Sir Liam told BBC's Sunday AM programme: "In a normal winter flu year ... flu actually kills in excess of 12,000 people every winter with the normal winter flu.
"But if we had a pandemic, the problem would be that our existing vaccines don't work against it, we would have to develop a new vaccine, and people don't have natural immunity because it hasn't be around before. So the estimate we are working to in the number of deaths is around 50,000 excess deaths from flu. But it could be a lot higher than that, it very much depends whether this mutated strain is a mild one or a more serious one."
Sir Liam told the programme that the lessons of history strongly suggest that the bird flu virus will combine with a human flu virus, becoming easily transmissible.
"It has happened before. It happened in 1918, it happened in 1958, and it
happened in 1968/69. These things come in natural cycles, once in a while,
every 10 to 40 years the flu virus mutates into a strain which we haven't
got natural immunity to. It is when (not if). But there is a lot we can do
to prepare. If we look back to the last pandemic in 1968/69, we didn't have
some of the measures that we now have, like anti-viral drugs."
Sir Liam acknowledged that the Government has some way to go before it meets its target for stockpiling vaccine. The Government is attempting to amass 14 million doses of the Tamiflu drug.
He told the programme: "We have got 2.5 million so far and they are coming in at the rate of about 800,000 a month. We are one of the few countries to have embarked on this stockpiling at a very early stage ... until now, we have been one of the countries which has been in the lead on this, and it is part of a comprehensive plan.
"We can't make this pandemic go away, because it is a natural phenomenon, it will come. But what we can do is to limit its impact, and that is the whole basis of the contingency plan that we will be releasing on Thursday. We have got some drugs so far, and those would obviously be deployed, but give us a little bit more time and we will be even better prepared."