Gloria Kennett is eager to take a long-awaited beach vacation, but she is keeping a watchful eye on new cases of the Covid-19 Delta variant.

Ms. Kennett, a hotel executive in Chicago, is vaccinated, but her 9-month-old daughter isn’t yet eligible. For now they’re planning to go. But if they see a big surge in cases, they’ll hold off. They’re reassured that they can get a refund if they cancel.

The more-infectious Delta variant has quickly spread around the world, and is now the most common strain of the Covid-19 virus circulating in the U.S. An analysis of genetic sequencing data as of June 27 showed that the Delta strain now makes up about 40% of positive Covid-19 test samples, according to Helix, a population genomics company that collects and analyzes test samples from several U.S. states.

The CDC has said that fully vaccinated Americans are well protected, even against variants. But children under the age of 12 aren’t yet eligible for vaccination.

Children are still at far less risk than adults for severe complications or death from the virus. Most children who get Covid-19 still have mild or asymptomatic cases. There is no indication that young adults and children are more vulnerable proportionately to the Delta variant than other age groups, and any pockets of increased transmission mostly reflect the fact that they haven’t been immunized, scientists say. “There’s no evidence it’s more severe in children,” says Monica Gandhi, an infectious-diseases doctor at the University of California, San Francisco.

Here’s what doctors say parents need to know about keeping unvaccinated children safe this summer.

Covid-19 strain known as Delta is in at least 60 countries including the U.S. and likely to spread world-wide, Covid-19 Genomics UK Chair Dr. Sharon Peacock tells WSJ's Betsy McKay at the WSJ Tech Health event. The Wall Street Journal Interactive Edition

Keep unvaccinated kids’ masks on indoors

Masking indoors, especially around unvaccinated adults outside a child’s household, is more important as the Delta variant becomes the dominant strain, says Andrew Janowski, a pediatric infectious disease physician at Washington University School of Medicine and St. Louis Children’s Hospital. While many parts of the U.S. have relaxed masking and social distancing rules for those who are vaccinated, unvaccinated children should continue the same precautions they took against earlier strains of the virus. “A lot of families and parents can apply the same rules they’ve been following,” he says.

Look out for regional hotspots

Local vaccination rates will be a big determinant of how strongly Delta takes hold in different regions. Places with fewer vaccinated adults are more likely to see cases rise, potentially creating a bigger strain on area hospitals, says Preeti Malani, chief health officer at the University of Michigan. For families taking part in camps or traveling to different parts of the country, Dr. Malani suggests tracking case numbers, test-positivity rates and deaths. “You have to find ways to move toward something that feels like normal, but to do it in a way that’s safe for their family,” she says.

Weigh travel plans carefully

Although some international destinations are opening up, Dr. Malani encourages parents of unvaccinated children to stay closer to home. The U.S. has higher vaccination rates and lower case counts than many other countries. Doing an outdoor-focused trip adds an extra layer of safety. “You can absolutely go on a vacation with your kids that is enjoyable and safe, especially in the U.S. right now,” she says.

Traveling internationally can be dicier, Dr. Malani adds, and not just because of Covid risks. Delta-related hospitalizations in low-vaccinated areas can pull resources away from other medical care that families may need in an emergency. “Some health systems are not fully functional and it may be hard to get care,” Dr. Malani says.

Consider extra precautions for higher risk children

Parents of children with cardiovascular or pulmonary problems or those who are immunocompromised may need to be more vigilant about precautions such as masking, staying outdoors and social distancing with the more transmissible variant, Dr. Gandhi says. In some cases, travel, playdates or group activities may not be recommended, though it’s important to talk with your doctor about weighing infection risk against socialization needs.

Children age 12 and older are eligible for vaccination against Covid-19, but younger kids aren’t yet.

Children age 12 and older are eligible for vaccination against Covid-19, but younger kids aren’t yet.

Photo: jeff kowalsky/Agence France-Presse/Getty Images

Get your family vaccinated as soon as they’re eligible

New variants of the Covid-19 virus will continue to emerge, especially in areas where vaccination rates are lower, says Dr. Janowski. To help keep variants from infecting younger children, it’s essential for people 12 and older to get inoculated. “We’re seeing evolution before our eyes,” he says. “This is going to be an ongoing issue, especially if we can’t get more people vaccinated.”

Be aware of local recommendations

As we learn more about the Delta variant, some places are reimposing precautions. In Los Angeles, public health officials are again recommending masking in public places indoors, even for those who are fully vaccinated, citing rising Delta cases. Internationally, World Health Organization officials have reiterated their recommendation that vaccinated people continue wearing masks indoors as Delta surges. For now, the CDC says vaccinated people still don’t need to wear masks in most settings, but also notes that local governments should make policies for their communities depending on their circumstances.

Write to Alina Dizik at alina.dizik@wsj.com