Lots to unpack here, most of which is based on incorrect premises.
Firstly: mRNA therapies have been approved by the FDA as early as 1998 (https://www.nature.com/articles/d41586-019-03068-4#:~:text=The%20first%20RNA%20therapy%2C%20fomivirsen,retinitis%2C%20inflammation%20of%20the%20retina.) With respect to their use as a vaccine platform, the earliest clinical studies go back to 2013, and no long-term adverse events were noted from these, as Professor Krammer notes: https://twitter.com/florian_krammer/status/1332077763809996801?s=20
The registration numbers of the studies are included so as to permit anyone interested in the details to examine them more closely.
mRNA is very difficult to work with. It requires ultracold storage, it requires precautions against RNases which are everywhere- it makes for a very intense and taxing process that results in a very high cost. Much of vaccine production is aimed at addressing diseases which are prevalent in lower and middle income nations, but an mRNA vaccination program is completely unfeasible because of cold chain requirements (let alone the freezers some RNA vaccines need) and the staggering cost relative to other technologies. They have some unique advantages though that make them well suited to responding to a pandemic. Firstly, it’s very easy to make huge amounts of mRNA very quickly, which is ideal for an emergent new virus. Secondly, the RNA is very easy to adjust quickly if needed, say, if a new variant emerges: https://investors.modernatx.com/news-releases/news-release-details/moderna-announces-it-has-shipped-variant-specific-vaccine
These vaccines aren’t safe “because, well, we want them to be.” They’re safe because they have undergone extensive safety evaluation and not demonstrated any signals. In their Phase 3 clinical trials, Pfizer and Moderna collectively examined over 70,000 individuals who received vaccine or placebo and no safety signals were noted. This a priori indicates any safety issues that may arise would represent exceptionally rare circumstances (if you go by the rule of threes in biostatistics, the events would occur at a rate between 0 and 1 in ~47,000 doses of vaccine. Post-marketing surveillance data spanning months and examining millions of doses demonstrates no safety signals: https://www.cdc.gov/vaccines/acip/meetings/downloads/slides-2021-02/28-03-01/05-covid-Shimabukuro.pdf
This is also the case in other countries e.g. the UK:
I’d seriously challenge you to find an example in the history of immunization of a long-term effect from a vaccine that fits the definition you have provided. It’s an incredibly broad definition but not even the vanishingly rare cases of varicella reactivation from inoculation with the Oka strain would meet it as this is not dependent on the vaccine effects on the immune system.
The question of the vaccine’s safety in pregnancy has been examined fairly extensively with the available data. I think reproductive immunologist Dr. Viki Male offers the best summary which you can read here:
https://drive.google.com/file/d/1_wHIYX-tGkGBPwuax7N8BxZPR4PTTCDm/view
She has also summarized the evidence in a comment in Nature Reviews Immunology, if the google doc source bothers you: https://www.nature.com/articles/s41577-021-00525-y
There’s also the matter of biological plausibility. It makes sense to be concerned about teratogenic effects from a vaccine when the virus causes those effects, especially if it’s a live attenuated vaccine. For instance, rubella and varicella are both known to cause congenital anomalies if contracted during pregnancy and there are live attenuated vaccines for both. These vaccines are therefore contraindicated during pregnancy because of the theoretical risks posed to the fetus (and in fact it is recommended to receive them at least 1 month before trying to conceive). However, that doesn’t mean that no pregnant people have ever received either vaccine. Yet in both cases, no teratogenic effects were noted: https://www.cdc.gov/vaccines/pubs/pinkbook/rubella.html
https://www.cdc.gov/vaccines/pubs/pinkbook/varicella.html
Now, indeed, this should not be used to justify vaccination with these vaccines during pregnancy, but the fact that even what would be the highest risk vaccination for the wellbeing of the fetus has failed to show evidence of harm makes this concern at best tenuous. These mRNA vaccines are not live and aren’t even adjuvanted (though pertussis antigen-containing vaccines which are standard of care in pregnancy are adjuvanted with aluminum salts, so I’m not sure why some have fixated on that as a criterion for a vaccine safe for use in maternal immunization as its completely irrelevant but I digress). COVID-19 has not shown evidence of causing congenital anomalies: https://pediatrics.aappublications.org/content/147/2/e2020015065
However, pregnancy is a massive risk for adverse outcomes in COVID-19: https://www.acog.org/clinical/clinical-guidance/practice-advisory/articles/2020/03/novel-coronavirus-2019
On the other hand, as ACOG summarizes, the potential for harm from these vaccines in pregnant patients and their fetuses is slim, but it could save their lives:
Furthermore, fetuses will receive antibodies through the placenta elicited by the vaccine which may play a role in protecting them from COVID-19. The theoretical and largely baseless risks of the vaccine on fetuses should not be used as justification to withhold a vaccine that may save both mother and baby. Maternal death crosses the placenta. This vaccine though? Doubtful given its highly local distribution:
https://www.cell.com/molecular-therapy-family/nucleic-acids/fulltext/S2162-2531(19)30017-4
That’s why ACOG recommends that pregnant patients be freely available to receive the vaccine and do not need to consult with their healthcare provider.
Comparing vaccination to chemotherapy is an inherently false analogy. Vaccines are a public health intervention. Chemotherapy is not. Every single person vaccinated takes us closer to eliminating COVID-19. Cancer incidence rates aren’t going to decline from giving more people chemotherapy. Therefore the calculus here is quite different. It is in fact much more stringent for vaccines because they have to be given to healthy people, but at this point we have data on nearly 100 million doses given in the US and 319 million given globally.
I would further challenge you to tell me who is at risk of severe COVID-19, because previously healthy people can and do die of this disease every day. For instance, one well characterized risk factor for fulminant, often lethal COVID-19 is antibodies against type 1 interferons: https://science.sciencemag.org/content/370/6515/eabd4585
If I gathered a random sample of people for you, could you tell me which of them, if any, have these antibodies? I couldn’t and I’d be genuinely shocked if you could find someone who could.
It is also a fallacy to claim that mortality is the only pertinent risk from COVID-19, but one that many so exasperatingly make. About 1 in 7 individuals who develop COVID-19 will develop post-acute sequelae of COVID-19 (PASC; long hauler syndrome) which can be disabling and debilitating. There is no apparent therapy (though there are hints that COVID-19 vaccines may, ironically, be helpful here, but far more rigorous study is needed to confirm this than what is currently available). I am young and in good health and I am getting the first vaccine offered to me as soon as it is offered to me because I am not taking these risks with COVID-19, and the millions of doses given without any safety signals make me extremely confident that these vaccines are the best possible option. I am not taking my chances with COVID-19. Because that is the risk here. It’s not get the vaccine or don’t get it. It’s get the vaccine or wait long enough to get COVID-19.
The belief that Johnson and Johnson and Janssen's vaccine is safer than the mRNA vaccines is completely baseless. In fact we have far more data on the mRNA vaccines with respect to both safety and efficacy (and effectiveness) than for JJJ. The JJJ vaccine hasn’t been in use for years- it’s roughly as old as the COVID-19 mRNA vaccines.