Originally Published: 19 MAY 22 06:48 ET

Updated: 19 MAY 22 09:27 ET

By Jacqueline Howard, CNN

(CNN) -- The United States is preparing to welcome more displaced Ukrainians now that the Biden administration has approved the first group to enter through the new Uniting for Ukraine program. Ukrainians began arriving through the program this month.

In early May, "the first notices authorizing Ukrainian nationals to travel to the United States to seek parole through the Uniting for Ukraine process were sent out," Liza Acevedo, a spokesperson for the US Department of Homeland Security, said in a statement.

As of Thursday, "more than 15,000 individuals are able to book their own travel" to come to the United States, Acevedo said.

To enter the United States, those travelers have to meet certain requirements -- including vaccinations and infectious disease screenings.

Uniting for Ukraine is a streamlined process that allows Ukrainian citizens and their immediate family members to come to the United States on humanitarian grounds and stay for a two-year parole, meaning they can stay on a case-by-case for no more than two years. They must have a sponsor in the US who agrees to provide them with financial support, according to the Department of Homeland Security.

In April, President Biden announced a commitment to welcome 100,000 Ukrainians to the United States. Displaced Ukrainians are trickling in, and US Citizenship and Immigration Services has received more than 23,500 requests from supporters in the US who want to help Ukrainians entering the United States.

Sponsors need to pass security background checks of their own. There's no limit on the number of individuals a person or group can sponsor, but administration officials noted that they'll be evaluating sponsors' means and ability to support Ukrainians.

Meanwhile, Ukrainian applicants will undergo rigorous security checks, including biographic and biometric screening, and must have been residents of Ukraine as of February 11. They must also meet certain public health requirements, including receiving a Covid-19 vaccine.

"They'll require these individuals to attest that they have received at least one dose of measles, polio and Covid vaccinations prior to coming into the country. If they have not, they must receive the vaccination abroad from the country that they are in," said Lori Tremmel Freeman, chief executive officer of the National Association of County and City Health Officials.

Those vaccinations could be provided by the World Health Organization or another agency or nonprofit group, she said. The Covid-19 vaccination requirement must be a vaccine that has been approved or authorized by the US Food and Drug Administration, or a vaccine with WHO emergency use listing.

People seeking authorization to travel to the United States through the Uniting for Ukraine process will be able to confirm vaccination in an account on the government's myUSCIS website.

Additionally, "they'll have two weeks once they get here to complete tuberculosis screening if they are 2 years or older," Freeman said. Any community provider could conduct the screenings -- but because the Ukrainians probably will not have health insurance, many will be done by local health departments.

People then submit those results on their myUSCIS account.

"The reason for the aggressive approach with the measles and polio vaccines in particular, and also TB, is because Ukraine is known as an area that in the past has had severe measles outbreaks, and we want to be careful," Freeman said. "From a health security standpoint in our country, it was important to set up the eligibility to come here based on this attestation."

Although the polio virus is still a threat in some countries, the US has been polio-free since 1979, thanks to vaccines. And despite occasional outbreaks, the country has maintained measles elimination status for almost 20 years.

These medical requirements are also nothing new for public health officials; people fleeing conflict and other refugees generally are required to get medical checks before coming to the United States.

As more people are approved to come to the US, health officials expect many of them to travel to two states, Freeman said: Washington and New York.

In Washington, "they have a very large Ukrainian community, and they do a lot of work in public health that's focused on that community," Freeman said. And New York state is home to the largest Ukrainian population in the nation, according to Gov. Kathy Hochul's office.

"It would seem like there would be opportunity for them to connect within their own community in those places," Freeman said. "That would be the draw."