Natural mRNA sequences tend to contain uracil, which the immune system is not a fan of (https://www.sciencedirect.com/science/article/pii/S152500161...). The innate immune system generally attacks extracellular mRNA because of this. The mRNA in vaccines are specially modified to bypass the immune system and be taken into cells, at which point they will be translated into spike proteins and be expressed on the cell’s surface. In conjunction with an adjuvant, the immune system will recognize this as a threat and begin producing antibodies.

It’s certainly possible that mRNAs could do harm: they could code for protein sequences that might disrupt cellular function, increase cancer risk (e.g. by disrupting DNA repair or even causing mutations), etc. The spike protein is none of these things - it just gets expressed on the cell’s surface - so the risk from the vaccine is low. And, crucially, once the mRNA runs out, no more protein will be produced: mRNAs are degraded after protein translation. This serves to generally limit the amount of lasting impact that an mRNA vaccine could possibly have.

Sure there is potential for harm. Most mRNAs just tell your cells how to make certain proteins, and there are tons of harmful proteins. You could be instructing your cells to make potent poisons, or instructing them to make all of the proteins necessary for an entire functional virus.

But the person he's responding to is citing methods of harm that aren't realistic.

I only got that he was arguing it won't make permanent changes to our genome, which was the claim he was replying to, and he's correct. The spike protein is damaging to cells when present at all, but still very apparently much less dangerous when exposed via vaccine than an actual infection. And, as you know, cells die and get replaced all the time. I think people who are aware of it sometimes gloss over how the spike protein does some damage simply because they don't want to stoke unreasonable fears.

No, he said that (in general) mRNA is flushed from your system within days and that (in specific) these vaccines don't permanently alter your DNA. That's nowhere near claiming that "mRNA can't ever do harm."

It is unclear to me (having just re-read Linus's post) where you see the claim that an mRNA cannot do harm. He says it cannot be incorporated into the genome, which is true of the Covid vaccine RNAs but not necessarily true in general (one could inject the RNA of a retrovirus). But the Covid19 vaccine mRNA simply makes the spike protein and (I'm sure they checked) does not get incorporated into the genome.