Here's what to know if you're unsure about your measles immunity status

Measles is the most contagious disease known. It can rip through a population without immunity like wildfire. Just look at what's happening in Pennsylvania, says Patsy Stinchfield, executive director of the Measles Collaborative, a public health effort led by the National Infectious Diseases Foundation.

"In January in Pennsylvania, they had 12 cases. Now we're seeing 100 plus cases per week and over 1,000 cases in Pennsylvania," she says. "If Pennsylvania can see it spread that fast, that can happen in any state that has low immunization rates."

As vaccination rates have fallen across the country, many communities have become vulnerable. Nationwide, more than 4,000 measles cases have been reported this year alone — the highest number in 35 years. Pennsylvania has reported five deaths. New York state declared a measles emergency.

Stinchfield says the recent measles surge has got many people wondering if they're at risk.

"It's a very common question for people to ask, 'am I immune? Am I OK? What do I need to do?' And that is exactly the right question everyone should be asking themselves right now," Stinchfield says.

The best defense is vaccination. Two doses of the measles, mumps and rubella vaccine is 97% effective — meaning 97% of people will have an immune response.

"The measles vaccine is safe. It's highly effective, and it confers lifelong immunity," says Dr. Adam Ratner, a pediatric infectious disease physician in New York City and author of Booster Shots, a history of measles.

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But what if you don't know your immunization history, or you only got one dose of the measles shot? Should you get another dose? Infectious disease experts say that depends on several factors.

If you were born before 1957

People born before 1957 are presumed to be immune because back then, measles was so common, just about everyone got it in childhood, says Dr. Larry Kociolek, a pediatric infectious disease specialist at Ann & Robert H. Lurie Children's Hospital of Chicago and an associate professor of pediatrics at Northwestern University Feinberg School of Medicine.

"It was thought that pretty much everyone in the U.S. was infected with measles as sort of a rite of passage in childhood," Kocielek says. He says anyone who's ever had measles can be confident they're protected.

That is of course, if you survived. Before vaccines, measles used to kill 400 to 500 people each year in the U.S.

If you were vaccinated before 1968

People who were born after 1957 and vaccinated before 1968 may need an additional dose. That's because a small group of people who were immunized during that time received an early, less effective version of the vaccine that used inactivated or killed virus.

If you're not sure which vaccine you got, the Centers for Disease Control and Prevention recommends talking to your doctor about getting one or two doses of the MMR vaccine.

If you only got one dose of the MMR vaccine

At first, children in the U.S. received just one dose of the MMR vaccine. But that changed in 1989, after public health officials determined that a single dose wasn't enough to prevent outbreaks. That year, the official recommendation switched to two doses of MMR vaccine for children, which is still in effect today.

Kids need the higher protection "because they tend to be at higher risk of exposure, since they're oftentimes in group settings like schools and daycares," Kociolek says.

If you're in a higher-risk group

One dose of MMR vaccine is 93% effective, and traditionally, Kociolek says, that was considered enough protection for most adults, unless they were at higher risk of being exposed to measles. Health care workers, people in a college setting and those travelling abroad are all in this group. If you live with or are in close contact with someone who is immunocompromised, you also need two doses of vaccine to protect them.

But given the recent, dramatic rise of measles across the country these days, "if you are living in areas with high measles activity, it does make a whole lot of sense for adults to go ahead and get that second dose," Kociolek says.

Ratner says it's definitely worth asking your doctor if you should get a second dose.

While measles is often considered to be a childhood disease, a large proportion of people sickened this year have been unvaccinated adults — and people over 20 who lack immunity are at higher risk of serious complications.

No matter your age, "measles is a serious disease," Stinchfield says.

She notes that getting two doses of the vaccine can also help others at higher risk who can't get the MMR shot, including people who are pregnant, severely immunocompromised and infants under 6 months of age.

If you really don't know your immunization history

If you're just not sure if you're protected against measles and you can't find records of immunization, you could get a test to check your blood antibody levels. But those tests aren't perfect, says Dr. William Moss, a professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health.

"It's possible that a person could actually be protected and test negative on one of those," Moss says.

Depending on your circumstances, your insurance might not cover the blood test. But most private insurance plans and Medicaid must cover another MMR vaccine at no cost.

Moss says if you can't confirm how many doses of MMR vaccine you've had, "I would say it is easier and less expensive to just get another MMR vaccine, and there is no harm in getting a second MMR vaccine or even a third, additional MMR dose."

After all, it's much better to be protected than infected.