system
Given the following post, respond as if you're commenting on Reddit:
human
How effective are vaccines?

I've heard that the vaccine for Covid-19 would only be about 60-70% effective. Not sure if that is 100% correct or not, but I know it wasn't 100% effective. So, what I was wondering, does that also mean that vaccines for small pox, measles, polio, etc. are also not 100% effective? Or even down to 60-70% effective? Or is that effective rate dependent upon what it is meant to go after? Or is it based on the person getting the vaccine and how their body accepts it? Or both? None? Other? Thanks!
gpt
Vaccines aren’t 100% effective. They will not guarantee that you won’t get the disease they’re designed for, and they generally don’t protect against diseases they aren’t intended for. 

However, they do reduce the likelihood of a vaccinated person catching the intended disease. Some of the common vaccines are almost 100% effective; it’s how we’ve eliminated polio. Other diseases like the flu are trickier and only run at 40% effective. It depends largely on how the human immune system interacts with the disease. For some diseases, like polio or smallpox, the body’s really good at remembering the vaccine’s signature and will kill any of the associated disease quickly. For flu, there’s lots of slightly different strains, and our body isn’t as good at fighting them, so the immune system might not recognize the specific flu as a bad thing. There are other reasons, but those get extremely complicated. 

But the important thing is how many people are vaccinated. For example, take a disease that, on average, one sick person will transmit it to one non-vaccinated person in a 10,000 person population with 1% with the disease already. It’s gonna get most of the population sick eventually if there’s no vaccine. But if everyone takes a 40% effective vaccine, only a small fraction of the population will get it because the first 100 will only infect 60, that 2nd group only 36, and so forth.