Previous data from the Centers for Disease Control and Prevention put that share of cases at about 10% as of June 5 and 2.7% on May 22.

The Financial Times estimate, based on available sequencing data, would mean the share of Delta-variant infections in the US tripled in just 11 days.

The CDC has not yet released data on the rate of Delta-variant cases after June 5 in the US.

Dr. Rochelle Walensky, the director of the CDC, warned last week that the Delta variant could soon become dominant in the US.

And on Sunday, the former Food and Drug Administration chief Scott Gottlieb warned that the Delta variant could lead to a surge of cases in the fall even if 75% of eligible Americans were vaccinated by then.

The variant, which was first identified in India and is also called B.1.617.2, appears to be 60% more transmissible than the variant that is now still dominant in the US, the Alpha variant.

Data from the UK, where the Delta variant is thought to make up about 90% of current coronavirus cases, also suggests that the risk of hospitalization is higher for unvaccinated people with this variant.

The Delta variant also appears more likely to evade the protection given by partial vaccinations.

Research suggests a single shot of the Pfizer-BioNTech or Oxford University-AstraZeneca coronavirus vaccines may give just 33% protection against symptomatic COVID-19 cases with the Delta variant, compared with at least 88% for other variants.

Two doses of the Pfizer-BioNTech or Oxford-AstraZeneca shot have been found to be protective against hospitalization from the variant, however.

It is not clear what protection other vaccines give against this variant.

On Friday, President Joe Biden warned that the Delta variant was "particularly dangerous for young people," who are less likely to be vaccinated than older adults and are more socially active.

He also urged all Americans to get second doses of vaccines that require them.

As of Monday, 45% of Americans were fully vaccinated and 53% had received one dose, according to data from the CDC. The pace of vaccination, however, has fallen in the past two months.