What do you know about avian flu?
by Philippa Bridge-Cook, PhD.
Scientific Insights® Consulting Group

What do you know about avian flu? And is what you think you know true? It seems like avian flu is a hot topic for media coverage recently, and articles range from saying we are on the verge of a pandemic, to saying that talk of a pandemic is just fear-mongering. What is the truth about the world's current risk of a flu pandemic?

One thing that is certainly true is that we are closer to another pandemic now than we have been since the last pandemic in 1968. Of three conditions required for a pandemic, two have already been met: the emergence of a new virus that humans have little or no immunity to, and the ability of that virus to replicate in humans and cause serious disease. The third condition, the transmission of the virus from human to human, has not yet been met with avian flu.

How would the avian influenza virus change in order for the third condition to be met? Influenza viruses are constantly mutating, and changes can happen by two different mechanisms. Reassortment causes rapid, large changes in influenza viruses and occurs when two influenza viruses coinfect one individual or animal. The two viruses can exchange genetic information. Avian influenza could change to become transmissible between humans if an avian influenza virus and a human influenza virus both infected the same person. The other mechanism by which influenza viruses mutate is by adaptive mutation. In this process, viruses accumulate a series of small changes gradually.

So what is the risk that one of these processes will change the avian influenza virus into a form that is transmissible between humans? Most experts estimate the risk of a pandemic of avian influenza at about 10 percent per year. However, the risk does increase as more birds in more geographical areas become infected, since this provides more opportunities for human infections and changes in the virus.

Is avian flu worth the media hype? The risk of avian flu developing into a pandemic is very real, and the consequences of that occurrence range from fairly mild, to catastrophic. The whole population of the world is vulnerable since it would be a new strain, and in a pandemic situation, likely no geographical area would be spared. Since the virus has to change in order for a pandemic to happen, the exact characteristics of what the pandemic strain would be are unknown. The virulence and transmissibility have a drastic effect on the number of people sick and the number of fatalities. The CDC has estimated 2 to 7.4 million deaths worldwide, based on the relatively mild pandemic of 1957. With a more virulent strain, the number of deaths could be much higher.

To date, avian influenza has infected 135 individuals (in laboratory-confirmed cases), and 69 have died [1]. However, an estimate of the mortality is impossible at present, because it is possible that milder cases also occurred in affected areas, and these were never reported. Even if that were the case, clearly the current strains of avian influenza can cause very severe illness. In fact, two surprising characteristics of avian influenza to date are its overall severity, and its ability to cause severe disease in previously healthy children and young adults. Once the virus mutates to become transmissible from human to human, it is possible and perhaps even likely that its severity will be somewhat lessened.

Is it possible that the start of a human avian flu pandemic could be contained at the source? The first possibility for achieving this was through control of the avian influenza virus in birds, using antiviral medicines and killing infected birds. Those efforts failed and avian influenza is now endemic in poultry in Asia, and also found in Europe. Two research papers modeling the spread in Asia of a putative avian influenza strain that is transmissible from human to human concluded that preventative treatment with antiviral medicines, quarantine measures, and vaccination could contain a pandemic at the source, if the transmissibility is relatively low [2, 3]. However, this would also depend upon the effectiveness of disease surveillance and early reporting, and the availability of large stockpiles of antiviral medicines.

Given the real threat and serious consequences of an avian flu pandemic, it is important for governments and medical agencies to be as prepared as they can. A vaccine against a pandemic avian flu strain would be an important tool to reduce the number of infections, and deaths from infections. Two companies in the US have had phase I clinical trials ongoing for avian influenza vaccines since April 2005, and the results of these early studies should be available in 2006. Although the pandemic strain might not match the strain used to develop this vaccine, the vaccine might still have some efficacy against the virus, and experience in this type of vaccine production should help expedite the availability of a more specific vaccine if a pandemic develops.

Stockpiles of antiviral medicines are also important tools, and can be used prophylatically for health care workers, or contacts of sick people, as well as for treatment of the disease itself. Of the four antiviral medicines currently available, avian influenza is resistant to two: amantidine, and rimantidine. The other two antivirals, oseltamivir (Tamiflu) and zanamivir (Relenza), should be effective against avian influenza, although these medications must be started within 48 hours of when symptoms start to be effective.

Given everything we have learned about pandemics from the past three pandemics in the 20th century, advances in public health knowledge and hygiene, advances in medical treatment and prevention with vaccines, is the world better prepared to deal with a pandemic now than in the past? Will knowledge reduce illnesses, deaths and suffering? It won't be until the aftermath of the pandemic, whenever it occurs, that we will be able to truly answer that question.

So what is the truth about the world's risk of an avian flu pandemic, and is the media hype justified? Although the risk in any given year is modest, the risk overall seems fairly high. And the media has a tendency to exaggerate the truth in both directions, so according to most articles either the pandemic is starting tomorrow, or anyone who says a pandemic is likely is paranoid. Maybe the sum of those two views actually paints a fair picture: the truth is probably in the middle.

References

1. Cumulative Number of Confirmed Human Cases of Avian Influenza A/(H5N1) Reported to WHO

2. Longini, I.M., Jr., et al., Containing pandemic influenza at the source. Science, 2005. 309(5737): p. 1083-7.

3. Ferguson, N.M., et al., Strategies for containing an emerging influenza pandemic in Southeast Asia. Nature, 2005. 437(7056): p. 209-14.