What is the measles vaccine schedule? Here's what age to get the MMR shot.

With measles cases in more than 20 states across the U.S., doctors and health officials are urging anyone who hasn't received the MMR vaccine to get vaccinated. 

"To be very clear, the number one, number two, number three things we need to be doing are vaccinating kids and others who have not been vaccinated against measles. That is the only way to prevent measles transmission," Dr. Céline Gounder, CBS News medical contributor and editor-at-large for public health at KFF Health News, said on CBS News 24/7. 

"Once somebody is sick with measles, there are limited things we can do, largely ICU kinds of supports, in order to help someone through that illness," she added.