From Jan. 1 to April 11, the Centers for Disease Controls and Prevention recorded 555 individual cases of measles across 20 states.

This has been the second-largest number of cases reported in the United States since measles was eliminated in 2000. Based on the health agency's observations, the majority of people who got measles were unvaccinated.

The CDC said the outbreaks are linked to international travelers who brought measles back from other countries such as Israel, Ukraine, and the Philippines, where large measles outbreaks are ongoing.

The viral disease is still common in many parts of the world including some countries in Europe, Asia, the Pacific, and Africa. This makes unvaccinated travelers more vulnerable and at risk. As shown by the recent CDC data, travelers with measles continue to bring the disease into the United States.

Two-Dose MMR Vaccines Against Measles

Experts continue to encourage the vaccination of children and adults to prevent the further spread of measles.

The measles, mumps, rubella vaccine is administered as a two-dose vaccine series that effectively protects against all three viruses. People who had the two-dose MMR vaccines are said to be protected against measles for life with 97 percent efficacy.

However, even vaccinated adults are at risk of contracting the viral disease if their immune system did not respond properly to the vaccine. Due to changes in the MMR vaccine, some adults may also not have received full protection, and so those who had their immunization as children should also consider getting a second jab.

"We assume anyone born before 1957 was exposed to several outbreaks of measles so they have immunity. It turns out about 98% of people who were born before 1957 are immune. That's how common measles was. Everyone got it so they are in the clear," said Dr. Allison Bartlett, a pediatric infectious disease specialist at the University of Chicago Medicine.

Who Are Advised To Get A Second Immunization?

Adults who do not have evidence of immunity to the virus, especially older people in their 50s and 60s who are not sure if they were actually vaccinated are advised to get two doses of the MMR vaccine.

Likewise, those who received the "killed" or inactivated version of the measles vaccine that was proven as not effective or those who don't know what kind of vaccine did they receive in the 1960s should be reimmunized.

An estimate of 600,000 to 900,000 people received the outdated vaccines when they came out between 1963 to 1967. Running a blood test can check a person's immunity level against the virus.

People traveling during an outbreak, those who are near travel destinations, and healthcare providers exposed to the disease could also be at risk if they had only one dose of the vaccine. Experts say one dose is still more than 90 percent effective at preventing measles, but not quite as good as the ideal two doses.

During an outbreak, people are advised to check and follow the measles vaccination schedules and recommendations from their local health departments.