But jut thinking about regular people, we have some 3 million cases now. The recovered have some immunity. Some people never knew they had it. Let us say there are 10-15 million that were affected. That is not enough for herd immunity. Even with vaccinations, we have 25-30 million flu cases every year. (Another roughly equal number was vaccinated.) That gives us herd immunity, and also, flu is seasonal. Coronavirus is not.
We could look at 10 infected people. In them we have 2 older people. They are very careful and alert. They will go and get tested at the slightest symptom. The old person will likely infect their spouse, as most of them live daily under one roof. They could infect one other person, as there is some chance that one of them had contact with some close by person. They wore the mask, but other people around them did not. It probably takes two masks on TWO people to prevent infection at some 90% level.
Some 5 people of various ages are in the group. They are working people, attempting to go about their jobs. Those 5 infected 10 people before they figured out to stay home. Then there are 3 young people. They are not worried. There are less than 300 people dead in the country in the under 24 group. They did infect some 20 people, but mostly young people. If they contacted friends, they had a chance to stop most of the spread to a third person in line. So some 20 people got sick, recovered from it.
How many total got sick from our 10 people? More than 10. Also those passed it on. Otherwise the pandemic would count fewer and fewer new cases day by day.