What do you think the current shots effectiveness are rated at? 90%? 80% 60%? What percentage would you consider a failure? How much time between boosters would you consider a failure? 6 months? 3 months? 30 days?
Initially – Moderna has a 95% efficacy which gradually reduces to 90% over 6 months. It claims to be effective against Beta, Delta, Eta, and Kappa variants.
Pfizer – starts out at 90% dropping to 77%. With two doses it is about 84% effective overall against COVID mutations
Johnson and Johnson single shot is 72% effective with a booster it is 94% effective
There are a few other brands available outside the U.S. that I haven't assessed here.
Time between boosters no doubt varies from one brand to another. Failure is subjective. IMO less than 6 months is unacceptable, preferably a booster would last a year. One problem is that the vaccines work better against some variants than others and how well they will work with future variants/mutations is unknowable.
What isn't effective is no vaccine because one cannot know if they have natural immunity until they are exposed to COVID. Some will not have any symptoms and others may get sick and possibly die, which makes this a hell of a crap shoot.