The vaccines have been shown to be almost 100% effective in the context of preventing serious illness, hospitalisations and deaths. In the context of infections though the waters become muddy and the figures drop somewhat below 60% so it depends what the outcome is.
On variants, don't forget that COVID was novel and it previously had never been seen before by the immune system. The vaccines do not kill COVID in the way that antibiotics kill bacteria which generates antibiotic resistance. The vaccines just teach your immune system that COVID viral particles are bad so they need to destroy it. Previously this wasn't the case and COVID ran free infecting cells until the immune system realised and started to respond, usually by then though it was too late and sometimes the huge immune response is what killed people (cytokine storm).
The virus cannot mutate to avoid the vaccine because it doesn't know what a vaccine is. The virus can only mutate to become more infectious and change the properties of how it infects us but this would require considerable mutated change where it almost becomes a new virus which is highly unlikely.
I wouldn't be too worried about variants, the prospect of a super deadly variant is extremely small, especially when case numbers begin to tumble globally as a result of the vaccines. It's actually more likely that we'll see new super infectious variants like Delta which are less severe but we may need boosters to protect the vulnerable. This is exactly how flu works, you never hear of a cold or flu season where young/healthy people are suddenly getting really ill and dying.
