As mentioned in the Nature article and in another poster's comment[1], MRNA vaccines have been known for a while now (decade+ ?), and studied on animals, with some limited phase I / phase II human studies. But nobody's gone through a full phase III before covid hit, so that kind of a wide fan-out is new and exceptional because of the emergency nature of the disease.

But I don't think they're unconcerned. I think it's a combination of lack of decades-long longitudinal studies, and also that previous phase I / II studies haven't throw up significant red flags.

[1] https://news.ycombinator.com/item?id=25403560

> But nobody's gone through a full phase III before covid hit,

Do you know why nobody did not conduct phase III before? Is it because of safety concerns or lack of necessity since concerned disease disappeared?

AFAIK some of the breakthroughs in the delivery mechanisms are relatively new. Some of the companies now making a covid vaccine were talking about running a trial for a flu vaccine in the next few years, but obviously that was far less urgent and all the steps (getting permission, getting participants, actually running the study without "unlimited" money, ...) would take longer.

(mRNA would be a good candidate for a flu vaccines because they are fast to produce, which in theory reduces the amount of guessing needed to make the "right" flu vaccine for the year, but of course a) flu vaccines already are a thing even if sometimes wrong and b) a flu vaccine isn't given as high priority overall)

They discussed this in the approval meeting. There was only one case ever in any vaccine or trial of a side effect showing up after 6 weeks, it was over 50 years ago in a live virus polio vaccine on an immunocompromised person.

Given the many billions of vaccines given of various types, it is about as close to certain as we get in this world to not expecting long term side effects ever.

The reason they aren't concerned about it is for good reason, not because they "haven't looked into it". You have to look at relative risk.

This decision to lift the total ban on this vaccine was probably the easiest decision the FDA ever had to make. This one isn't even a question.

Now they just have to lift the ban on the rest. The standard for EUA is supposed to be "net expected benefit", which they are not following. They are currently only approving EUA when they hit "no conceivable downside", which is a too high standard.

> There was only one case ever in any vaccine or trial of a side effect showing up after 6 weeks

That is not true: https://www.cdc.gov/vaccines/vpd-vac/rotavirus/vac-rotashiel...

It was approved August 1998 and removed from market October 1999 - more than a year later.

Edit: Oh, I see what you are saying - the intussusception happened within 2 weeks of receiving the vaccine.

But that's not really comforting, if it took more than a year to understand that it was linked to the vaccination.

In addition to your other responses, I'll add this. There's not really a plausible mechanism that could cause problems to happen long after the fact. Although it sounds exotic, the mRNA vaccine is just using the same process every cold or flu you've ever had used, but in a more limited way since the proteins created by the vaccine cannot reproduce.

By 2 months all traces of vaccine are long gone and you're just left with an immune system trained to fight COVID (just like millions of people have already had happen naturally). So, while everybody has their eyes open, nobody can really explain the mechanism where something might happen.

> There's not really a plausible mechanism that could cause problems to happen long after the fact.

Well there are some limited circumstances, but I don’t think they justify not going forward with the vaccine. For instance there is a possibility in some individuals that the mRNA itself (not just the spike protein) as well as the lipid encasing it to protect it and transport it into the cell can cause an immune response. It’s theoretically possible that in some individuals that could result in auto immune responses to other forms of messenger RNA in their cells. [1]

1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5906799/

From the article

“A possible concern could be that some mRNA-based vaccine platforms induce potent type I interferon responses, which have been associated not only with inflammation but also potentially with autoimmunity”

Compared to old traditional vaccines, is mRNA-based vaccine safer considering long term side effects?

They are concerned, but it's a balancing act. COVID itself is a new virus with unknown long term side effects, and it's killing millions while causing massive economic harm triggering political destabilization in some areas. The vaccine is new and may cause issues, but current data doesn't indicate it will cause issues and it prevents death and sickness by COVID very effectively.