The Centers for Disease Control and Prevention (CDC) has confirmed 228 cases of measles in 12 states so far this year.

The CDC has identified six outbreaks, which it defines as three or mores cases, in New York, Washington, Texas, Illinois and California.

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The cases are linked to unvaccinated American travelers bringing measles back into the U.S. from other countries where large measles outbreaks are occurring, such as Israel and Ukraine, the CDC says.

The outbreaks have renewed the focus on states with lax vaccine exemption laws. Every state requires children to have certain vaccines before entering school, but most allow exemptions for religious or personal beliefs.

Public health officials blame the spread of misinformation online and on social media about the risks and efficacy of vaccines. The CDC considers the MMR vaccine, which protects against measles, to be safe and effective.

The number of measles cases so far in 2019 puts this year on track to surpass previous years.

The CDC confirmed 372 cases of measles in 2018. The highest number of cases since 2010, when the disease was considered eliminated in the U.S., was in 2014, when 667 cases were reported.