The United States is still trying to get people vaccinated, Freeman said, so it will be important to compare vaccination rates against other Covid-19 data, such as cases, hospitalizations and deaths.

Anthony Fauci said Tuesday in a White House Covid-19 briefing, adding it's "such a powerful argument" to get vaccinated.

Fauci, the National Institute of Allergy and Infectious Diseases director, warned that the Delta variant "may be associated with an increased disease severity, such as hospitalization risk, compared to" the Alpha variant, B.1.1.7, which was first identified in the United Kingdom and was dominant there before Delta is believed to have recently taken over.

The Delta variant is susceptible to available two-dose vaccines from Pfizer and AstraZeneca, he said, but protection from these vaccines requires following a two-dose schedule.

"There is reduced vaccine effectiveness in the one dose," Fauci said.

Its effects seem to be a little reduced against the Beta variant first seen in South Africa and the Gamma variant that spread rapidly across Brazil, but the immune response appeared fully effective against the Alpha variant first seen in Britain and a variant identified in California.

Fauci added that variant-specific boosters may be on the horizon.

Even those who've already had coronavirus should get vaccinated because research shows immunity achieved through vaccination is better than immunity through previous infection, Fauci said.

Meanwhile, the United States has averaged almost 14,380 new Covid-19 cases a day across the past week -- the second-lowest average since March 28, 2020, according to Johns Hopkins University data.

Rochelle Walensky said Tuesday.

But health experts warn that a recent lag in vaccination rates leaves millions unprotected against Covid-19 variants that have made their way to the US from other parts of the world.

Over the past week, the US averaged more than 1.07 million Covid-19 vaccine shots administered per day -- well below the peak seven-day average of 3.38 million shots per day reached on April 13, according to CDC data.

Vaccine maker says it is working to extend shelf life

Johnson & Johnson -- maker of the only single-dose Covid-19 vaccine authorized in the US -- says it is working to extend the shelf-life of its product amid reports that doses in the country may expire before they're used.

Of the 21.4 million Johnson & Johnson doses delivered in the US, about 11 million have been administered, according to CDC data.