KEY POINTS:
Dr Donald Burke has been working on an Aids vaccine since 1986.
"At the time I started," says the dean of the University of Pittsburgh's Graduate School of Public Health, "the usual question was, 'How long is it going to be before we have an Aids vaccine'?"
Being conservative, he would say, "It'll probably take 10 years."
Then 1996 came around and he was still saying saying 10 years.
"Now 2006 has come and gone and I don't say anything anymore because it has proven much more difficult than we imagined."
A recent article in Scientific American suggested that an effective Aids vaccine has been hard to produce because the virus mutates at an incredible rate and directly attacks immune system cells that would normally help fight it.
Burke, who spent much of his career as an infectious disease specialist in the military, says both those things are true, but that still doesn't explain why a vaccine has been so elusive.
After all, flu viruses mutate constantly, he said, and yet scientists have figured out ways to immunise people against the flu.
"If I knew the answer, we would have solved the problem. And so if you find anybody who says they know the answer, they don't."
The one thing he does know is that in the world of infectious diseases, it is the slow, chronic infections that present the biggest challenges in terms of creating vaccines.
That is true whether the disease comes from a virus (Aids), bacteria (tuberculosis) or a parasite (malaria).
"The chronic diseases are chronic because they have found ways to evade the immune system or have mechanisms to actively alter or suppress or divert the immune system," Burke says.
HIV, the virus that causes Aids, changes once it gets inside a person's body, so the virus the immune system is initially trained to attack is not the same one it sees days or weeks later. Vaccines work best against diseases that tend to hit people's bodies hard and then leave, such as flu or measles.
The struggle to come up with an Aids vaccine contrasts sharply with the progress that has been made in diagnosing and treating the disease, Burke says.
In industrialised nations, doctors now can tell how much of the virus an infected person has, what major subtype it belongs to, and then can prescribe drugs that keep the patient alive, if not completely healthy.
One reason people in wealthy Western nations haven't been up in arms over the lack of an Aids vaccine is that the main burden of the disease today is falling on other people, particularly in Africa and Asia.
Because of drug treatments, people falsely believe the epidemic is under control, but Burke says: "The notion that we've got it solved couldn't be a worse misrepresentation of what's going on.
"There are millions of new infections a year around the world, and even in the United States, 40,000 to 50,000 new cases a year."
It is hard to know when a disease will mutate in animals or humans so that it spreads rapidly inside borders. Burke says the West Nile virus sat in Africa for 60 years and then all of a sudden it swept through the United States unannounced. Most epidemics start with germs that make the leap from animals to humans when people come in contact with animals' body fluids.
Using a supercomputer, Burke and his colleagues have simulated what would happen if an avian flu epidemic began in densely populated Thailand.
By modelling how all 85 million people in the region interact with each other, the computer program estimated that 95 per cent of the spread could be halted by using 300,000 courses of antiviral drugs.
As a result of such calculations, Roche has donated more than five million treatment courses of its drug Tamiflu to the World Health Organisation for such emergencies.
Pitt's Graduate School of Public Health currently has a US$10 million ($18.5 million) grant from the Bill and Melinda Gates Foundation to improve its computer modelling of infectious disease epidemics.
Burke believes that "if we can monitor the viral chatter - it's like terrorism chatter. When you monitor the airwaves and you know something is happening - and we can see a virus is jumping species - that's where we should be positioning ourselves to quench epidemics before they begin."
Mark Roth writes for the Pittsburgh Post-Gazette
