system
Given the following post, respond as if you're commenting on Reddit:
human
Can diseases mutate and infect vaccinated people?

So I’ve been thinking about this for a while, and I can’t seem to find any good/fulfilling answer to this question. 

I believe it would be easier for me to explain with a scenario, so here goes nothing:

Person 1 - vaccinated for X
Person 2 - not vaccinated for X

Can X mutate in 2’s body, so 2 can infect 1 even though 1 got the vaccine for the “original” x?

Hope it makes sense. Thanks!

Edit: thank you so much for the answers, helped me a lot!
gpt
As everyone says, yes, this can happen. It does happen for influenza. 

But it doesn’t happen for the vast majority of viruses, even for viruses that mutate as fast as influenza. Polio vaccines are effective after 60 years of use. Measles vaccines haven’t had to change since the late 1960s. Yellow fever vaccine has been effective since the 1930s. Smallpox vaccine worked for centuries. Mumps and rubella vaccines haven’t had to change. In the veterinary world, most vaccines have been highly stable - parvovirus vaccine has been widely used since the 1970s with no issues. 

Vaccines aren’t like antiviral drugs that target one specific site.  Vaccines drive thousands of different antibodies, typically attacking dozens of different viral targets. That makes it very hard for a virus to mutate away from vaccine control - they have to simultaneously mutate a dozen different sites at once. If a single site change happens at 1/100,000 frequency - which would be common for a virus - that change will happen fast, within a handful of generations. But it means that two changes only happen 1/10,000,000,000 times, and 3 simultaneous changes 100,000 times less than that, and so on. That’s very hard for most viruses to do. 

So it’s a legitimate concern, but the history of vaccines says it’s not a huge concern.  

And if SARS-CoV-2 does mutate, it’s very unlikely that a modified vaccine to handle it would need the time-consuming trials that the first batch have done. More likely, as with annual influenza vaccines, regulatory agencies would treat them as similar vaccines and would allow shorter trials, so that new effective vaccines could be made within a matter of weeks.