Measles cases in the United States have surpassed the highest number on record since the disease was declared eliminated nationwide in 2000.

Overall, there have been 681 measles cases across 22 states this year, according to CNN's analysis of data from state and local health departments.

The states reporting measles cases are Arizona, California, Colorado, Connecticut, Florida, Georgia, Illinois, Indiana, Iowa, Kentucky, Maryland, Massachusetts, Michigan, Missouri, Nevada, New Hampshire, New Jersey, New York, Oregon, Tennessee, Texas and Washington.

As of Monday, the US Centers for Disease Control and Preventionreported 626 individual cases of measles confirmed in those 22 states. This includes illnesses reported by state health departments to the CDC through April 19 and therefore does not include cases reported since then.

The agency updates the number of measles cases each Monday.

Previously, the highest number of reported cases since elimination was 667 in 2014.

'Most of the cases that we're seeing are in unvaccinated communities'

Measles is a highly contagious disease caused by a virus that can spread through the air when an infected person coughs or sneezes or if someone comes into direct contact or shares germs by touching the same objects or surfaces. Measles symptoms may include fever, cough, runny nose, watery eyes and a rash of red spots.

Most cases in the United States have emerged in communities with low rates of vaccination against the virus, according to public health officials.

"I do believe that parents' concerns about vaccines leads to undervaccination, and most of the cases that we're seeing are in unvaccinated communities," Dr. Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases at the CDC, said in February at a congressional hearing about measles outbreaks.

Nationally, the United States has high measles vaccination coverage. The CDC says 91.5% of US childrenaged 19 months to 35 months received at least one dose of the measles, mumps and rubella vaccine in 2017, the most recent year available.

"However, there are pockets of people who are vaccine-hesitant," Messonnier said.

"Outbreaks of measles occur when measles gets into these communities of unvaccinated people," she said. "The only way to protect against measles is to get vaccinated."

A source familiar with the measles situation in the United States previously told CNN that of the 626 cases of measles that federal officials counted as of last week, 72% are unvaccinated, and 18% have an unknown vaccination status. Among those who are unvaccinated, it may be because of personal beliefs and medical reasons. The other 10% were vaccinated with either one or two doses.

Of those 626 cases, 487 were in people 19 and younger.

Measles outbreaks -- defined as three or more cases -- have been ongoing this year in Rockland County, New YorkNew York CityWashington stateSanta Cruz County, CaliforniaNew JerseyButte County, California; and Michigan.

The CDC has noted that those outbreaks are linked to travelers who were infected and brought measles back from other countries, including Israel, Ukraine and the Philippines.

For instance, the outbreak in New York, which was declared a public health emergency last month, began when an unvaccinated child became infected while visiting Israel, according to health officials.

A person from New York who was unknowingly contagious with the measles then visited Southeast Michigan, spreading the illness to at least 38 people there, according to Lynn Sutfin, public information officer for the Michigan Department of Health and Human Services.

The history of measles in America

In 1912, measles became a nationally notifiable disease in the United States, meaning it was required that health care providers and laboratories report diagnosed cases. In that first decade of reporting, an average of about 6,000 measles-related deaths were reported annually.

In the 1950s, researchers isolated the measles virus in a patient's blood, and in the 1960s, they were able to transform that virus into a vaccine. The vaccine was licensed and then used as part of a vaccination program.

Before the measles vaccination program was introduced in the United States in 1963, an estimated 3 million to 4 million people got the disease each year nationwide, according to the CDC. Afterward, cases and deaths from measles in the United States and other developed countries plummeted. There were 963 cases reported in the United States in 1994 and 508 in 1996.

By 2000, when there were only 86 cases, measles was declared eliminated from the United States, meaning there was no continuous transmission of the disease for more than 12 months.

Since 2000, the annual number of reported measles cases has ranged from 37 people in 2004 to 667 in 2014.

The measles, mumps and rubella vaccine -- known as the MMR vaccine -- is very effective. One dose is about 93% effective at preventing measles if you come into contact with the virus. Two doses are about 97% effective.

Experts recommend that children receive the vaccine in two doses: first between 12 months and 15 months of age and a second between 4 and 6 years old.

