(DENVER) — Children make up more than a quarter of new COVID-19 cases in Colorado, bolstered by the spread of more-contagious variants and in-person school activities.

In Colorado, children between 0 to 19 account for 26.4% of all cases reported the week of April 25, according to state data. Overall, children make up 16.57% of all infections in the state since the start of the pandemic.

Dr. Sean O’Leary, a pediatrician and professor at the University of Colorado School of Medicine, said highly transmissible variants, the lack of vaccine for kids and loosened COVID-19 restrictions are contributing to the numbers.

“Kids under 16 right now are not eligible for vaccination so that’s a group that is completely prone to getting infected at this point,” O’Leary told ABC News.

The state has reported the presence of four variants with Centers for Disease Control and Prevention data showing that 49% of confirmed cases, regardless of age, are the B.1.1.7 variant, which first emerged in the United Kingdom.

Children in the state returned to in-person learning at the beginning of the 2020 academic year, though most with masks and social distancing.

“Kids have been a little bit more out there, there’s been some outbreaks in schools. There’s lots of after-school activities happening as well, we’ve seen some outbreaks in sports,” O’Leary said. “Who is getting infected in schools has also shifted. More infections are happening in students as opposed to the staff.”

Last week, the state reported 210 active outbreaks in schools — the highest number since Dec. 2 when there were 211, according to the Denver Post. Outbreaks fell in January, but began to increase in March and April. The state defines an outbreak as two or more cases connected to the same location or event.

O’Leary said University of Colorado hospitals have seen an uptick in hospitalizations, including of children, but it’s not as dire as last winter’s levels.

So far, 847 people between the ages of 0 to 19 in Colorado have been hospitalized and 13 have died since the start of the pandemic through April 29, according to a report by the American Academy of Pediatrics and the Children’s Hospital Association.

The fact that children are not eligible for the vaccine is another reason behind the spread. Currently, no authorized vaccine is available for those 16 and older.

Pfizer-BioNtech requested the Food and Drug Administration authorize its vaccine for children between 12 to 15 in late March. The FDA is projected to authorize the vaccine for that age group by next week.

And it’s not just Colorado where children are increasingly getting the virus.

Children now make up 22% of recent COVID-19 cases in the nation, according to the American Academy of Pediatrics and the Children’s Hospital Association report.

There’s been an increase in new reported cases among kids nationwide in March and April and about 72,000 new child virus cases were reported from April 22 to April 29, the report stated. That’s about a 4% increase in the cumulative number of child cases from the two weeks prior.

Since the start of the pandemic over 3.78 million children have tested positive for COVID-19, or about 13.8% of all national reported cases.

While severe illness due to COVID-19 is rare among children, experts in the report warned the virus “may harm the long-term physical health of infected children, as well as its emotional and mental health effects.”

Data pulled from 24 states and New York City show children make up between 0.1% to 1.9% of all child COVID-19 cases resulting in hospitalization, the Children’s Hospital Association report said.

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