SANTA MONICA (CBSLA.com) — A number of Disney employees have contracted the measles virus, even though some of them say they had previously received vaccination.

The figures of the virus continue to increase after visitors to the Disneyland park started falling ill last month.

Angela Vassallo of Providence St. John’s Infection Prevention and Epidemiology says she thinks she may know why these individuals may have still been stricken, despite claims of vaccination.

“My suspicion is that, if they were vaccinated, they only received one dose, the first dose of the vaccine, and didn’t check their immunity and didn’t get the second dose of the vaccine,” Vassallo said.

The Centers for Disease Control recommends two doses of the measles vaccine. Most children born after 1971, the year the vaccine was created, received the first dose at age 1, and the second dose at age 4.

Vassallo says that once the vaccine is administered, it lasts for a lifetime.

Those born before 1957 are likely already immune, according to Vassallo. This is because they’ve been exposed to the measles. However, she recommends people born between 1957 and 1971 to check their immunization records.

“Anyone born between 1957 and 1971, you definitely want to check, because the vaccines that were created in the 60s, at least one we found later, didn’t work,” Vassallo said.

Doctors are able to perform a simple test for anyone who is in question over their immunity.

“They take blood specimens and look at your titers to see if you have immunity to the disease,” Vassallo described. “If not, then they start that two-dose regimen.”

Recent figures show the total of measles cases in California to be 53.

This number, Vassallo says, is cause for great concern.

“For one sick person, about 90 percent of people around them who don’t have immunity will pick up the disease.”

Vassallo says that pregnant women, women trying to get pregnant or people with weakened immune systems should not receive vaccinations.

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