A vaccine is special in the sense that it lets you get the disease without incurring the downside of the disease. So it lets you get to the herd immunity withou

A vaccine is special in the sense that it lets you get the disease without incurring the downside of the disease. So it lets you get to the herd immunity without incurring the deaths. Similarly, a new effective medicine can change the expected outcome (it's possible the Vitamin D supplementation has that property, for example). Herd immunity doesn't mean the disease disappears completely or that everyone is immune. It means, however, that if someone does get, the number of people that they can transitively infect is limited by a constant independent of the population size. If US, UK, Australia, Sweden, ... have herd immunity due to disease, and NZ doesn't, it means that NZ cannot afford travel (and many kinds of exports) to or from the rest of the world. > Doesn't one universe have half as many people infected? You insist on misreading and/or misrepresenting my claim. I specifically said "other than vaccine", which you obviously did read because you asked "what makes a vaccine special". Why would you do that? Take those two universes, but in which there is no vaccine in two years, or in which "x" does exceed the number of infectable people. Does lockdown, or even masks, change the overall outcome? Or just the speed over time in which the dead and long-term residuals accumulate? A handful of countries (New Zealand, Vietnam, and a few more) have some chance to stay in your universe. Most of the world already has, after 6 months of serious (and economically catastrophic lockdowns), over 10% infected. It is not unreasonable, that if an effective vaccine is not effectively administered within those two years, we'll have 50% infected.