On the Philippines, they gave the dengue vaccine to schoolchildren, a substantial number of whom were seronegative. The disease in that population was much more serious than it would have been otherwise, there were unnecessary deaths, and a good number of parents avoided the measles vaccine for their children, and a measles outbreak promptly followed.
The rushed vaccine apparently substantially increased the risk of Guillain-Barré syndrome in those receiving it. The syndrome is fatal in up to 5% of those that contract it and can cause permanent damage to the peripheral nervous system.
First, do no harm.
Clearly a significant issue, but less so if it was likely to prevent a serious illness. The real issue was the vaccine was administered for a disease that never gained a significant foothold.
Only about 200 cases of swine flu and one death were ultimately reported in the U.S., the CDC said.
More than 500 people are thought to have developed Guillain-Barre syndrome after receiving the vaccine; 25 died
https://www.latimes.com/archives/la-xpm-2009-apr-27-sci-swin...
So in this case, the cure was literally worse than the disease.
Which is my point, the problem was mass vaccination for a non issue. Saving 500,000 lives at the cost of 100 lives is a really easy tradeoff to make. Costing 25 lives to save 0 is a problem.
From another practical concern, we need to preserve the public perception that vaccines are low risk to them, not just that they are lower risk to society than the hypothetical alternative, before healthy people will get vaccinated.
I'm not saying that is how we SHOULD act. I'm definitely not saying that's how we DO act. Clearly, taking into account all of the factors that you mention, we have a very high standard for action, and that's a rational choice given the realities. It's still not a rational choice from the economics perspective, which strictly looks at cost/benefit.
When we get farther along in testing we will have more information and should re-evaluate. Maybe something will prove to work well (we can do a new flu shot in a few months because we have approved the process to create a new one), maybe everything else will fail. Maybe this will fail. We don't have enough useful information to work with now though.