Still Around and Still Deadly
The occasional story about another appearance here or there hardly raises any eyebrows, although it may cause the share prices of the "bird flu" companies (including BioCryst (BCRX), Vical (VICL), Roche (OTC:RHHBY), GlaxoSmithKline (GSK), Novavax (NVAX), Sanofi-Aventis (SNY), Gilead (GILD), Cel-Sci [CVM], AVI BioPharma (AVII), etc.) to rally for a day or two. Despite the reduced interest, H5N1 remains as serious a threat as it ever was.
The infection rates for birds are still high, and the virus keeps showing up in new places and re-appearing in areas where it was thought to be long gone. People in contact with infected birds are at risk, and the lethality rate is high and has not budged. The virus is very deadly even to young adults in good health prior to infection. Just recently a new human case appeared in Lagos, Nigeria and killed a 20-year old female. It was the first such case in that country.
As long as the virus moves around in bird populations, the risk of a mutation is there. If such a mutation would permit rapid human-to-human transmission, an epidemic would surely follow, and a pandemic would be a real possibility. Once authorities were notified of such change, development of a vaccine would take a minimum of 6-8 months. During that time, the virus would have a free reign that could only be curtailed by antiviral drugs and quarantine-like measures. Travel restrictions would be one such option.
Current antiviral drugs include Tamiflu from Roche and Relenza from GlaxoSmithKline. Older drugs such as amantadine (generic) and rimantadine (generic) could also be effective. There have been rumors of resistance, and resistance cannot not be ruled out once the virus mutates. The current model assumes the drugs are effective at stopping the virus.
Modeling A Flu Pandemic
To better understand what would happen in the case of a pandemic, a group of researchers from the U.S., Italy, and France has developed a model to study how such a pandemic would spread and what could be done to stop it. The model was published in the current issue of the free online journal PLoS Medicine.
The model takes into account airline travel flow among urban areas. Because of its speed and reach, airline travel is considered the main channel for spread of the virus around the world. The model also considers a ban on air travel as a means to contain the pandemic. It does not consider variations in travel frequency between individuals or viral spread through rural areas. The model is based on worldwide air travel data and census figures from urban centers near airports.
A key parameter is the so-called reproductive number, a measure of how many people an infectious individual infects on average. Many simulations were done with low reproductive numbers. Seasonal variations matter too because influenza clearly does better in winter, when individuals tend to crowd together, offering many more opportunities for infection. The hypothetical starting site is Hanoi in Vietnam.
A virus with a reproductive number of 1.1 poses a very mild global threat. If the rate jumps to 1.5 however, roughly half of the population of more than 100 countries would be affected. Given lethality rates of 50% or higher, that could mean a lot of deaths. Strict travel restrictions would have little effect on the evolution of the pandemic. That is because most of the spread happens very quickly, before any measures can take effect.
Antiviral drugs would mitigate pandemics if the reproduction rate was below 1.9 and every country had drugs sufficient to treat 5% of its population. We note that this is not currently the case. Many poor countries do not have adequate supplies. But a virus reproduction rate of 2.3 or higher could not be contained even if 20% of the population could be treated.
Another interesting observation is that when only a few countries have stockpiles, a "selfish" strategy where countries are only using the drugs for their own populations does worse than limited worldwide sharing. The latter would slow down a low reproductive virus by more than a year and benefit both donors and recipients.
The model shows that even viruses and conditions that lead to low reproductive rates could seriously affect many people worldwide. A highly contagious virus in an environment that allows for multiple contacts would be nearly impossible to stop.