NEW YORK (NYTIMES) - Paediatric hospitalisations for Covid-19 in the United States have soared over the summer as the highly contagious Delta variant spreads across the country, according to two new studies from the Centres for Disease Control and Prevention (CDC).

From late June to the middle of last month, hospitalisation rates in the US for children and teenagers increased nearly fivefold, although they remain slightly below January's peak, one new study found.

But vaccination has made a difference. During this summer's wave, the hospitalisation rate was 10 times as high in unvaccinated adolescents as in those who were vaccinated, researchers found.

Paediatric hospital admissions were nearly four times as high in states with the lowest vaccination rates as in those with the highest rates, according to a second study.

The studies, released on Friday (Sept 3), do not provide clear answers about whether Delta causes more severe disease in children than earlier versions of the virus. The rise in paediatric hospitalisations could also be because of the variant's high infectiousness.

Indeed, one study concluded that the proportion of hospitalised children with severe disease had not changed in late June and July, when the Delta variant became dominant in the US.

The rates reported in the CDC studies are based on data from two national surveillance systems, including hospitals in 49 states and Washington, DC.

In one CDC study, researchers found that since July, the rate of new coronavirus cases increased for children 17 or younger, as did Covid-19-related emergency room visits and hospital admissions.

In a second study, researchers analysed data from the Covid-NET surveillance network, which includes information on hospitalisations in 99 counties across 14 states.

Over the course of the pandemic - or from March 1 last year to Aug 14 this year - there were 49.7 Covid-19-related hospitalisations per 100,000 children and adolescents, the researchers found.

But the weekly rates have been climbing since July.

During the week ending Aug 14, there were 1.4 Covid-19-related hospitalisations for every 100,000 children, compared with 0.3 in late June and early July. (That remains slightly below the peak weekly rate of 1.5 hospitalisations per 100,000 children, in early January this year, in the post-holiday wave of cases.)