The general feeling is that the reaction will almost certainly be an order of magnitude over the threat, given our knowledge and expertise with pandemics, and that most deaths will be from human response as opposed to the pandemic itself (as would be the case with a dirty nuclear bomb).
I'm not disagreeing, I just don't see any obvious ways that the human response would cause a lot of deaths - so I'm curious.
To be ultra conservative, assume there's a 0.1% chance that this becomes 10% as bad as the 1918 flu. The 1918 flu killed ~3% of the world population - if swine flu is one tenth as bad it means ~300K deaths in America. Therefore the expected number of (American) deaths is 300. As long as the vaccine is expected to cause less than a 300 deaths, we should do it, right?
But in the 1976 case: there was only 1 death from the feared strain... and 25+ deaths from the vaccination program, which was halted early. Overreaction can be out of proportion to the danger.
And there are all sorts of other harder-to-measure costs, including excess deaths, from overreaction. If a collapse of commerce driven by fear retards economic growth, more people die of other causes. If funds are diverted from other health interventions, more people die of other causes. Even in trying to vaccinate the whole nation: some small but nonzero number of people will die in the extra road travel to the vaccination sites.
Your 'ultra conservative' numbers are still way on the alarmist side, as far as I'm concerned. We may never have another natural pandemic even 1/10th as bad as the Spanish Flu. People are healthier; medicine and public health is far more advanced; the same WW1 disease-breeding-overcrowding is unlikely to recur; we have antibiotics (and many of the Spanish Flu deaths may have actually been from coincident bacterial infections).
There was a good article at The New Republic during the bird flu mini-panic that emphasized how unique the Spanish Flu was to its time. It appears to have disappeared from their archives, but fortunately I quoted it in its entirety to another forum at the time. See here:
http://www.xent.com/pipermail/fork/Week-of-Mon-20051003/0380...
TV news stations will of course hype this thing beyond all proportion, as they always do, but that doesn't mean the threat isn't real and that, as the article suggests, the death toll from Swine 'flu itself won't run into tens if not hundreds of millions.