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)?

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?

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?

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)?