Which vaccines are required is decided on the state level. But generally, they include: polio, meningococcal disease, MMR (measles, mumps, rubella), DTaP (diphtheria, tetanus and pertussis [whooping cough]), varicella (chickenpox) and hepatitis A & B. Some states also offer the HPV vaccine to middle and high school girls (since the HPV2 vaccine only works on females).
I don't think we really know how many people are unvaccinated. That's something that would be very hard to figure out because 1) It's done by self-reporting, 2) many Americans who were vaccinated may not be up-to-date on their shots, and 3) it'd be almost impossible to to account for illegal immigrants, whose population ranges from 7 to 20 million people. But we do have an idea of how many school-aged children are vaccinated, and what the rates are by state/region:
- The CDC found that 94% of kindergarteners had the MMR vaccine, 95% had the DTaP vaccine, and 93% were vaccinated against chickenpox
- Mississippi has the highest rate of MMR vaccination with 97.7% (Congrats to Mississippi for being no.1 in something other than obesity and teen pregnancies) and Colorado had the lowest with only 81.7%. Seven states + Washington DC had rates less than 90%
- DTaP vaccination rate was 95% or over in 25 states, but less than 90% in five states and DC
- Chickenpox vaccination rate was at least 95% in nine states, but less than 90% in eight states and DC
- In certain parts of Oregon, the vaccination rate among schoolchildren is less than 50%. In some areas, only a quarter are actually vaccinated
- According to a recent study, 6 out of 7 schools (public and private) has an MMR vaccination rate of 90%+
- Ohio, Colorado and West Virginia are tied for the worst MMR vaccination rate, with an average of only 86% of all children in those states being immunised
California overall has a 90.7% vaccination rate (which is well below the needed 95%, but it's not
too horrible). The issue is that the anti-vaxxers tend to be concentrated in certain areas. Los Angeles, which is only 27 miles from Disneyland, has a lot of the anti-vaxxers and alternative medicine types.. Obviously, a lot of Los Angeles residents visit Disneyland due to the short distance, so it's really not surprising at all that the outbreak is so close to one of the major "hubs" for anti-vaxxers.