DALLAS (AP) — A Texas megachurch linked to at least 21 cases of measles has been trying to contain the outbreak by hosting vaccination clinics, officials said.

The outbreak started when a person who contracted measles overseas visited Eagle Mountain International Church in Newark, about 20 miles north of Fort Worth, Texas. Health department officials said those sickened ranged in age from 4-months to 44-years-old. All of the school-age children with measles were homeschooled.

"If it finds a pocket of people who are unimmunized, and the majority of our cases are unimmunized so far, then if you are around a person with measles, you will get sick," Russell Jones, chief epidemiologist for Tarrant County Public Health, said Monday.

In Tarrant County, where the church is located, 11 of the 16 people with measles were not vaccinated while the others may have had at least one measles vaccination. None of the five people infected in nearby Denton County have been vaccinated.

In a sermon posted online, senior pastor Terri Pearsons encouraged those who haven’t been vaccinated to do so, adding that the Old Testament is "full of precautionary measures."

"I would encourage you to do that. There’s absolutely nothing wrong with doing that. Go do it. Go do it. Go do it. And go in faith," said Pearsons, whose father is televangelist Kenneth Copeland. But she added, if "you’ve got this covered in your household by faith and it crosses your heart of faith then don’t go do it.

"The main thing is stay in faith no matter what you do."

Measles is spread by coughing, sneezing and close personal contact with infected people; symptoms include a fever, cough, and a rash on the face. The Centers for Disease Control and Prevention recommends that children get two doses of the combined vaccine for measles, mumps and rubella, called the MMR. The first dose should be given when the child is 12 to 15 months old and the second at 4 to 6 years old.

Vaccination opt-out rates nationwide have been creeping up since the mid-2000s, spurred in part by the belief that the vaccinations routinely given to infants could lead to autism, despite scientific evidence to the contrary.

Pearsons’ father is a pioneer of the prosperity gospel, which holds that believers are destined to flourish spiritually, physically and financially — and share the wealth with others. He has built a vast ministry with a worldwide reach. Eagle Mountain International Church is located on the grounds as the Kenneth Copeland Ministries.

Robert Hayes, risk manager with Kenneth Copeland Ministries, said they have held several vaccination clinics since the outbreak. He said the church has never advised against immunization against measles or seeking medical care.

A statement from the organization said their position is to "first seek the wisdom of God, His Word, and appropriate medical attention from a professional that you know and trust."

Hayes did not know why some people had not immunized their children.

"Some had for whatever reason chosen not to. Some depending on the age of their child were advised by their physician to wait until the child was a certain number of years old. I don’t get a sense for why after that they did not," he said.

He said if people are still choosing to not be immunized and they’ve been exposed to the measles, they are being asked to isolate themselves until it is clear they are not infected.

The incubation period for measles is about two weeks from exposure to fever. People are contagious from four days before getting the rash to four days before breaking out in a measles rash.

Texas Department of State Health Services spokeswoman Christine Mann said so far across the state there have been 27 cases of the measles this year, and that five of those cases were not connected in any way to the latest outbreak. She said it is unclear whether a case recently diagnosed in Harris County, where Houston is located, is tied to the outbreak. There were no cases in the state last year and six the year before.