The New Vaccines, DNA "damage", and Infertility

With the previous discussion on basic vaccine mechanics under our belts, we can now consider what sorts of dangers these vaccines might entail.

Can a vaccine cause a systemic problem in the body because of DNA damage of some sort?

The oldest type of vaccine, which is simply a milder disease, replicates throughout the body and--as viruses do--modifies and "damages" DNA in cells throughout the body as it does so.  So, there is some danger of systemic damage throughout the body.  But DNA damaged cells are disposed of throughout the body all the time, so this isn't usually a big deal.  Cells die and are disposed of naturally; even mild diseases like the cold "damage" DNA and cause more cells to die and be disposed of all the time and no one thinks twice about it.

More modern vaccines are not a milder disease, but rather an attenuated form of the disease: neutered virus particles.  It is *intended*  that the effects of these virus would be limited to a smallish number of cells because the virus replication ability has been (hopefully) mostly destroyed by whatever the neutralizing procedure was (UV lighting or something else).  How well you can trust that only a limited number of cells will be effected depends on how well you trust this attenuation process.

Adenovirus vector vaccines (AstraZeneca and Johnson & Johnson, among others) certainly do "damage" the DNA of cells at the injection site, by design.  They are limited to a set number of cells which they effect because their replication ability has been genetically removed.  How well you can trust that only a limited number of cells will be effected depends on how well you trust this genetic alteration.

Finally, the mRNA vaccines (Pfizer and Moderna) don't affect DNA at all.  They cause the production of spike proteins without needing virus particles at all; they deliver mRNA to the cell via mini lipid drops rather than via a virus of some sort.  mRNA vaccines are therefore absolutely incapable of reproduction, no matter what the circumstance.

Bottom line: new vaccines are less likely to cause systemic problems than older vaccines, because they are more precisely targeted to specific effects.  None of them, though, have a built-in mechanism that would cause a systemic problem throughout the body, despite some of them working via DNA.

How could a vaccine cause problems with infertility?

DNA "damage" is therefore not an issue, but that doesn't mean there is no mechanic by which a vaccine might cause some body-wide problem.  Almost all plausible mechanics of that sort are related to the immune response to the vaccine, however, not the vaccine itself (which only exists in the body for a short time).    Here's how something like infertility could potentially be caused by a vaccine:

As mentioned, the body's immune systems learns how to produce virus-specific neutralizing antibodies.  As we have also seen, sometimes these antibodies are not actually specific to a single virus--and this is sometimes the point.  The cowpox antibodies happen to match the smallpox viruses as well, which is why cowpox infection ends up in smallpox immunity.

The danger then exists that the body will produce antibodies in response to a virus, or in response to a vaccine, that will attack something good as well as the virus that it is intended to attack.  This is the mechanism by which it has been theorized that a vaccine could interfere with fertility.  There is a protein called "syncytin-1" that is used in the placenta, and it happens to have small stretches of genetic code that are similar to parts of the code for the spike protein of a coronavirus.  What if this similarity is enough that the antibodies produced against the spike protein are also able to attack syncytin-1?  This would, in effect, create an auto-immune response to the placenta and prevent or imperil pregnancy.

While not impossible, this scenario is very unlikely, for several reasons:

1. The amount of similarity between the proteins is actually quite small.  I know that scientists nowadays are actually pretty good at determining how likely it is that you can get cross-reactions between different proteins based on genetic structure.  I'm not aware of an actual study in which the similarities between syncytin-1 and the spike protein were computer-analyzed to estimate *how* similar they would be, but if you hear scientists say that the similarity is too small to matter, they probably have good reason for saying that.
2. Pfizer actually has some data on pregnancy from its phase 3 trial data.  No trial participants were recruited who were pregnant, but some became pregnant in the course of the trial.  Pfizer had 12 people who took the vaccine end up getting pregnant and 11 people who took the placebo end up getting pregnant.  This is a good indication that no effect on fertility is to be expected from the antibodies.
3. If the vaccine caused infertility, it would be because of the immune response and not the vaccine per se.  This means that recovering from Covid-19 itself would also cause infertility, because it would be the immune system's antibodies that would attack syncytin-1.  Worldwide, there have been about 95 million reported cases of Covid-19.  We're pretty sure that this is a large understatement of the total number of cases, and that there are a lot of people who had mild or no symptoms who are not accounted for by that number.  These people would also have generated these neutralizing antibodies.  So there should be something like 200 million people out there at this point who have neutralizing antibodies to the Covid-19 spike protein.
   
   If that many people were out there with antibodies that cause a fertility problem, we should have already started seeing some of these problems materialize by now.  No greatly increased reports of fertility problems since the widespread proliferation of Covid-19 have, however, been forthcoming.
4. Covid-19 isn't the only virus that has a spike protein.  Many common colds / flus are caused by coronaviruses that have very similar spike proteins.  Presumably, these virus spike proteins *also* are similar to syncytin-1, and yet there have been no indications over the years that people who have suffered from a coronavirus of one sort or another have had any sort of fertility problems more than other people.

Therefore, it is extremely unlikely that the new vaccines cause any sort of fertility problem.