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More than a quarter of US coronavirus cases are being caused by the super-contagious Delta variant, but highly vaccinated New England is seeing a lower proportion than the national average, the US Centers for Disease Control and Prevention says in a new estimate.

Adjusting an earlier estimate upward, the CDC said Tuesday that 26.1 percent of cases nationally were caused by the Delta variant as of June 19.

In addition to a national estimate, the CDC releases numbers for various regions of the country, as defined by the US Department of Health and Human Services. In Region 1, which comprises the New England states (Maine, New Hampshire, Vermont, Massachusetts, Connecticut, and Rhode Island), 18.9 percent of cases were Delta variant cases. The region had the fourth lowest Delta proportion among the 10 HHS regions.

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Experts say the Delta, or B.1.617.2, variant, is much more transmissible than the alpha, or B.1.1.7, variant, which reached its peak this spring. There is also some indication that the Delta variant makes people sicker, experts say.

Concern about the rapid spread of the Delta variant has led to discussion of reinstating mask requirements. The World Health Organization last week reiterated its long-standing recommendation that everyone — including the inoculated — wear masks. Health officials in Los Angeles County on Monday recommended that “everyone, regardless of vaccination status, wear masks indoors in public places as a precautionary measure.”

According to the CDC estimates, the areas where the Delta variant has made the most inroads are: Region 7 (Iowa, Kansas, Missouri, and Nebraska) where the variant was at 57.5 percent, and Region 8 (Colorado, Montana, North Dakota, South Dakota, Utah, and Wyoming), where the variant was at 52.0 percent.

Current vaccines are effective against the Delta variant. New England has been a national leader in getting people shots.

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But experts are warning that the arrival of the Delta variant in places that have vaccinated fewer people is a recipe for trouble.

“We have always said that this virus is an opportunist, and in areas where we still have rates of low vaccination, that is where the virus is likely to take hold,” Dr. Rochelle Walensky, director of the CDC, said on ABC-TV’s “Good Morning America.”

Dr. Anthony Fauci, the federal government’s top infectious disease expert, warned Tuesday on CNN of “two Americas” emerging because of differences in vaccination levels in different places.

“When you have such a low level of vaccination superimposed upon a variant that has a high degree of efficiency of spread, what you are going to see among undervaccinated regions -- be that states, cities or counties -- you’re going to see these individual types of blips,” he said. “It’s almost like it’s going to be two Americas.”

He emphasized that spikes in coronavirus cases are “entirely avoidable, entirely preventable” if people get their shots. “If you are vaccinated, you diminish dramatically your risk of getting infected and even more dramatically your risk of getting seriously ill,” he said. “If you are not vaccinated, you are at considerable risk.”

Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine said in a column in the Daily Beast that experts had warned of spikes in unvaccinated areas of the South and “right on cue” COVID-19 was “resurfacing in force in the Ozark states of Arkansas, Missouri, and Oklahoma.”

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Dr. Ashish Jha, dean of the Brown University School of Public Health, posted a tweet Monday that included a chart comparing the case rates in Missouri, Nevada, Wyoming, and Utah with the much lower rates in Vermont and Massachusetts, which, in that order, lead the nation in percentage of people fully vaccinated.

“Vaccination rates are increasingly the major factor why infection rates vary so much across US,” he wrote.

Material from Globe wire services was used in this report.


Martin Finucane can be reached at martin.finucane@globe.com.