From the National Health Service (UK):

http://www.nhs.uk/Conditions/vaccinations/Pages/measles-outb...

"Can you still get measles after the MMR vaccination?

It’s extremely unlikely, but you need two doses of MMR to be fully protected. The first dose of the MMR jab protects 90% of those who receive it, and the second dose tops this up to 99% protection. Almost all of the children in the Welsh outbreak who caught measles were either completely unvaccinated or only partially vaccinated."

Assuming everyone was vaccinated then:

2000 people exposed, with 99% effectiveness of vaccine, yields about 20 people who will contract measles despite being vaccinated.

Being vaccinated does not ALWAYS mean that you are individually immune, even if it is a lot better than not getting any vaccinations.

Many vaccines build "herd immunity" and will not work as well if people decide to opt out for some frivolous reason.

Also of course some people cannot receive a specific vaccine because they are pregnant or immunocompromised. Others should be vaccinated to prevent spread to those vulnerable populations, not just to protect themselves.

Think about the children!

But seriously, children don't usually get vaccinated for measles until they are 1yo.

Seriously. I have a 3 week old right now and was taking BART in everyday. People who know they have contracted a highly contagious disease and decide to travel on mass transit the day after should really consider the consequences of their malicious (even if unintentional) actions.

More than that; they should be arrested.

The effect of most vaccines wanes with time; however, because most people receive them, even those with waning or no resistance gain benefit (herd immunity). In this circumstance, though, once you're directly exposed, you're at risk.

If you are wearing a bullet proof vest then why should you care that you were shot?

Because the vaccine doesn't work in everyone.

Because young children can not be vaccinated, so he should stay away from them for a time (don't know how long).