Sorry to get all "Nerdy" on you, but the following equation and data set I threw together from the attached Lancet article is important for understanding 'WHY'...
Epidemiologists use the expression Pc = [1-1/R0]/E, to determine what proportion of the population needs to be immune from a pathogen through vaccination, or prior antibodies (e.g. survival from infection).
Pc = % of the population that needs to be vaccinated (or otherwise immune)
R0 = the "Reproduction #", or the degree of 'contagiousness' (the higher, the more dangerous)...can be diminished with "mitigation" (e.g. wearing masks, etc.)
E (Epsilon) = Vaccine efficacy
Ok...here we go...
E R0 Pc
----- ------- -------
100% 2.5 60%
95% 2.5 63%
90% 2.5 67%
95% 3.0 70%
90% 3.0 74%
95% 3.5 75%
90% 3.5 79%
-----------------------------------------------
95% 5.0 84%
90% 5.0 89%
While the Moderna and Pfizer vaccines promise to be highly effective (95% and 90%, respectively), the "R0" factor is NOT static...the current estimate of ~2.5 is based on mitigation through mask wearing, social distancing, etc...in the early days of the pandemic, estimates of R0 were on the order of 5.7, so from the equation and chart above, you can readily see the risk we face if a) folks don't mitigate, or b) don't want to take the vaccine. If we fail to "Make our Numbers", we'll be stuck in 'Pandemic Land' until hundreds of thousands more die here in the USA.
Please read the article itself, in case I mis-represented something, and get the word out to as many friends and family members as you can...
Link: https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)32318-7/fulltext