The 1918–1919 influenza outbreak, often misnamed the Spanish Flu, stands as the most devastating pandemic in modern history, sweeping across the globe in a series of deadly waves. Within a mere two years, the H1N1 virus infected an estimated 500 million people, approximately one-third of the world’s population at the time. The resulting death toll ranged from 50 million to perhaps as high as 100 million lives worldwide, far exceeding the fatalities of World War I. This immense scale of suffering pressured the scientific community to deliver a medical defense against the rapid killer, leading researchers to quickly deploy a preventative shot, often referred to as a vaccine, in an attempt to halt the disease’s spread.
Scientific Context of 1918
The medical understanding of infectious disease in 1918 was governed by germ theory, which had successfully identified bacteria as the cause of illnesses like tuberculosis, cholera, and typhoid. Consequently, researchers believed influenza was also caused by a bacterium. This belief stemmed from German bacteriologist Richard Pfeiffer, who isolated a rod-shaped organism, Bacillus influenzae, during the 1889 influenza pandemic.
By 1918, this organism, now known as Haemophilus influenzae, was still widely considered the primary pathogen causing the flu. The technology to conclusively identify the true causative agent—a virus—did not yet exist. Viruses were poorly understood, sub-microscopic entities, sometimes observed as “filterable agents” because they passed through filters designed to trap bacteria. Since viruses could not be cultured in a lab, Bacillus influenzae remained the most plausible, though incorrect, suspect for the disease’s origin.
Development of the Anti-Pneumonia Shots
Based on the consensus that a bacterium was the cause, researchers and pharmaceutical companies rapidly synthesized what they believed was a true influenza vaccine, which was structurally an anti-bacterial shot. These injections were polyvalent, meaning they contained several types of inactivated bacteria, or “killed cultures,” thought to be linked to the disease. The main components were heat-killed cultures of bacterial species commonly isolated from the lungs of deceased patients. These included Streptococcus pneumoniae, Haemophilus influenzae, Streptococcus pyogenes, and Staphylococcus aureus.
The US Army, following the successful implementation of Major Frederick Russell’s typhoid fever vaccine program, was aggressive in mass-producing and deploying these anti-bacterial shots. The typhoid vaccine experience provided the logistical framework and institutional confidence for rapid deployment. Public health officials authorized the distribution of these shots to military camps and civilian populations across the country. The shots were distributed urgently, despite being unproven against the unknown primary cause. The volume was immense, with researchers like E.C. Rosenow at the Mayo Foundation producing hundreds of thousands of doses for widespread use.
The Fatal Misidentification
The fundamental flaw in this massive public health effort was the misidentification of the infectious agent. The shots were designed to stimulate an immune response against various bacteria, but the illness was caused by the H1N1 influenza A virus. Consequently, the preventative injections offered no protection against the initial viral infection that was spreading so rapidly through the population.
Modern research, however, provides a nuanced understanding of why this bacterial approach targeted the overall mortality. The vast majority of deaths during the 1918 pandemic resulted from a sequential infection, not the influenza virus alone. The H1N1 virus would first inflict damage by destroying the protective cells lining the bronchial tubes and lungs, essentially creating an open wound in the respiratory system. The shots targeted the common bacteria that would then opportunistically invade the damaged lungs, leading to severe, often fatal, secondary bacterial pneumonia. These secondary infections accounted for an estimated 90% or more of the pneumonia deaths. While the bacterial shots failed to prevent the flu itself, they theoretically aimed at the correct mechanism of death, although their actual effectiveness in the chaotic 1918 environment remained highly contested.
Impact on the Pandemic and Future Research
The widespread use of the bacterial shots yielded mixed and confusing results, leading to a public health paradox. The overall pandemic continued its deadly trajectory, clearly demonstrating that the injections failed to stop the disease’s spread or significantly reduce the total number of cases. This failure directly challenged the long-standing theory that Pfeiffer’s bacillus was the cause of influenza.
The controversy over the shots’ efficacy prompted scientists to re-examine the nature of the illness and the limits of bacteriology. This experience accelerated the collective realization that a smaller, filterable agent—a virus—must be responsible for the primary infection. This shift spurred significant research into the nascent field of virology immediately following the pandemic. The scientific community learned a definitive lesson: a vaccine cannot be developed without first identifying the true causative pathogen. The failure of the 1918 bacterial shots created the foundation for modern influenza research, ultimately leading to the isolation of the influenza virus in the early 1930s and the subsequent successful development of true influenza vaccines.