With any medicine, including vaccines, there is a chance of reactions, according to the CDC. These are usually mild and go away on their own, but there is a "remote chance" of side effects and even serious injuries.

Experts say the benefits outweigh the risks when it comes to the measles vaccine.

Are you protected from measles? It may depend on when you were born

— Michael Nedelman, CNN

Nearly two decades after measles was declared eliminated in the United States, the country and the globe have seen an upsurge of cases — including adults who thought they were protected by the vaccine. Now, some are questioning whether they are properly vaccinated and whether they are still at risk for getting measles.

In a video posted to YouTube last week, a rabbi in Detroit who said he was "fully vaccinated" nevertheless came down with measles — "a brutal three weeks," he said. In another recent case, an Israeli flight attendant has been unable to breathe on her own after experiencing a dire complication of measles: inflammation and swelling of the brain, called encephalitis. Health authorities believe she also received a vaccine.

"It's just so darn contagious. And the way that people travel nowadays, it just doesn't really take much for it to spread," said Dr. John Cullen, the current president of the American Academy of Family Physicians and a family physician in Valdez, Alaska.

"I have never seen a case of measles," he added. "But it's just a matter of time."

Adding to the uncertainty, some adults simply don't know their vaccination status or have long lost their documentation, Cullen said. The history of the measles vaccine has also prompted questions about how people of different ages have been vaccinated: Not only has the recommended number of doses changed over the years, but so has the vaccine itself.

Several years ago, he recalled, a case of measles popped up in Fairbanks, Alaska -- the state's first case in years. Worried about having been exposed and uncertain about whether they were properly vaccinated, patients of his who had traveled there asked for a dose of the vaccine, just in case. (When given within three days of exposure to the virus, the vaccine may offer some protection or make the illness milder, according to health officials.)

Although Alaska hasn't seen any cases this year, health officials issued an alert following a declaration of emergency in nearby Washington state, which has confirmed dozens of cases in 2019.

"If we see a substantial increase in the number of infections of measles, then there's going to be a whole lot more people who are going to be wondering what their vaccine status is," he said.

What are the current measles vaccine recommendations?

Doctors recommend two doses of the MMR vaccine, so called because it covers measles, mumps and rubella. Doctors give the first dose between 12 and 15 months, the second between 4 to 6 years.

The current recommendation was issued in 1989 by the US Centers for Disease Control and Prevention. Prior to that, a single-dose recommendation had been in place from 1963.

Before we had a vaccine, the agency says 3 million to 4 million Americans were infected yearly, including 48,000 hospitalizations and 400 to 500 deaths.

In some recent years, there have been fewer than 100 cases nationwide. But the virus has made a comeback in other years, including 2019 — largely due to anti-vaxers, experts say.

"The reason why we have vaccine hesitancy in this country is because people don't remember how bad it was," Cullen said.

One vs. two vaccine doses: What's the difference?

If you were vaccinated with two doses, in line with the latest guidelines from 1989, the CDC says you have a 97% chance at being protected against measles. And if that last few percent happen to come into contact with the virus, they're less likely to spread it to others, and their illness is often milder.

One dose is still about 93% effective at preventing the disease.

"Most virologists only dream of a solution as successful" as today's measles vaccine, CDC's principal deputy director, Dr. Anne Schuchat, wrote in 2015. "Success can breed complacency, skepticism or even attack," she added.

The second dose, however, is not a booster per se. In research studies, nearly everyone developed signs of immunity to measles with a single dose — and the vast majority of the time, this immunity is lifelong, experts say.

"That was in the clinical trials where everything was managed very, very tightly," said Dr. William Schaffner, an infectious disease specialist at the Vanderbilt University School of Medicine and an adviser to the CDC on vaccines.

But out in the real world, despite major gains against the virus, health experts were still finding measles outbreaks among kids and adolescents who had supposedly been vaccinated with one dose.

A second dose seemed to do the trick, but the question remained: Why had the vaccine failed the first time around?

"When that was examined further, it was discovered that, in the hurly burly of busy [medical] practice, the vaccine was not always handled optimally," Schaffner said.

