With 555 cases of measles in the United States reported so far this year, 2019 is shaping up to be one of the worst years in decades for the highly contagious disease.

And it’s only April.

Most of the cases have been linked to people who became infected overseas and, upon returning to this country, transmitted the virus to others, the U.S. Centers for Disease Control and Prevention said.

The total includes 13 cases in New Jersey but none in Pennsylvania, though Philadelphia saw an outbreak of mumps this year at Temple University. Why one disease and not the other?

The answer involves vaccines — and, to some extent, luck, say infectious-disease experts from Temple and Drexel Universities.

The combination MMR vaccine that protects against the two diseases, along with rubella, is more effective and longer-lasting against measles than mumps. But measles is more contagious. (It also can have more serious complications. A flight attendant from Israel’s El Al airline is reportedly in a coma after being infected with measles.)

The question of when a disease-causing microbe takes hold in a community comes down to math. Public health officials consider a community to be protected from measles if the vaccination rate is 92 to 95 percent, whereas the threshold for mumps is 90 to 92 percent, said Neal Goldstein, an assistant research professor at Drexel’s Dornsife School of Public Health.

Yet even if the vaccine rates in a given city or state exceed those numbers, as they do in Pennsylvania (97 percent of kindergartners for the 2017-18 school year) and New Jersey (96 percent), public health officials cannot let down their guard. When vaccine rates fall below the threshold in “pockets” or neighborhoods, residents are more vulnerable if someone brings the disease home from abroad, said Thomas Fekete, chair of the department of medicine at Temple’s Katz School of Medicine.

“Those little pockets are now tinderboxes that are catching fire,” he said.

Some of this year’s measles cases have occurred in ultra-Orthodox Jewish communities, where a handful of leaders have expressed doubts about vaccines despite overwhelming scientific evidence of their safety and ability to save lives. But most Jewish leaders have emphatically supported vaccines. The outbreaks may be due to the fact that certain communities are very close-knit, allowing for easy disease transmission upon exposure, said Goldstein, an infectious-disease epidemiologist with the Christiana Care Health System.

Among people who get the recommended two doses of the combination vaccine, 97 percent are protected from measles in the event of exposure, the CDC says. That number drops to 93 percent among people who receive just one dose.

The mumps component of the vaccine is less effective, reducing a person’s risk by 88 percent with two doses, and by 78 percent with one dose, the agency says. What’s more, the measles protection is considered lifelong, while the mumps portion appears to wane with time. That’s why Temple, where the mumps patients had been immunized as children, was distributing booster shots to students and faculty.

While mild cases of measles can resemble the flu, the disease can have serious complications such as pneumonia and encephalitis — swelling in the brain. For every 1,000 children who are infected, one or two will die, the CDC says.

Of the 555 cases of measles nationwide, more than 70 percent occurred in people who had not been vaccinated, and nearly 20 percent occurred in people who did not know if they had been vaccinated, CDC spokesperson Jason McDonald said. The percentage of cases in vaccinated people was in the single digits, he said.

In addition to the 13 cases in New Jersey, measles has been reported in California, Michigan, New York, and Washington.

As for the mumps, 155 cases had been associated with the Temple outbreak as of Thursday -- 26 confirmed and 129 probable cases, the city Department of Public Health said. Twelve are from surrounding counties, while the 143 others are in Philadelphia.

Efforts by Democratic lawmakers to tighten vaccine exemptions in a few states recently have been met with resistance from Republicans, Politico reported. But a recent study led by Drexel’s Goldstein found worrisome anti-vaccine sentiment among lawmakers from both major parties.

The last big measles outbreak in Philadelphia occurred in 1991, when more than 1,200 cases were reported, including hundreds in two church groups that rejected the concept of vaccination. Six people in those groups died from the disease between November 1990 and March 1991. Outbreaks were reported elsewhere in 1991, too, including more than 1,700 cases in New Jersey.