The problem is that the virus mutates over time, perhaps to the point that the vaccinated immune system doesn’t recognize the new mutant. Learn the word endemic; you’ll be hearing it more often.
From The Guardian in March, 2021:
The planet could have a year or less before first-generation Covid-19 vaccines are ineffective and modified formulations are needed, according to a survey of epidemiologists, virologists and infectious disease specialists.
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The grim forecast of a year or less comes from two-thirds of respondents, according to the People’s Vaccine Alliance, a coalition of organisations including Amnesty International, Oxfam, and UNAIDS, who carried out the survey of 77 scientists from 28 countries. Nearly one-third of the respondents indicated that the time-frame was likely nine months or less.
Source: Covid: new vaccines needed globally within a year, say scientists | Coronavirus | The Guardian
Another issue is that we don’t know how long the vaccine’s effectiveness lasts. Pfizer’s CEO recently suggested a booster by be needed as soon as six months after the last shot.
Steve Parker, M.D.
Advanced Mediterranean Diet 
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