Health officials in South Carolina confirmed 124 new measles cases since Friday, as the outbreak in the northwestern part of the state balloons following the holidays.

There is a second, separate measles outbreak along the Arizona-Utah border, where 418 people have been infected since August.

Last year was the nation’s worst year for measles spread since 1991, according to the Centers for Disease Control and Prevention. The U.S. confirmed 2,144 cases across 44 states. Three people died, all of them unvaccinated.

Measles is caused by a highly contagious virus that’s airborne and spreads easily when an infected person breathes, sneezes or coughs. It is preventable through vaccines and has been considered eliminated from the U.S. since 2000, though the country is at risk of losing that achievement.

Measles first infects the respiratory tract, then spreads throughout the body, causing a high fever, runny nose, cough, red, watery eyes and a rash. Most kids will recover, but infection can lead to dangerous complications such as pneumonia, blindness, brain swelling and death.

The CDC defines an outbreak as three or more related cases.

South Carolina has logged 434 cases as of Tuesday in an outbreak centered in Spartanburg County.

The outbreak has rapidly grown in the last month to one of the worst in the nation. Hundreds of children have been quarantined because of school exposures, some more than once. A person with measles also exposed people at the South Carolina State Museum in Columbia on Friday.

The case count could soon rival an outbreak last year in Texas, where 762 cases were reported and two children died. Experts believe that was likely an undercount.

The outbreak in the border towns of Hildale, Utah and Colorado City, Arizona — an area nicknamed Short Creek — continues to grow.

Arizona health officials added nine new cases Tuesday, for a total of 217 in Mohave County. Utah officials added two cases Tuesday for a total of 201.

Experts in both states have said they are concerned about undercounts.

“We did see our cases slow and then pick back up again after the holidays,” said Nicole Witt, of the Arizona Department of Health Services. “We’re hopeful we’ll see the end of this outbreak soon but, right now, we continue to see the same trickle of cases week over week.”

The best way to avoid measles is to get the measles, mumps and rubella vaccine. The first shot is recommended for children between 12 and 15 months old and the second between 4 and 6 years old.

After two doses, the shot is 97% effective against measles and its protection is considered lifelong.

Measles has a harder time spreading through communities with high vaccination rates — above 95% — due to “herd immunity.” But childhood vaccination rates have declined nationwide since the pandemic and more parents are claiming religious or personal conscience waivers to exempt their kids from required shots.