By Megan Marples, CNN

(CNN) -- Children may be spreading Covid-19 through extracurricular activities like sports, health officials say.

After-school activities are creating clusters where coronavirus can spread among children, said Dr. Rochelle Walensky, director of the US Centers for Disease Control and Prevention.

"We know that these increases are due, in part, to more highly transmissible variants, which we are very closely monitoring," Walensky said Monday at the White House COVID-19 Response Team briefing.

The virus was linked to high school wrestling tournaments in Florida last December where 38 people tested positive, according to a CDC report published in January.

In Minnesota, the B.1.1.7 variant of SARS-CoV-2 spread through Carver County with at least 68 cases linked to youth sport activities including hockey, wrestling and basketball, according to the Minnesota Department of Health.

This variant of the novel coronavirus, originally found in the United Kingdom, is about 50% more transmissible than other strands of the virus in the US, researchers reported in the journal Cell in March.

The CDC recommends children limit youth sport participation and follow specific guidelines when engaging in those types of activities. The organization listed nearly a dozen different recommendations, including minimizing the time spent indoors and reducing the amount of time players spend in close contact with each other.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, warned about the spread of Covid-19 among children who participate in youth sports.

"We're finding out that it's the team sports where kids are getting together, obviously many without masks, that are driving it, rather than in the classroom spread," Fauci told ABC's George Stephanopoulos Tuesday on "Good Morning America." "When you go back and take a look and try and track where these clusters of cases are coming from in the school, it's just that."

Staying safe on the field

There may be a higher risk of Covid-19 transmission in informal settings like after-school activities and sports compared to formal settings because people may be more likely to let down their guard, said CNN Medical Analyst Dr. Leana Wen.

She recommends children play outdoors whenever possible and not share water bottles. Kids also need to be on high alert in locker areas and changing rooms.

Parents should educate themselves on their sports league's protocols if a player tests positive, Wen said. Some questions she advises parents ask are, "Will they be contacted promptly if there is a positive case? How is contact tracing done?"

Wen said it's important to stay vigilant as Covid-19 variants evolve.

"With a more contagious variant, that means the activities we thought were safer are now going to be higher risk," Wen said.