The number of Colorado teens getting vaccinated against the coronavirus increase steadily over the past month, which Gov. Jared Polis said earlier this week helped the state reach the milestone of nearly 71% of eligible residents having received at least one shot.

The rise in vaccinations is a positive development as the delta variant is driving an increase in COVID-19 infections statewide, just as school is about to start. Cases are also increasing among children and teens across the U.S. amid the new surge of illnesses.

“It’s trickling back up and what we are seeing — we’re seeing sicker kids,” said Jerrod Milton, chief clinical officer with Children’s Hospital Colorado.

Physicians are also seeing more cases of other respiratory viruses, including respiratory syncytial virus, or RSV. In some cases children are testing positive for more than one virus, including COVID-19, he said.

About 15% of the patients arriving in Children’s Hospital Colorado’s emergency department have respiratory symptoms.

COVID-19 infections among children began increasing in the U.S. in July. There were about 72,000 COVID-19 cases among American children in the week ending July 29, which was up from 39,000 cases reported the previous week, according to a report by The American Academy of Pediatrics.

Immunizations among Colorado teens also began rising each week over the last month. During the week of July 4, 5,092 doses were administered to those between the ages of 12 and 17. That rose to 7,934 doses the week of July 25, according to the Colorado Department of Public Health and Environment.