Phase 1 clinical trials began March 16th 2020. Since that time, between the US and the UK, over 100,000 people have participated in the Phase 1, Phase 2, and Phase 3 clinical trials for Moderna and Pfizer. Tens of thousands more for J&J. Animal trials using rhesus monkeys were also carried out successfully. That's how we know.
"Without wanting to commit the logical error of “post hoc ergo propter hoc,” there are disturbing reports of bad things happening to people who take these vaccines."
You've correctly identified the common fallacy for people looking at those reports as evidence of a problem. Put another way, if you gave anything (Tylenol, gummy bears, a glass of fresh water) to 105,000,000 people - starting with the oldest and most frail - and then watched them all like a hawk, some would have adverse medical events. Do gummy bears cause broken hips? Does drinking a glass of water cause death? Of course not, and we know that. But people have health issues and people sometimes die. If we attempt to paint them all with a broad brush trying to find a singular thing to tie all that data together (e.g. "IT MUST BE THE VACCINE!") then we will almost certainly not arrive at the truth. Each event must be individually examined. And they are.
To date, there have been zero medical investigations which have determined that a person in the US or the UK died because of a COVID-19 vaccine. Literally not a single death has been causally linked to any of the available COVID-19 vaccines in either of those countries (the only two I'm really tracking data for). Out of 105,000,000 people who've had at least one dose of a COVID-19 vaccine. That's pretty telling.
"There are also reports of people contracting the virus after vaccination."
The vaccines aren't 100% effective. No vaccine is. The polio vaccine comes close after you've had four doses of it. The smallpox vaccine that wiped smallpox off the face of the Earth is only 95% effective. You can achieve a perfect end result with an imperfect product.
It's a bit simplistic to say this, but if the vaccine is 95% effective, that means 1 in 20 can still contract it (yes I know it's more complicated than this, but this is the easy math example). So if 105,000,000 people have been vaccinated, we would expect that about 5.25 million could still contract the disease (along with 99.75 million who can't). This gap can come because of an immune system problem (like chemo or AIDS), or because of existing sickness, or because there are some people whose immune systems don't react to a particular antigen. But again, it doesn't have to be perfect in all cases to do its job.
“Phase 1 clinical trials began March 16th 2020. Since that time, between the US and the UK, over 100,000 people have participated in the Phase 1, Phase 2, and Phase 3 clinical trials for Moderna and Pfizer. Tens of thousands more for J&J. Animal trials using rhesus monkeys were also carried out successfully. That’s how we know.”
I am told that more time is required for adequate testing than has passed since testing began. Q.E.D.
“To date, there have been zero medical investigations which have determined that a person in the US or the UK died because of a COVID-19 vaccine.”
I very much doubt that TPTB would allow any such investigation to reach any other conclusion.
Or the difference between safety and efficacy, and the trade-offs between them?