Wrong how, spicerack? A virus spreads until it can no longer spread. Perhaps you were thinking that I meant that "runs out of people" means that "it infects the whole population", and, if so, I should clarify that I didn't mean that. Depends on the rate of transmission, for example. But as a rule a virus won't disappear until it runs out of people to infect. Whether that's the entire population or merely a large part of it, it's clearly preferable to intervene if possible.
It may be of course that a vaccine is developed "too late", or is ineffective except seasonally, but it's also clear that without a vaccine the only strategy to prevent the virus spreading is to restrict social contacts (or give up and wait for herd immunity to be a thing).