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I'm aware that some people don't want to take a vaccine for Covid-19, and in fact some countries have suspended rollout of some vaccines due to some young people getting blood clots. Also some people have had adverse reaction from the virus e.g. anaphylaxis. My question is not about any of these rare, short-term reactions to the vaccine (which I think are poor justification to refuse the vaccine, because the probability of death or serious illness from the virus is far greater than these rare reactions from the vaccine).

People in different countries are weighing the advantages vs the disadvantages of taking the vaccine. A large proportion of people in France are choosing to delay taking the vaccine, probably for various different reasons, whereas most people here in Britain are choosing to take the vaccine.

It seems to me that one of the main reasons that many people including myself are choosing to delay taking the vaccine, is: "fear of the (unknown) long-term potential health effects of taking the Covid-19 Vaccine." Many people express this opinion in the comments here:

https://www.reddit.com/r/science/comments/jpxv55/only_58_of_people_across_europe_were_willing_to/

and in the comments here:

https://www.ft.com/content/c576e15f-e5b1-4369-a5f0-073b4466036f

In fact, even in the above article itself, it says:

Among the 2,305 survey respondents, many feared long-term side effects...

I suspect that these fears are irrational and that scientists took the unknowns of the long-term effects of the vaccine into account before rolling out the millions of vaccines. Furthermore, people have to make decisions on incomplete information, and everyone does this all the time.

Now I am a layman when it comes to virology and medicine in general. I believe I have some vague understanding of how the vaccines work based on for example this video.

So my question is, what is the science behind why it is so unlikely (<1%?) that the long-term health effects of the vaccine will be harmful to a significant proportion of the population?

Or is this the wrong way of viewing things, and a better way of viewing things is: Whatever long-term harmful effects that you get from the vaccine if you take it, you are very likely to get even worse long-term effects of a similar kind the when you (inevitably at some point) catch the virus if you didn't take the vaccine. Is this correct?

Addendum: One point made by the only current answer is that, "There is no way to completely rule out the possibility of totally unexpected side effects that don't show up until much later." My response to this is that you can't completely rule out anything ever, so "completely ruling out" is irrelevant. I am asking why it is believed to be so unlikely to get side effects from vaccines - in particular the Covid ones - years later (in particular, less likely than getting seriously ill from Covid)?

This website says:

ARE THERE LONG-TERM SIDE EFFECTS? Long-term side effects following any vaccination are extremely rare. Millions of people have received COVID-19 vaccines safely.

Why does no serious side effects in the population after a few months imply no side serious side effects in the population after 10, 20, or 30+ years? Linear extrapolation of the data is surely insufficient justification (extrapolation of data is unreliable). Wouldn't one have to do long-term simulations of multi-variable analysis?

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  • Yes BrenBarn, that's very relevant. Thanks for that link. – Adam Rubinson Mar 20 at 20:17
  • Something to think about: With hundreds of millions of people already benefitting from at least one dose of a COVID-19 vaccine (nytimes.com/interactive/2021/world/…), & billions more doses likely scheduled to be shipped, if there winds up being serious long-term problems with the vaccines, humanity will experience an upheaval likely unparalleled in recorded history. Do you really want to be a survivor in that grim scenario? Similarly, would you really want to survive a global nuclear holocaust? There's something to be said for being at ground zero. – RockPaperLz- Mask it or Casket Mar 22 at 9:19
  • "Do you really want to be a survivor in that grim scenario?" Yes of course I do. Do you not want to survive that scenario? If so, why not? "would you really want to survive a global nuclear holocaust?" Yes! That's preferable to dying in one (unless you want to die for whatever reason)! Note that in the first part of your comment you say "already benefiting", which kind of implies that you think there won't be any long-term effects of taking the vaccine, i.e. you're assuming the answer to the question is no, one shouldn't fear the long-term effects... - without justifying this first. – Adam Rubinson Mar 22 at 9:47
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This general-audience article from Children's Hospital of Philadelphia gives a good overview of reasons to believe that long-term side effects are unlikely. To summarize its two main points:

  • Previous vaccine research and development has found that most side effects occur within about two months. The FDA approval process for COVID vaccines required the trial to follow participants for eight weeks before submitting the vaccine for approval.
  • Although the Pfizer and Moderna vaccines are the first mRNA vaccines in wide use, previous mRNA vaccines were developed and tested for Zika, influenza, and other diseases. Thus although the technology is "new" it is not as if this is the first time anyone has been injected with mRNA.

