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My on-air "swine flu" update was trimmed back last night. Reports of Michael Jackson's heart attack and possible death were beginning to filter through the news ether just as Newsnight went on air. So here's the bit I didn't have time to say.

It's about the vaccine against this new flu virus, which the government sees as its most powerful weapon in holding back a potentially overwhelming tide of infections - whether mild or severe. The Health Secretary, Andy Burnham, yesterday confirmed that the Government has contracts with Glaxo SmithKline and Baxter for vaccine for the whole UK population, and that some of this will arrive as early as August.

But in the very next breath we learned that only 60 million of the 120 million doses on order will be ready by this winter. At the estimated two doses apiece to provide protection, that's enough for only half of the UK population. I think many people are expecting a vaccine out there sometime soon with their name on it, should they want it. That will not now be the case for everyone.

It may be that trials show some people gain protection from only one dose, so it's possible there may be more doses to go around. But the Government is clearly going to have to decide who gets the vaccine first, as it comes off the production line. That judgment's a fine balance between the needs of the individual against those of the public at large. High-risk groups such as those with asthma, diabetes or suppressed immune systems, for example, have a strong case for early vaccination. But there's also logic in early vaccination for children, because they're notorious "super spreaders" of influenza.

Sir Liam Donaldson, the Chief Medical Officer for England, made it sound as if this was all part of the master plan, pointing out that pandemics typically last two to three years, so we're going to need vaccine stock in subsequent years. The public may not see it like that if we really do reach the tens of thousands of cases a week he's predicting for this autumn and winter, especially if the virus changes in a way that makes it more dangerous.

As , reminded us at the start of all this, flu viruses are notoriously unpredictable, and can change rapidly. One piece of news I was told last night is typically ambiguous. Flu specialists in the US now estimate that as many as 30 to 40 per cent of infections may be completely symptom-free. Good news if you happen to be one of those with such a mild response, but tricky for scientists trying to track and model the spread of infections to help officials who are planning ahead.

Scientists learn more about this virus every day, and plans put in place assuming the pandemic virus would resemble H5N1 "Bird flu", with its 60% mortality, are having to evolve. So what may appear to many of us to be policy-making on the hoof, could turn out to be common-sense tweaking as the world tries to make sense of who's getting infected, and how badly.