This article was amended on 18 November 2020 to clarify that results of vaccine trials at this stage refer to “efficacy” – the performance of an intervention under ideal and controlled circumstances – not “effectiveness”, which describes performance under real-world conditions" (The Guardian).
"In Moderna's trial, 15,000 study participants were given a placebo, which is a shot of saline that has no effect. Over several months, 90 of those people developed Covid-19. Another 15,000 participants were given the vaccine, and five of them developed Covid-19" (CNN).
News reports from other sources are very similar.
Okay, so 90% or 95% effectiveness or “efficacy” means that of the people who got the vaccine, 90% or 95% of them did not get Covid-19. This doesn’t answer the question of how many of them were enough exposed to the coronavirus to get Covid-19. If the people in the clinical trials wore masks, washed hands often, and minimized social contact, then it should not be surprising if effectiveness falls when people who get vaccinated during the big roll-out are not so precautious. Of course, effective rates depend on the degree of presence of the virus, and herd immunity will be reached eventually.
I heard a doctor say on television that he expects effective rates will fall as the vaccine is more widely distributed. He wasn’t clear about why.
Both vaccines are type mRNA. There are many articles on the Internet about this, such as this one.