In assessing long-term side effects, one problem is you have to wait a long time, but another problem is that the longer you wait, the more difficult it becomes to track the results and determine which events can be attributed to the vaccine. Answers to this question discuss such issues with the oral polio vaccine.

There is no way to completely rule out the possibility of totally unexpected side effects that don't show up until much later. You can't really know what will happen in ten years until ten years have passed. However, all that is known from previous vaccines suggests it is unlikely that some catastrophic side effect will suddenly arise years later.

Your last question is also relevant: any decision about the use of the vaccine has to take into account the risks of not using it. Given the massive public health crisis and other societal damage caused by COVID-19, there would probably need to be significant justification for waiting, say, an extra year "just to be sure" that no side effects emerged in that time. Also, preliminary data on "long COVID" mean that it is also possible there will be long-term "side effects" of COVID-19 itself, meaning that delaying the vaccine to avoid its possible side effects could just result in people suffering different long-term effects from the disease. So even if a catastrophic vaccine side effect did arise years later, that could still be better than not getting the vaccine. (Of course, we won't know the long-term effects of COVID until time passes, just as we won't know the long-term effects of the vaccine.)

Another thing to remember is that people's subjective feelings about risk levels are often only vaguely connected to reality. I'm sure there are people out there who worry about the risk of the COVID vaccine but smoke a pack of cigarettes a day; three guesses which one of those is likely to pose a greater health risk. Some people worry about the risk of dying in a crash when they get on a commercial airline flight in the US, despite the fact that annual total deaths from such events rarely exceed zero. (Here is one classic paper on risk perception.) Fear of long-term side effects is as likely to be due to the human tendency to exaggerate unfamiliar risks as it is to be due to any actual risk from the vaccine. In other words, even apart from the scientific evidence for the safety of this vaccine, there is a fair amount of scientific evidence that how safe you think something is may not be a good predictor of how safe it actually is.

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  • Thanks for the answer: many good points made. My response: "In assessing long-term side effects, one problem is you have to wait a long time". Is this necessarily true? Can insight into the long term effects not be approximated using the studies done on mice and seeing the "long-term" effects on mice, using the fact that mice live much shorter than humans? "...from previous vaccines suggests it is unlikely that some catastrophic side effect will suddenly arise years later." Firstly, can you cite any research papers or studies with evidence that the Rubella... – Adam Rubinson Mar 22 at 10:38
  • ...vaccine doesn't increase risk of certain types of cancer? Have scientists carried out research/analysis to determine this? And assuming the above quote is true, is there good scientific reason for this? After taking the vaccine, is the only thing that lingers in the body the antibodies created by your body, caused by a reaction from the virus part of the vaccine? And basically do the antibodies just sleep in your cells and not really affect/interact with other parts of your cells or body? Or how does it work? Talking about the mechanisms might help me to understand what's going on... – Adam Rubinson Mar 22 at 10:38
  • My Rubella example might come across as anti-vaxxer-y. But I ran out of space and time to edit the comment. What I was really asking was: do scientists track and analyse the long-term effects of vaccines? Or do they not even see a need to do this? – Adam Rubinson Mar 22 at 10:44
  • "There is no way to completely rule out the possibility of totally unexpected side effects that don't show up until much later." You can't completely rule out anything ever, so "completely ruling out" is irrelevant. I am asking why it is believed to be so unlikely? – Adam Rubinson Apr 12 at 21:17
  • @AdamRubinson - Your peppering of the user posting this answer is inappropriate. You asked a question and received an answer, which likely took a long time to write. Your comments make it look like what you want is a debate. SE is not a debate site. I would answer it were it not for this perception. – anongoodnurse yesterday

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