Health officials around the world are racing to vaccinate enough people to stop the spread of COVID-19 — but what qualifies as “enough” is still an open question. The goal is to get to “herd immunity,” which is when enough people are protected from a virus, either from vaccination or a past infection. That makes it hard for the virus to jump from person to person. Many experts say at least 70% of the population needs to be protected. Health officials will watch infection trends as vaccinations roll out. And restrictions are likely to begin relaxing long before herd immunity is reached.