Last spring, on April 9, 2013, the Centers for Disease Control and Prevention (CDC) raised its Emergency Operations Center to Level II, the second-highest alert level. The Chinese government reported increasing numbers of humans falling dangerously ill due to a powerful new flu strain, H7N9, that had never been seen before in animals. By the end of June there were at least 132 confirmed cases, including 43 deaths.
The World Health Organization quickly launched a full effort toward creating a vaccine for this new strain. The vaccine is now complete and will be moving into clinical trials any day now.
So far it appears H7N9 has a difficult time moving between humans. Most cases transfer from an animal host to human. However, there is a quality that makes the H7N9 strain particularly dangerous. SmartPlanet spoke with Dr. Richard Webby, Director of the World Health Organization Collaborating Center for Studies on the Ecology of Influenza in Animals and Birds at St. Jude Children's Research Hospital.
SmartPlanet: We know that the H7N9 flu has infected humans, and some of the cases were fatal. But has it shown any human-to-human transfer (as opposed to the typical animal-to-human transfer)?
Richard Webby: No, not yet. There is some evidence of very limited human-to-human within family groups. But there is no evidence that this thing transmits well between humans. It’s essentially still a disease of chickens.
I imagine there are a great number of flu strains that are carried by animal populations at any given time. So if a strain is not yet moving easily between humans how do you assign risk to it? Or what warranted a vaccine for this particular virus?
This is one of the big problems. Trying to predict which of these viruses we ought to be most worried about. In the animal population there is a whole soup of flu viruses. We are not good at determining which of those are most likely to jump into humans.
This particular virus had features that concerned us. There are specific mutations in its genome that we associate with an increased ability to grow in mammals. And our laboratory studies found an elevated ability of this virus, as compared to most other avian viruses, to transmit.
Was there any overt indication that this strain of flu was more of a risk?
Consider the epidemiology that was going on in China at the time and compare it with H5N1 -- which is the other bird flu that has been going for a decade or more now –- there has been 600 cases in humans.
But with H7N9 there was a hundred within a space of a few weeks.
Why is there an acceleration of cases of H7N9?
Well, we know that there are genetic elements of a virus associated with growth in mammals. This particular virus had some of those.
Also, the hemagglutinin (H) protein in this virus attaches the virus to the host cell. We saw some significant changes in that protein that looked like it might be able to bind to a human host cell.
There are subtle differences between the avian host cell and the human host cell in terms of what viruses bind to. So there are signatures that avian viruses have that are associated with binding to those avian cells. This virus is primarily avian still but it did have some signatures that we associate with mammalian virus.
Where does this virus rank among the flu viruses we know about already?
I think it is more infectious than H5. It's probably more infectious than H9 which is another virus we sporadically see in humans.
But if we rank it highest amongst the avian flu viruses, what does that mean? The next leap is: Does it have a ten percent chance of being able to go human-to-human? Is it one percent or less? We really don’t know. I think the chances are still most likely that it is going to remain a chicken virus that will spread human infections.
But flu viruses do change. If you give them enough opportunity they will adapt to a new host.
So it may have the ability to turn into a real bona fide human pathogen. That is why we continue to monitor this in the bird populations, control it and reduce the number of human infections.
I understand that of those infected last spring with H7N9, 30 percent died.
[Thirty percent] of those confirmed infected. There are probably many, many more people that were infected and probably asymptomatic. We have to be careful with that 30 percent figure. It is just 30 percent of those that we know were infected.
How does it manifest itself in humans? Is it stronger than a regular flu? Are infected humans much sicker? Is there a higher chance of pneumonia?
The H7 is variable. The one thing we know is what stops a lot of the severe disease of the normal human flu strains is immunity. Essentially anyone over the age of 10 years has probably had the flu a couple of times, and at least in the U.S., most have been vaccinated a couple of times. So we have quite a bit of immunity to the human flu viruses.
And that probably stunts a lot of the ability of that virus to cause disease. It can still get in and make you a little bit sick. It can still transmit, but the severity is limited by your immunity.
The H-type is the most variable part of that virus. That is why we call them H1, H3, H5 and H7 because they are very different. We are not expected to have immunity to the H5 or we are not expected to have much immunity to the H7. That is one reason they cause disease.
It is a little bit of a mystery as to why this H7N9 causes such severe disease but it does.
Why is it a mystery?
We group flu viruses into pathogenic types. Highly pathogenic or low pathogenic. Some viruses of the H5N7 type fall into a very virulent form. And we know how they do this. They accumulate additional amino acids in their H protein. The H5N1 is one of these highly virulent forms. If it has that extra amino acid, it kills chickens. And it's associated with severe disease in humans.
Even though H7 can also do that, this particular virus does not have that extra bit.
So that is a good thing?
No. This is the problem. It does not cause a lot of disease in chickens, quails, etc. So with the H5N1 you see a lot of disease in your birds and you know the virus is there.
With the H7 you cannot tell just by looking at the chickens. You have to be swabbing these birds regularly to find it. That obviously makes it harder to control. It can spread a lot quicker.
So the chickens are contagious before they show any symptoms. Isn’t that the case with most viruses?
With any flu viruses there is a period when you’re infectious before you start to get clinical signs. With the H5 you [eventually] see sick birds.
With the H7 they do not get sick at all. Through the whole infectious course the animals can be wandering around, unprotected.
That's pretty serious. What do you think avian flu will do over the next 10 to 15 years? What is the potential for a possible pandemic?
I think it is inevitable that there will be another pandemic.
But if you look at the pandemics we've seen they have all been from viruses of the H1, H2 and H3 type. In 1918 with H1, 1957 with H2, 1968 with H3 and in 2009 with H1 again. So in terms of H1, H2 and H3 types I think it is a matter of when it will happen again rather than if it will happen again.
There is still a question with these other more classical avian types like the H5 and H7. I personally believe that they probably have the ability to become a human pathogen.
What can we do to prevent a pandemic?
The best hope is to monitor what is going on in these animal populations. If we find viruses like the H7 that we perceive as high-risk then we start making viable vaccine strains.
We would like to be on course toward a universal flu vaccine.
We have vaccinated against the H1 but essentially you are only protected from a portion of these H1 viruses. And you are not protected against the H5 or H7. There is some hope that perhaps we can target other parts of the virus. There is a lot of work toward creating a universal flu vaccine. We are not close right now. But hopefully in 10 or 15 years we will have a much better handle on how to do that.