Were the measles outbreaks of this decade caused by people not vaccinating?

Question

There are several movements against vaccinating children in the USA. There were several measles outbreaks in the 2010's.

Are they related?

Answer 1

Yes, there is a correlation between groups of people not vaccinating against measles and outbreaks within the United States in the current decade. The Center for Disease Control (CDC) explicitly states "the majority of people who got measles were unvaccinated" concerning the record number of cases in 2014 on their page for measles outbreaks. There are links to their reports on multiple outbreaks. The following is their most recent outbreak specific paper:

Measles Outbreak — California, December 2014–February 2015

Among 110 California patients:

• 49 (45%) were unvaccinated
• five (5%) had 1 dose of measles-containing vaccine
• seven (6%) had 2 doses
• one (1%) had 3 doses
• 47 (43%) had unknown or undocumented vaccination status
• one (1%) had immunoglobulin G seropositivity documented, which indicates prior vaccination or measles infection at an undetermined time.
• Twelve of the unvaccinated patients were infants too young to be vaccinated. Among the 37 remaining vaccine-eligible patients, 28 (76%) were intentionally unvaccinated because of personal beliefs, and one was on an alternative plan for vaccination.

As can be seen, only 12% of infected patients had a confirmed MMR (Measles Mumps Rubella) vaccine dose administered to them, compared to 45% confirmed with no dose of the vaccine. At most, a mere 66% of infected had an MMR vaccine in their medical history. To compare, 91.5% of California children aged 19-35 months had received at least one dose of the MMR vaccine according to the CDC's 2012 Vaccine Coverage Report

Populations receive protection via vaccines through herd immunity, which is a medical concept that if a certain threshold of the population is vaccinated against a disease, even those who are not vaccinated are protected from the disease (as it has difficulty finding a large pool of humans to infect and multiply in). Measles had a very high threshold, as 95% of the population needs to be vaccinated to protect the unvaccinated from the disease, so only 5% of a population needs to not be vaccinated to significantly increase the risk of outbreak. As unvaccinated children tend to be clustered together geographically and socially, the risk for outbreak is higher as those clustered populations are further from the 95% threshold, and the threat of a sustained outbreak is higher, as can be seen in the stats quoted above.

Disclosure notice: I am a contract employee of Pfizer's Vaccine Research Unit. Any statements made do not necessarily reflect the opinions of Pfizer or its subsidiaries

Answer 2

Yes. The three most severe measles outbreaks of 2013 all, interestingly, began when an unvaccinated member of a small religious group with low vaccination rates traveled overseas to a country where measles remains endemic, and returned with a nasty souvenir that spread.

These were

• an Amish missionary to the Philippines

  The real story behind the 2014 outbreak isn't on the West Coast. It's in Ohio Amish country, where a missionary returning from the Philippines turned an otherwise unremarkable year for this virus into one of the worst in recent history.

• an ultra-orthodox Jewish student who went to the UK, and

  On March 13, 2013, an intentionally unvaccinated adolescent aged 17 years returned to New York City from London, United Kingdom, while infectious with measles. This importation led to the largest outbreak of measles in the United States since 1996.

• a member of a small charismatic (and antivax) Evangelical church in Texas, who went to Indonesia.

  The outbreak began when a man who contracted the virus on a recent trip to Indonesia visited the Eagle Mountain International Church in Newark, about an hour and a half northwest of Dallas.

EDIT: The largest epidemics as of August 2013 (and the vax status of the index patients) can be found at CDC. In the remaining months the Amish outbreak moved into first place. Also, while the Texas index case is given as unknown-status in the link above, the church itself was against vaccinations, and other reports list the person who brought the virus back as unvaccinated.