Isn't one of the great advantages of mRNA vaccines that they can be quickly refined in this scenario? Would there be any reason why people already vaccinated couldn't also receive the updated vaccine?
The vaccine payload can be modified, the question is can the immune system of people who are already vaccinated with an older vaccine be modified successfully with an updated vaccine using the same mRNA platform? or will the upgrade brick the system?
That is something nobody knows at this point, hence hedging and waiting to see. I have thought about it quite a bit and can see some pitfalls and don't know any answers either.
Modified improved vaccines, Someone who has been vaccinated with a narrow antigen such as the spike produced by the mRNA vaccines has antibodies to the original spike in the original vaccine. These antibodies will bind to the modified spike in the "new improved upgrade vaccine" (TM), not all of them or as maybe not as well, but they will bind sufficiently and the B cells that make these sub-par antibodies will be stimulated to make more and more of the original antibodies. If these are reasonably effective, then they lead to clearing the new improved spike (vaccine antigen) quickly before there is time for much selection. Perversely existing antibodies prevent the immune system from developing the new improved antibodies, reducing the ability to adapt, causing failure of the service pack upgrade to install. That is the concept of original antigenic sin. It was seen in 2009 with people who had the normal flu shot not producing very good antibodies to the pandemic H1N1 vaccine and in many other situations. They were good enough to bind the vaccine antigen but the vaccine is not the virus, at least some antibodies to a real live virus need to be neutralizing or they can be worse than useless (ADE and similar).
mRNA vaccines only produce spike for a short time and is self clearing through the liver I guess, perhaps a week at the outside but more likely a couple of days, (hey Pfizer Moderna where is this really basic data???) because the mRNA vaccine is self clearing there is no need for antibodies to be neutralizing, they just have to stick pretty well, so there is no selection for neutralizing antibodies over high affinity antibodies.
there are some other issues too, where nobody has any answers.
The transport lipids used in the mRNA vaccines appear to be immunogenic themselves, so reactive and potentially anaphalactic type reactions and other reactions to the vaccine delivery packaging is then likely to become more common. Anaphalactic type reactions are already much higher with both Pfizer and Moderna than any previous vaccine ever deployed and dose 2 is more problematic than dose 1, dose 3 of an upgrade would likely be even worse. I would be surprised based on the current data if the mRNA vaccine platforms get regular product licenses rather than just the limited pandemic emergency use approval they have now but we are truly in a brave new world where anything goes.
The viral vector vaccines can be adapted practically as fast as the mRNA ones but they have an additional problem, immunity builds against the viral vector usually an adenovirus which makes them less and less effective on repeat dosing the immune system just wipes out the vector virus, this has already been seen with AZ Oxford vaccine and will also be an issue with J&J. Sputnik uses 2 different adenoviruses for the 2 doses so has a slight advantage but they don't seem to be able to culture both adenoviruses efficently. So again there are pitfalls.
This is an aside, but coronavirus vaccines are not comparable to polio vaccines. Polio vaccine is complete inactivated or live attenuated virus, therefore people vaccinated have a broad range of antibodies to many parts of the virus, for the virus to escape all of those antibodies would be quite a feat of evolution even though polio mutates at quite a high rate. This is in contrast to single antigen coronavirus vaccines where only a small part of spike has to evolve to evade the current vaccines.
The immune system is stupidly and beautifully complex and the level of knowledge and understanding about it is highly incomplete, but that is not the story immunologists and people selling vaccines want to tell anyone. It is a minefield.
