As more countries lift travel restrictions for those who have been vaccinated, the world is slowly opening to travelers again.

Travelers without kids, that is.

To date, no Covid-19 vaccine has been approved for use in children, said Dr. Sharon Nachman, chief of the pediatric infectious disease division at Stony Brook Children’s Hospital.

Clinical trials are different for children, and factors such as dosage amounts and preexisting childhood vaccination schedules need to be examined before vaccines are approved for them, she said.

Countries, such as Estonia and Seychelles, have opened borders and eliminated quarantine requirements for vaccinated travelers. Greece and Thailand have indicated similar policies are in the works.

Other countries are granting vaccination exemptions to certain types of travelers. Georgia requires they enter by air from specific countries, and Slovenia’s policy only applies to those who have taken American- and European-made vaccines.

Iceland is allowing vaccinated travelers to bypass quarantine requirements, only if they are currently allowed to enter — and most people aren’t.

Covid-19 vaccines made by Moderna, Johnson & Johnson and Oxford-AstraZeneca are authorized for people aged 18 years and older. Those aged 16 years and over can take Pfizer-BioNTech’s vaccine.

Vaccine trials in children have begun, but the ones that are farthest along involve older children and teenagers, said Dr. Jeremy Levin, chairman of the Biotechnology Innovation Organization, a trade association for the biotech industry.

“Pfizer and Moderna are testing their vaccines on children 12 and older and may have data by the summer,” Levin told CNBC Global Traveler. “Johnson & Johnson, Novavax and AstraZeneca are further behind, but also plan to test their vaccines in children.”

Last month, The University of Oxford announced AstraZeneca trials are starting for children as young as 6 years old. Johnson & Johnson is also launching vaccine trials in older children first, with infants and newborns following quickly thereafter. “It’s critical to understand that trials in children may have to be different,” said Levin. “These trials may require longer to get a read-out because adverse effects of Covid-19 are rarer in this population.”

Infected children are often asymptomatic and don’t tend to get severely ill from Covid-19. Deaths in children are also rare.

Vaccinations in time for the summer travel season aren’t expected, however, they are likely to be available to high school students by the fall, said White House Chief Medical Advisor Dr. Anthony Fauci.

“I’m not sure it’s exactly on the first day school opens, but pretty close to that.” Elementary school children will have to wait a little longer.

“If you project realistically, when we will be able to get enough data to be able to say that elementary school children will be able to be vaccinated … I would think that would be, at the earliest, the end of the year and very likely the first quarter of 2022,” Fauci said.

Fauci explained that companies are testing vaccines in a process known as “age de-escalation,” which entails testing older children first, then working incrementally down to infants.

“Almost all of the vaccine companies will need to study from infants on up,” said Nachman, although she called conducting trials by age groups “somewhat [of] an artificial plan” that is “not mandated by science.”

“In many studies, there is no indication of any increased safety … as we age de-escalate,” she said. “The result is instead of protecting children and their families, we are increasing their risk by not getting them [vaccinated] sooner.”

Slovenia allows children under the age of 13 to avoid quarantine and testing requirements, if they cross the border “with a close family member who has not been put under home quarantine,” said Sabina Langus Boc from Slovenia’s ministry of the interior.

However, most countries that have loosened travel restrictions for vaccinated travelers aren’t granting exceptions for unvaccinated kids. But this could happen as countries reeling from catastrophic losses in tourism revenue seek to attract family holidaymakers this summer.

“Countries which exempt children from vaccination are doing so in the absence of data,” said Levin. “It’s important to realize that we know that children can and are harmed by Covid-19.”

While vaccination policies may not open avenues for kids to travel abroad this summer, families can consider destinations, such as the Maldives and Mexico, that are open to everyone — vaccinated or not.