As measles cases continue to spread across the United States, many Americans may be asking themselves if they need a measles vaccine “booster” to enhance protection.

An outbreak in western Texas has grown to 279 cases, mostly among those who are unvaccinated or whose vaccination status is unknown. Meanwhile, an outbreak in nearby New Mexico has also increased, reaching 38 cases as of Wednesday.

The Centers for Disease Control and Prevention (CDC) currently recommends that people receive two vaccine doses, the first at ages 12 to 15 months and the second between 4 and 6 years old. One dose is 93% effective, and two doses are 97% effective, the CDC says.

Most vaccinated adults don’t need another vaccine dose, said Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia. He also said it’s important to call another shot of the MMR vaccine “a dose” as opposed to a “booster.”

He explained that the measles vaccine used to be a single-dose vaccine before a second dose was recommended in the late 1980s.

“In the late ’80s, there were sort of big outbreaks of measles,” Offit told ABC News. “But if you looked at the epidemiology of those outbreaks, it was in people who never got a vaccine.”

He went on, “So it wasn’t that the immunity faded, that the vaccine wasn’t good enough. It’s an excellent vaccine as a single-dose vaccine. The problem was people didn’t get it. So, the second dose recommendation really was to give children a second chance to get a first dose.”

Depending on the year you were born

If someone was born before 1957, they are presumed to have life-long immunity against measles, Offit said.

Before the MMR vaccine was available, nearly everyone was infected with measles, mumps and rubella during childhood, according to the CDC.

Those with a confirmed laboratory diagnosis of measles are protected from the virus, the agency adds.

In 1963, the first measles vaccine became available, followed by an improved vaccine in 1968, said Dr. Gregory Poland, a vaccinologist and co-director of The Atria Research Institute — which focuses on disease prevention.

A very small number of people, representing less than 5% of Americans, may have received the inactivated measles vaccine from 1963 through 1967 during childhood, which may not have offered sufficient protection against the virus. These people would be eligible for re-vaccination with one or two doses, the CDC says.

“So, the first measles vaccine licensed in the U.S. was in 1963 and it was an inactivated vaccine,” he told ABC News. “That inactivated vaccine had two consequences to it. One, it did not produce protective immunity and, number two, it led to — when people did get exposed and infected — it led to atypical measles, and that can be very severe.”

Poland said, at the time, there was also a live attenuated measles vaccine, similar to what is used today “but it was not very attenuated or weakened, and so it caused a lot of side effects.”

He explained that to decrease side effects, physicians would give a patient a vaccine and then a shot of immunoglobulin, or antibodies. While this decreased side effects, it also tended to kill the vaccine virus, not giving people adequate immunity.

For those who were vaccinated with the single-dose vaccine similar to the one used today — or received the MMR vaccine — Offit said another dose is likely not needed.

In 1989, the Advisory Committee on Immunization Practices, the American Academy of Pediatrics and the American Academy of Family Physicians recommended children receive a second MMR dose.

Offit and Poland said anyone who has received two doses of the MMR vaccine does not need to receive another dose.

If someone is unsure if they are immune to measles, they should first try to find their vaccination records. If they cannot find written documentation, there is generally no harm in receiving another dose of the MMR vaccine, according to the CDC. A health care provider can also test blood to determine whether someone is immune, but this is generally not recommended.

On its website, the CDC says older children, adolescents and adults need one or two doses of MMR vaccine if they don’t have evidence of immunity.

Infants in a high-risk area

In the face of the growing measles outbreak, the CDC issued an alert on March 7 saying parents in the outbreak area should consider getting their children an early third dose of the MMR vaccine.

Texas health officials have also recommended early vaccination for infants living in outbreak areas.

This would result in three doses overall: an early dose between age 6 months and 11 months and then the two regularly scheduled doses.

Poland says it’s important to note that this dose is only for infants living in high-risk areas or going to visit high-risk areas and not recommended for most children.

“Generally, the reason we don’t give [the vaccine] at an early age is that, if the mother was immunized or had disease, the antibodies that she has are passed through the placenta to the baby — those last around 12 months,” he said. “If you give the vaccine prior to that, then some amount of that live virus vaccine will be killed by the mother’s antibodies circulating in the baby, and so it’s not long-lasting, high-titer immunity.”