CNN  — 

More adolescents have received Covid-19 vaccinations in the past two weeks – representing about a fifth of all new vaccinations in the United States, data suggest.

Public health experts call the increase in vaccinations good, but the cause may be grim.

The rise in Covid-19 vaccinations among adolescents parallels an increase in Covid-19 cases and hospitalizations, mostly fueled by the Delta variant – leading some experts to speculate that parents who were once hesitant to get their children vaccinated are now reconsidering their decision after seeing more young people falling ill.

Over the course of the summer, the Covid-19 case rate among adolescents grew nearly five times. In June, there were 3.4 cases per 100,000 adolescents ages 12 to 15, which nearly tripled to about 9 cases per 100,000 adolescents in July and up to 14.6 cases per 100,000 adolescents in August, according to data from the US Centers for Disease Control and Prevention, last updated on Monday.

At the same time, that age group represented 30.9% of people receiving at least one dose of Covid-19 vaccine in June, rising to 38.9% in July and 44.9% in August. Children ages 12 to 15 account for just 5% of the population overall.

The share of fully vaccinated people who are ages 12 to 15 also saw an increase, climbing from 21.5% in June to 28.7% in July to 32.2% in August.