For example, if a doctor or nurse were vaccinating multiple children in a row, they may have kept the vaccine out of the refrigerator too long, causing it to deteriorate. Even placing it in the door of a fridge, which is slightly warmer than the inside, may in some cases be enough to degrade the temperature-sensitive vaccine, he added.

That's less of a threat now that health care providers are more educated about how to handle the vaccine. In addition, we have shifted to single-dose bottles from multidose ones that might have been left out between shots, Schaffner said.

Still, giving two doses has allowed health systems to "fill in" a small percentage of kids for whom the vaccine didn't take the first time around.

"That is a wonderfully American solution: We'll double the cost of the vaccine program in order to protect this small group of children from getting measles and its complications," Schaffner said. "That strategy worked brilliantly and has been adopted in the developing world.

"And the only way it's been undermined is when children are withheld from vaccination."

Which vaccine did I get?

If you were born before the 1960s, you may have never been vaccinated against measles because it was assumed you'd been exposed to the virus. In fact, the CDC says that most people born before 1957 don't need the vaccine because "before vaccines were available, nearly everyone was infected with measles, mumps, and rubella viruses during childhood."

Then, in 1963, two types of measles vaccines were introduced: One was "killed" and another "live attenuated." The difference is that the first inactivates the measles virus, whereas the other has a weakened form of it.

The killed vaccine, which was given to an estimated 600,000 to 900,000 people, was itself killed off in 1967 "because it did not protect against measles virus infection," the CDC says. The agency also recommended that people who got that vaccine, or aren't sure of which one they got during those years, should get vaccinated anew.

The following year, in 1968, a new version of the live vaccine hit the market. It was just as effective as its live-attenuated predecessor and even safer than the first, as it had been further weakened. It's still used today, and has been available in combination with the mumps and rubella vaccines since 1971.

Experts say many middle-aged adults who were vaccinated in the early days won't know which one they got. Whether one received the "live" or "killed" vaccine was not always documented, Schaffner said -- and these days, those decades-old records might be nowhere to be found.

"Back then, the entire profession was not as nuanced about giving vaccines," he said.

What's more, neither the CDC nor any national organization has kept vaccination records. Individual states have immunization registries, but those didn't come along until later. For many Americans, "the records that exist are the ones you or your parents were given when the vaccines were administered and the ones in the medical record of the doctor or clinic where the vaccines were given," according to the CDC, which recommends that people search among baby books, school records and previous employers that may have collected this information, such as the military.

But many adults will be out of luck.

Older records "simply don't exist anymore because the doctors closed their practice, they retired, they moved away," Schaffner said. "It would be nigh on impossible to actually resurrect those records."

What do the experts recommend?

If you've received two doses of MMR or have had lab results showing immunity or previous measles infection, the CDC says you're all set.

If you've had one documented dose of the live vaccine and aren't at high risk of exposure, the agency says that's also adequate.

Those at higher risk -- whom the agency advises to get two doses for good measure -- might work in health care, travel internationally or be more likely to be affected by an outbreak.

Some people can't get vaccinated at all or need to wait: for example, people with weakened immune systems and babies who are too young to respond to the vaccine.

Born before 1957 and don't work in health care? The CDC considers that "presumptive evidence" that you're immune from having been exposed to the virus in a pre-vaccine era.

But what if you don't have written documentation of the right vaccine? For anyone who's unsure, the CDC says you can simply roll up your sleeve for another dose or two.

"The MMR vaccine is safe, and there is no harm in getting another dose," the agency says.

Cullen, however, acknowledged that the number of people uncertain about their vaccination status could be very high.

"If we were to have an epidemic in our area, would we have enough MMR to go around for the people who are concerned?" he wondered. "Probably not, under those circumstances. Unfortunately, this is kind of the new reality that we're dealing with."

There is a blood test doctors can use to check immunity as well, but it may take some time to get results back, he added.

For the general population, Schaffner said that blood test is "expensive, impractical, and it only rarely produces an actionable result."

"The juice ain't worth the squeeze," he said. "The way to prevent those older people from getting measles is to make sure all the kids are vaccinated."