Analysis of the 1918 strain revealed several mutations also found in H5N1. The findings suggest that both strains share an ability to jump directly to humans from other animals without having to first combine with a flu strain already adapted to humans.
"The sequence evidence from 1918 suggests that the virus is from completely avian origins," said Adolfo Garcia-Sastre, a microbiologist at Mount Sinai School of Medicine in New York City.
Human immune systems would likely be caught off guard by a purely avian virus to which they have no acquired immunity.
The century's other great pandemics, of 1957 and 1968, were sparked by hybrid flu viruseshuman flus that acquired some genes from an avian source.
"This [finding] suggests that pandemics can form in more than one way, which is an important point," said the AFIP's Taubenberger. "We've been identifying a series of mutations that we believe are important in the way that an avian virus would adapt to become a human virus," Taubenberger added.
Though the evidence is compelling, only pre-1918 samples would allow scientists to be 100 percent sure that no aspects of the Spanish flu were previously present in contemporary human flu strains and that the flu was of completely avian origin.
"What makes a virus able to go from an avian reservoir to humans?" Mount Sanai's Garcia-Sastre asked. "That's important. Influenza is mainly a virus of birds. There are many different strains in birds and only a few affect humans. Pandemics occur when one of these jumps into the human population and can affect people by propagating from person to person."
Deadly Virus May Save Lives
CDC officials are taking extreme care to safeguard the re-created 1918 virus, though it would not likely lead to a major pandemic if the strain spread among the general population.
Tests have shown that mice that had been injected with current flu vaccines were protected from the 1918 strain.
"We know that all documented pandemic episodes have involved the emergence of a completely new strain," Garcia-Sastre said. "That happened in 1918, 1957, and in 1968."
"There is also evidence that people who were 40 years or older in 1918 were spared," he added. "One hypothesis to explain that is that persons 40 years old or older were exposed to an H1 virus before and had some immunity."
Now, decades after its outbreak, the 1918 virus may be a weapon rather than an opponent.
"I think we've been able to unmask the 1918 virus, and it's revealing to us some of the secrets that will help us prepare for the next pandemic," said Julie Gerberding, Director of the CDC.
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