By Fergus Walsh
BBC News Medical Correspondent
Human trials of an experimental vaccine against the H5N1 bird flu virus have begun in Belgium.
The jab is aimed at protecting against the virulent avian infection which has killed more than a hundred people worldwide.
The BBC was given exclusive access to the trial of 400 volunteers, designed to test what protection the vaccine will give.
Stefaan Vervaet says he wasn't worried about taking part in the trial
Half on the trial will receive the prototype H5N1 vaccine, the other half will be given a dummy, or placebo. So why did the volunteers sign up?
Stefaan Vervaet said: "I guess it's a noble thing to do."
He has heard about the men in the UK who fell ill during a drug trial, but says it did not put him off taking part in this research.
"This is not a new drug so the risk is zero, or so they tell me."
Priming the immune system
All the volunteers are taking part for humanitarian reasons; just one admitted another motive.
Mireille Smets said she was being paid 300 euros. "If I'm honest, it's the money and the experience," she said.
In time, we may all have reason to be grateful to these Belgian volunteers.
It is only by running experiments now that scientists can work out how best to create a vaccine once a pandemic has begun - a vaccine that could save millions of lives.
The vaccine works by using fragments of the H5 "spikes" on the virus surface and the N1 protein. These are inactivated in the vaccine.
The jab triggers the immune system to create antibodies, and these will attack the virus in case of a real infection.
But flu viruses mutate - they change. That is why people need a flu jab every winter.
So there is no guarantee this prototype vaccine will work when a pandemic finally happens.
And H5N1 is still a bird disease - the only humans who have caught it had direct contact with sick animals.
We do not know if, or when, the virus might become an infectious human disease.
So, there is a question as to whether it is worth immunising whole populations with an experimental vaccine.
GlaxoSmithKline (GSK), the company producing this vaccine, says it is.
The fee led Mireille Smets to decide to take part
The company believes a chemical agent, or adjuvant, in the jab will help boost the body's immune response to H5N1.
Dr Ripley Ballou from GSK, said: "We believe that using an adjuvented vaccine strategy will allow us to prepare the immune system so that it will be able to respond and recognise a virus that may not be exactly the same as the virus in our vaccine.
"That gives us the ability to seriously propose pre-pandemic vaccination."
But the professor of immunology who is carrying out the GSK vaccine trial disagrees.
He says we simply do not know if the prototype will work against another flu strain, so it would be wrong to use it for mass vaccination now.
Professor Geert Leroux-Roels says: "That would not be the right strategy.
"The prototype vaccine we are evaluating may show some resemblance to the pandemic strain, but, since we do not know what the pandemic strain will be, it would be premature to vaccinate everyone with that vaccine."
Many governments are stockpiling small amounts of prototype H5N1 vaccines, but none is planning to use them until a pandemic starts.
This Belgian trial may encourage some countries to immunise sooner and offer the jab to everyone.