Researchers from Switzerland have reconstructed a complete genome of the 1918 Spanish flu virus from preserved lung tissue of a young man who died more than a century ago, providing new insights into how pandemics evolve and how deadly viruses adapt to humans.

The findings, published this week in BMC Biology, mark the first-ever fully dated and complete influenza genome from Europe. What makes this achievement even more remarkable is the condition of the sample: it came from a formalin-fixed wet lung specimen, stored in a medical collection since July 1918.

“This genome, dating to the early first wave in Switzerland in the summer of 1918, already showed many genomic signs of adaptation to the human host,” the researchers wrote.

The patient, an 18-year-old, succumbed to the virus during the very beginning of the first wave of the pandemic in Zurich. At that time, the world was grappling with the spread of a mysterious and deadly illness that would eventually kill between 20 to 100 million people worldwide, more than any other flu outbreak in recorded history.

While previous efforts to sequence the virus relied mostly on samples recovered from permafrost graves or paraffin-embedded tissues, the Swiss team used a less invasive and safer method to unlock the secrets of viral RNA from fragile tissue.

“We present a new user-friendly ancient RNA (aRNA) workflow, which yielded reliable results from formalin-fixed wet specimen samples linked to the 1918 influenza pandemic,” the authors explain. The team’s technique avoids toxic chemicals like phenol and chloroform, and instead uses a “ligation-based” method that preserves the direction and structure of RNA fragments.

The implications are far-reaching. One of the most important discoveries is that this Swiss strain already contained three mutations—G16D, L283P, and T222A—that are associated with human adaptation. These changes, the scientists believe, helped the virus become more efficient at infecting people.

“In fact,” the study says, “the first precisely dated and complete genome (97% coverage ≥ 3) from Europe is currently the only first wave genome with the substitutions G16D and L283P, which potentially gave this strain an advantage during the course of the pandemic.”

What does that mean in simple terms? These mutations may have helped the virus become deadlier or spread more easily during later waves. In fact, all high-quality virus genomes from the second wave of the pandemic – the most lethal phase – also carry these same mutations.

Perhaps most importantly, this work offers more than just a look into the past, it also offers a roadmap for the future. Understanding how the virus mutated and adapted to humans in 1918 may help scientists predict the behavior of future pandemics, especially those caused by zoonotic viruses—pathogens that jump from animals to humans.

“A better understanding of historical pandemics can provide relevant information for the prevention of future pandemics,” the authors write.

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