COVID-19 Day 64: Why the 2nd Wave of Spanish Flu was more deadly


We’ve been warned to be careful about re-opening because the 2nd wave of COVID-19 could be worse. Experts remember the devastating Spanish Flu of 1918 which killed an estimated 675,000 people in the United States. More than half of these deaths occurred in the 2nd wave of infections that occurred in the Fall of 1918. 

These were the early days of modern medicine. They wouldn’t discover penicillin until 1928 and they wouldn’t identify the flu virus until 1933. In 1918 people thought the Spanish flu was caused by bacteria.

Why was the 2nd wave so much more deadly than the 1st or 3rd wave? Some think it was because people who survived the 1st wave in the Spring, let their guard down and stopped following safe hygiene practices. Others think returning soldiers from WW1 fueled the rise in cases. Some modern virologists hypothesized that the flu virus may have mutated into a more deadly strain. Unfortunately no live virus samples existed from 1918 to test this hypothesis. Until nearly a hundred years later, samples were unearthed. Literally.

In 1997, microbiologist Johan Hutin was able to recover flu virus from a body frozen in the Alaskan permafrost. He got permission from tribal elders to exhume the graves of tribal members who died of the flu in 1918. Hutin was able to find retrieve frozen samples and bring them back to the CDC for research. The full story is even more amazing and spans over 40 years of failed attempts. You can read it on the CDC website, but I’ll summarize the results here. 

A team was able to retrieve flu viral genes from Hutin’s sample and reconstruct a live virus. Under Level 4 safety protocols (the highest level reserved for the most deadly pathogens like Ebola), they tested the revived 1918 virus on special lab mice with human-like lung cells. They discovered the 1918 strain replicated 50x faster than the normal seasonal flu virus.


Fig A. Seasonal flu infecting lung tissue Fig C. 1918 flu infecting lung tissue

Fertile chicken eggs are used to breed normal flu virus to develop our yearly vaccines. When the 1918 strain was injected into live eggs, it killed the chick fetuses. Molecular biologists found a unique set of mutations in both the virus’ shell and in its RNA that made it more deadly than the seasonal flu.

The scary thing that keeps virologists awake at night, is that these random mutations might happen again. Flu viruses mutate all the time, which is why we have to get a new flu shot every year.  There’s no guarantee another flu strain won’t randomly assemble the same combination of genes that made the 1918 strain so deadly. 

Thankfully, evolution favors less-deadly virus strains. That’s because deadly strains tend to kill their hosts before they are able to infect more people, which is how the 1918 pandemic burned itself out. The less-deadly strains are able to keep going. COVID-19 is now too widespread and will probably stay with us year after year but become less deadly too. In fact, it’s mostly harmless to people under the age of 45.

That doesn’t mean we can expect the 2nd wave of COVID-19 to be less deadly. Epidemiologists expect that the 2nd wave will likely occur this Fall, along with the return of flu season. Combined cases could more easily overwhelm the hospitals, which is why we need to prepare now, restock PPE, and remain vigilant. Let’s just hope history doesn’t repeat itself.


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