Flu experts are always on the lookout for new strains of the flu, but it’s not often that they find one before it mushrooms to a potential pandemic. That’s the feat an international group of researchers might have pulled off.
After the latest outbreak earlier this year from a new bird-flu virus, H7N9, the team collected samples from the mouths and throats of more than 1,300 chickens, geese, ducks, pigeons, partridges and quail in the area surrounding Shanghai to determine the size of the H7N9 strain’s reach. What concerned experts — led by scientists at the University of Hong Kong — about the virus was that it hadn’t been seen in people before, so it was critical to understand how it made the jump from animal to human hosts.
By sequencing the genes in the flu viruses they found, the team was able to trace the journey that H7N9 took, and the likely genetic changes it made as it traveled from animal to animal. In the past, that type of pedigree work has only been done on influenza strains after they caused pandemic outbreaks, such as occurred in 2009 with H1N1, when scientists traced the genesis of H1N1 back to pigs. This is the first time that scientists have been able to sketch out such a lineage for a strain that could, but hasn’t yet, caused widespread and sustained disease in humans.
Reporting their results in the journal Nature, they concluded the H7N9 that appeared last spring and sickened more than 130 people, killing 40, was likely born in ducks. It jumped from wild ducks to chickens, blending with flu strains already circulating among the chickens and triggering the formation of two new strains — H7N9 and a new influenza, H7N7.
Such genetic marriages between existing strains — especially those that jump from one species of animal to another — are the driving force for mutations in influenza, as the virus adapts to make itself more at home in its new host and more adept at spreading from, say, one chicken to another.
H7N9 so far meets two criteria for a pandemic virus: it wasn’t seen before in humans, and infections cause major illness and even death. So far, it has limited ability to transmit from person to person — another pandemic prerequisite — but that ability could only be a few mutations away. Unfortunately, it’s impossible to predict what those mutations might be, or if they will even occur.
So how does the newfound knowledge about H7N9’s origins help? For now, it doesn’t, according to Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, which funded the study.
“I’d love to say that yes [this information helps intervene and prevent outbreaks], but I don’t think so,” he said. “What could we possibly do except get rid of all the animals incubating this evolution of the virus?”
That’s certainly not practical, nor desirable. And for now, the knowledge isn’t going to be applied to the new flu vaccine, since H7N9 still doesn’t transmit readily between people. Protecting against other strains that are more actively circulating and infecting people takes priority in the latest seasonal flu shot.
In addition, because flu vaccines take several months to develop and manufacture, the strains contained in each season’s shot are predicted about eight months before the flu season starts in October, and it would be too late to include H7N9 in the vaccines that have already shipped to doctors’ offices and hospitals in anticipation of the coming flu season.
“It’s nice to trace the steps that allowed a virus to make the jump to people, but there is no way of predicting what is going to happen or what must happen for an infection to become sustainable,” Fauci said.
But there may be one way in which knowing an influenza’s pedigree could prove useful. If a pandemic virus circulates and the existing seasonal flu shot does not protect against it, some experts are exploring the idea of a prepandemic vaccine. Such immunizations, which contain strains that are similar to — but not exactly like — the circulating pandemic flu could be used to prime people’s immune systems to better defend against the virus while scientists develop a vaccine more tailored to the pandemic strain.
That’s what could happen if H5N1, a bird flu that caused illness in more than 600 people beginning in 2003, with a death rate of 60%, becomes more virulent. For now, it does not spread easily from person to person, but if it does, Fauci said, the U.S. has a stockpile of H5N1 vaccines that could serve as a prepandemic priming vaccine. Immunizing people with that vaccine could help to slow the spread of H5N1 until scientists develop one that is more specific to the version that eventually breaks through and jumps from person to person.
The same could be done with H7N9 — if it becomes more virulent in coming flu seasons, and if scientists develop a vaccine against the recently circulating H7N9 strain.
With the knowledge of how viruses journey from host to host, flu experts can be smarter about advising public-health experts on how to control pandemics and possibly even avoid them, by identifying new strains early and tracking their evolution. Since influenza evolves to become more effective at infecting victims, so should our methods for staying on top of where the next pandemic strain might emerge.