Case Study: 1918 Influenza Pandemic

Image from Arnett, Boston Globe 3/2020 - print edition.

Prior to COVID-19, the 1918 influenza pandemic was the most severe pandemic in recent history. First identified in military personnel in the spring of 1918, the influenza was an H1N1 virus of avian origin. It is commonly referred to by scientists and historians as “the Mother of all Pandemics.” This pandemic is often referred to as the “Spanish Flu” in the lay press, though this name is a misnomer, and the virus likely originated elsewhere. Contemporary reporting focused heavily on Spain, as it was one of few places at the time that did not have restrictions on the press during World War I.

Fast Facts about the 1918 Flu

  • Infectivity: 500 million people, or ⅓ of the world’s population. Note that this is smaller than estimates for SARS-CoV-2, as the population had some pre-existing immunity from prior influenza exposure.

  • Death rates: at least 50 million people worldwide, with 675,000 deaths in the United States.

  • Mortality: highest in people age <5, 20-40, and 65+ years. It is thought that this unusual age-distribution of cases may be due to differences in prior influenza exposure across different age groups.

  • More soldiers died from the 1918 flu pandemic than were killed in battle during World War I in 1918.

  • In 1918, there were no flu vaccines, antiviral drugs, antibiotics, or mechanical ventilators. Treatment options were limited to supportive care and unproven remedies.

  • There were 3 waves of the epidemic, which lasted from January 1918 - December 1920 (CDC):

    • First wave: March 1918 - Summer 1918

    • Second wave: Fall 1918 (peak of epidemic)

    • Third wave: Winter 1918 - Spring 1919

  • According to a 2007 study in the Internal Journal of Epidemiology (Vynnycky et al.), the Re for the 1918 influenza virus was in the range of 1.2-3.0 for community-based settings.

    • The study estimates that, in a totally susceptible population, a single infectious case could have led to 2.4-4.3 cases in a community-based setting.

How Three US Cities Tried to Stop the Spread of the 1918 Flu

Philadelphia held a parade

By mid-September 1918, the second wave of the flu epidemic was in full effect, spreading from Boston to New York and Philadelphia before traveling west to St. Louis and San Francisco. Without a vaccine or known cause for the outbreak, mayors and city health officials were grappling with how to implement social distancing and reduce community transmission. They asked themselves the following questions:

  • Should they close schools and ban all public gatherings?

  • Should they require all citizens to wear a gauze face mask?

  • Would shutting down financial centers during a time of war be unpatriotic?

Wilmer Krusen, Philadelphia’s public health director, advised citizens they could lower their risk for flu by: staying warm, keeping their feet dry, and “loosening their bowels.” Krusen refused to cancel the Liberty Loan parade on September 28, 1918, even as cases steadily increased up to this point. Infectious disease experts warned Krusen that the parade (likely to attract several hundred thousand people) would be a “ready-made inflammable mass for a conflagration.” Krusen kept the parade on because it would raise millions of dollars in war bonds. The parade took place: soldiers, Boy Scouts, marching bands, and local dignitaries processed two miles through downtown Philadelphia past sidewalks teeming with spectators. Just 72 hours after the parade, all 31 of Philadelphia’s hospitals were full. By the end of the week, 2,600 people were dead.

St. Louis flattened the epidemic curve

Before the first case of 1918 flu appeared in the city, health commissioner Dr. Max Starkloff wrote an editorial about the importance of avoiding crowds in the St. Louis Post-Dispatch, putting local physicians on high alert. When a flu outbreak from nearby military barracks spread to St. Louis, Starkloff closed schools, movie theaters, and pool halls, and banned public gatherings. When infections surged, thousands of sick residents were treated at home by a network of volunteer nurses. George Dehner, author of Global Flu and You: A History of Influenza, writes that because of these precautions, St. Louis public officials flattened the curve and prevented the flu epidemic from exploding overnight like in Philadelphia. According to a 2007 NIH analysis in PNAS of 1918 flu death records (Hatchett et al.), the peak mortality rate in St. Louis was only ⅛ of Philadelphia’s death rate at its worst.

San Francisco required face masks

California governor William Stephens declared it the “patriotic duty of every American citizen” to wear a gauze face mask and eventually made it the law. Citizens found in public without a face mask or wearing it improperly were arrested, charged with disturbing the peace, and fined $5. San Francisco’s low infection rates were likely not due to the face masks, but instead due to:

  • Well-organized campaigns to quarantine all naval institutions before the flu arrived

  • Early efforts to close schools

  • Bans on social gatherings

  • Closing all places of “public amusement”

San Francisco did well in the second wave of the epidemic through the fall of 1918. When the third wave struck in January 1919, businesses and theater owners fought back against closings, as they believed masks were what saved them the first time. The 2007 NIH analysis found that if San Francisco had kept up the same flu protections in the third wave as it did in the second wave, it could have reduced deaths by 90%.

CORE TEXT: Barry, J. The Single Most Important Lesson From the 1918 Influenza. New York Times, (March 17, 2020).

Supplementary reading:

Thought Questions:

  • If you were a public health official in the early 1900s trying to determine the cause of the outbreak, what methods would you use to find the answer? (Assume you can only use resources available from the early 1900s.)

  • How could public health officials in San Francisco have probed deeper into whether it was gauze face masks that reduced transmission or closing of public spaces?

  • What are the challenges for trying to determine R0 and Re values for the 1918 flu epidemic? What assumptions can be made in modeling? What challenges do these assumptions bring?