Trying to understand it
You start off with 40,000 people injected. Half have been injected with the vaccine, half with a placebo. Other than that, the 40,000 are a diverse mix of all types with varying behaviors in all types of environments. Then you start tracking infections in the group. You expect to have significantly more infections in the placebo group. Once you hit 170 infections, you check to see which group has more, and indeed, fully 162/170 infections occurred among the 20,000 who received a placebo. Of the 20,000 who received the vaccine, only 8 became infected. Each passing day, additional people become infected and you watch them fit neatly into that pattern as well (or adjust as necessary).
Do you understand how having 95% of infections occur in a the group that consists of half of the 40,000 people is significant? Think about that. Only 5% of the infections occurred in the randomly selected group that consisted of half of the experiment.