The 1918 Spanish Influenza Pandemic

Approximately 50 million people died after being infected with the Spanish influenza dur- ing the pandemic of 1918. The high infection and mortality rate among young people was one of the baffling mysteries of the 1918 flu pandemic. The elderly are generally the most vulnerable during outbreaks of infectious disease, but they were largely unaffected by the Spanish influenza virus. A recent reconstruction of the timeline of the dominant influenza strains dating back to the 19th century has offered a possible explanation of the severity of the 1918 flu pandemic, particu- larly towards young people.

Strains of influenza can be identified by the proteins on the surface of the influenza virus that allows it to enter and exit the cells of your body. These proteins are hemagglutinin and neu- raminidase. Hemagglutinin acts as the key to unlock the doors to the cell and allows the virus to enter the cell and begin using the cell’s machinery to replicate itself. Then the neuraminidase is used to unlock the door from the inside and allows the replicated virus to leave the cell and infect other cells in the body.

There are 16 subtypes of hemagglutinin and 9 subtypes of neuraminidase. Different combinations of these subtypes account for all of the different strains of influenza and each of these subtypes have a number of different antigenic variants, which leads to possible thousands of strains that are possible.

Using preserved tissue samples, scientists have constructed the genomes of the dominant influenza strains of the 19th and 20th centuries. Beginning with the H1N1 subtype in 1830, it is believed that N8 inserted itself into the genome and from 1847 to 1889, the dominant subtype was the H1N8 subtype.

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H3 jumped into the genome around 1889 and the H3N8 strain was domi- nant for the next twelve years. H1 returned and the H1N8 subtype was dominant from 1900 to 1918, when N1 came back into the fold. A new strain of H1N1 was responsible for the pandemic of 1918.

Following this timeline, the H1N1 subtype that was dominant from 1830 to 1847 would have exposed people to a strain of influenza similar to the strain of H1N1 that was responsible for the flu pandemic in 1918. The H1 subtype was also present in the dominant subtype from 1847 to 1889. Those born between 1830 and 1847, who would have between 71 and 88 years old in 1918 would have been inoculated by the similar strain that they were exposed to as children and young adults. Scientists hypothesize that this is why the elderly were mostly immune to the H1N1 strain that caused the pandemic in 1918.

On the other hand, young people were devastated by the influenza in 1918. Looking at the timeline of the dominant influenza strains, between 1889 and 1900, the dominant influenza sub-type was H3N8, which does not include any of the subtypes of the H1N1 subtype that caused the pandemic in 1918. The unfamiliarity of the immune systems of those born between those years to the H1N1 subtype is speculated by scientists as the reason why the 20-40 age group suffered disproportionately high mortality rates during the 1918 pandemic. Those born between 1889 and 1900 when the H3N8 subtype was dominant fall almost directly into that age category in 1918.

Another reason young people with robust immune systems were effected so harshly is that scien- tists have identified the 1918 flu strain to cause a cytokine storm, where the infected person’s own immune system goes into overdrive, causing a cascade of an overabundance of inflamma- tory factors that overwhelm the body and leads to death. Further exacerbating that age group’s innate vulnerability to the 1918 pandemic flu strain was the ongoing First World War. That age group made up the bulk of the enlisted soldiers that fought in the war. As a respirator disease that spreads easily through the air, the influenza would infect thousands of soldiers who lived in close proximity with other soldiers.

The influenza pandemic of 1918 disappeared as quickly as it appeared. After nearly 50 mil- lion deaths and nearly half of the world’s population becoming infected with the virus, the pan- demic eventually came to an end naturally. One of the enduring mysteries of the 1918 flu pan- demic was its proclivity to claim the lives of young people who would have had robust immune systems. New evidence is showing that young people were not exposed to and familiarized with the 1918 flu strain, while older groups were exposed to a similar strain to the H1N1 subtype when they were children and were largely immune to the flu strain that caused the 1918 pandemic.