Let's say that 99 percent of people who get the disease will survive. Percent literally means per one-hundred, that means out of 100 sick people, one will die. So if 1000 people get sick, 10 will die, if 10,000 get sick 100 will die, if 100,000 get sick 1000 will die and so on. Are you okay with that? What about long time health affects for people who recover?
If a vaccine is 90% effective, let's say we are talking about 1,000,000 million people who got sick before the vaccine, and 10,000 die, with the vaccine then 90% of 1,000,000 is 90,000 so now there are only 10,000 people who get sick, and now 100 die instead of 10,000. That's worth it. This is all very simplified and doesn't take into account that 90% effective means that there are less sick people to spread the disease so the numbers get even better. i must say I am hopeful about the prospects of a vaccine, although it might take longer than we would hope for it to widely distributed.