system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Does the immune system get weak or “rusty” having not had to fight off human spread infections for over a year while in quarantine?
gpt
It really depends upon the organism being bacteria or virus.  Nobody knows how long Covid-19 vaccines will offer immunity, yet.  Also different vaccines will vary.  Chicken Pox vaccine (varicella) lasts \~40-50 years, then immunity lessens, that's why there is a Shingles vaccine for elders.  Measles last your lifetime.  Tetanus should be re-vaccinated every 6-10 years.

The yearly flu (influenza) lasts only about a year, but not because the immune system weakens, it's because the virus changes and the body no longer recognizes it.

Vaccines or the natural infection itself (such as chicken pox) give the same immunity.  With chicken pox whose immunity lasts about 40 years, it is the immune system getting weaker.  With influenza, it's "I don't recognize this enemy."  With Tetanus it's "I forgot how to fight this."

Not all mutations are bad.  Back around 2009 the "swine flu" hit the world, another pandemic, but the virus mutated so much that it pretty much was no longer a big issue.  Unfortunately Covid-19 is not being so polite, but it also could mutate into a friendlier form, or as you've heard a "worse" one.  In this case it's not more harmful, it simply spreads easier.

Covid-19 vaccines are most likely going to become an annual thing, maybe even mixed with the flu shot.  Remember though that vaccines can give "partial immunity."  This means that while you might become ill, you might not become seriously ill.