In randomized clinical trials, the original mRNA vaccines were just as effective as modified versions at preventing symptomatic disease and hospitalization due to SARS-CoV-2 infection
Effectiveness of the original vaccines against Omicron infection, in particular, is essentially zero after a few months:
https://www.medrxiv.org/content/10.1101/2021.12.20.21267966v...
It should be noted that the vaccines appear to remain highly effective against severe illness, even without boosting. The mRNA vaccines, in particular, seem to raise a robust, long-lasting cellular immune response, even though antibody levels rapidly fall. One of the downsides of the unusually rapid approval process for the Covid vaccine was that it was impossible to see this pattern in a randomized trial that lasted only a few months. This led "experts" -- people who really should have known better -- to oversell the shots as a form of sterilizing immunity. They are not.
There are good reasons vaccines take a long time to develop.
Since the immune response is for the older versions of the virus, getting the updated vaccine still ends up having the same effectiveness of the original vaccine.
[1]: https://arstechnica.com/science/2022/02/monkey-study-casts-d...