In 1918, as one global devastation in the shape of World War I came to an end, people around the world found themselves facing another deadly enemy, pandemic flu. The virus killed more than 50 million people, three times the number that fell in the Great War, and did this so much faster than any other illness in recorded history. But why was that particular pandemic so deadly? Where did the virus come from and why was it so severe? These questions have dogged scientists ever since. Now, a new study led by the University of Arizona (UA) may have solved the mystery. Michael Worobey, a professor in UA College of Science’s Department of Ecology and Evolutionary Biology, and colleagues describe their findings in the Proceedings of the National Academy of Sciences. They hope the study not only offers some new clues about the deadliness of the 1918 pandemic, but will also help improve strategies for vaccination and pandemic prevention, as Prof. Worobey explains: “If our model is correct, then current medical interventions, especially antibiotics and vaccines, against several pneumonia-causing bacteria, could be expected to dramatically reduce mortality, if we were faced today with a similar set of pandemic ingredients.”

The 1918 pandemic killed predominantly young adults One of the questions that has been particularly vexing is why the 1918 pandemic human influenza A virus killed so many young adults in the prime of life, he says, adding: “It has been a huge question whether there was something special about that situation, and whether we should expect the same thing to happen tomorrow.” Share on Pinterest Researchers reconstructed the origins of the 1918 pandemic virus, the classic swine flu and the postpandemic seasonal H1N1 flu virus lineage that circulated between 1918 and 1957, to find out why the 1918 pandemic was so deadly. Usually, the human influenza A virus is deadlier to infants and the elderly. But the 1918 strain killed many people in their 20s and 30s, who mainly died from secondary bacterial infections, especially pneumonia. For their investigation, the researchers developed an unprecedentedly accurate “molecular clock,” a technique that looks at the rate at which mutations build up in given stretches of DNA over time. Evolutionary biologists use molecular clocks to reconstruct family trees, follow lineage splitting and find common ancestors of different strains of viruses and other organisms. Prof. Worobey and his team used their molecular clock to reconstruct the origins of the 1918 pandemic virus, the classic swine flu and the postpandemic seasonal H1N1 flu virus lineage that circulated between 1918 and 1957.