But the basic reproductive number alone doesn’t tell the whole story. To really understand what we can expect from the delta variant, we need to consider vaccinations, natural immunity and any precautions people are taking to lower their chances of being exposed, such as wearing masks and social distancing. To do that, epidemiologists calculate another measure, called the effective reproduction number, or Re (pronounced arr-eee). The Re helps us estimate how much a disease may spread when a population has at least some immunity. Technically speaking, the Re measures how many new people a single person infects, accounting for whatever precautions people are taking and overall immunity levels. Whether we’re talking about the Re or the R0, any value larger than 1 means trouble, because if each person can infect more than one other person, any disease outbreak will continue to spread exponentially unless we take action.