I tried everything to convince her she's completely wrong, without success.
What else can I do?
Unfortunately I think the harder you try the more she'll entrench - that's what's so insidious about this mindset because (in my opinion) it appeals to people who have suffered traumatic disappointments in their lives, and gives the balm of belonging to a select group of the "awakened".
The only way I've managed to cope with my own deteriorating mental health in this situation is to steer clear of the subject as much as possible, give non-committal responses amd change the subject, and very occasionally drop in the odd question like "How would you ever know if you were wrong about this?" and "Says who, and why should you believe them?"
Sorry not to be more constructive - if I figure out anything more I'll follow up.
Careless rhetoric can lock people into their beliefs. If you make it so this person's only choices are admitting she was a dunce or continuing in her belief then she will likely continue in her beliefs if only to avoid looking and feeling stupid.
People tend to change their minds over time rather than in the heat of an argument. Don't try to defeat her in a debate and convince her to admit she was wrong. Do try to listen and raise meaningful issues and then let the matter lie. If you're successful you will raise doubts and thoughts that will let her reach the right conclusions on her own in her own time as she thinks through what you said.
Ask questions, listen to her perspective, raise points to make her think. Wait, be patient, and repeat.
I would also suggest you consider whether it's really worth it. It may take lots of time and effort to bring her around and even then you'll probably fail. I'd ask what it costs to let her believe the wrong thing. Probably an increase in her risk of death and for those of her kids if any are under her car, but the increase won't be that substantial. If you learned she wanted to become a skydiving instructor or climb Mount Everest I assume she'd face an even larger increase in mortality risk.
Maybe it is bowling, maybe it is cooking-class, bike-repair, yoga, painting, philosophy whatever. Just something sort-of meaningful where she can feel she is contributing in some minor way.
The threat of the pandemic is very abstract. She knows that previously things where normal. Now there is all sorts of hurdles to fulfilling her basic human needs.
It is a threat she can't fight head-on and win once and be done with it. Fleeing does not seem to work either, there is constant talk about the threat being imminent.
She has probably lost at least some sense of meaningful community. That applies to all of us, but does not lessen her suffering.
We might do better by listening and acknowledging the sacrifices she has made during the pandemic.
We can let her talk and explain her views. There might be a lot of feelings like confusion, anxiety, and frustration that needs to be processed. It will for sure not come out as a coherent argument so we should not attack it when it contains logical inconsistencies. Instead let us just listen.
There is sure to be common ground with her in how challenging being away from friends/family has been etc. Focus on shared experiences. Eventually perhaps there is a possibility to try to make sense of the world together. Discuss future plans, move the agenda away from the threat and into a tangible tomorrow.
There is sure to be dead-ends. Of course it is frustrating to see loved ones not just understand us. But letting this take some time might help ourselves learn both about our loved ones and also what really matters in life.
Fact#1 There is a respiratory disease abound and a lot of people are not immune to it. Evidence: hospitals are full of patients hooked on respirators. She is able to see them.
Fact#2 Most people who catch the disease are unvaccinated. She can confirm this by talking to patients.
If you can confirm these two facts with her, the rest isn't really that important. You can claim that vaccines are made from devil horns and she will realize she cannot argue belief and perhaps even empathize.
Try thinking the other way around, how do you convince the vaxxers that experimental vaccines can still give you health problems after 10 years? And that you have to make that choice yourself
There are leaked emails from Pfizer talking about used aborted fetuses cells to develop the vaccine. They are not being killed on purpose, but some people object to their use
Did you also think that this might be something painful to deal with? Perhaps your "lol" could have been avoided, no?