus.
  • COVID-19 takes longer to develop in the body than Spanish Flu and much longer to pass through.
  • Combined with social distancing, will take longer for COVID-19 to pass through the community (Spanish Flu waves second wave was about 14 weeks).
• COVID-19 and the common flu – making the wrong fatality rate comparison:
  • COVID-19 fatality rate may end up being well below 1%, maybe as low as 0.1%.
  • People think this is close to the oft quoted fatality rate for the flu of about 0.1%.
  • But, for the flu, the 0.1%rate is the case fatality rate.
  • For COVID-19, the quoted rate is usually the infection fatality rate. 
    • Case fatality: fatalities per reported cases of infection.
    • Infection fatality rate: fatalities per infected persons.
  • The common flu infection fatality rate is not known and likely much lower than 0.1%
    • We don’t know how many people actually get infected with the flu.
    • Many people that are infected are immune and will not show up in denominator.
    • The infection mortality rate of the flu is likely an order of magnitude below 0.1%.