Health experts warn that the particularly contagious delta variant may soon become the dominant strain of the coronavirus in the United States and could create "two Americas"—comparatively safe areas with high vaccination rates, and other areas with low vaccination rates and the potential for Covid-19 surges.

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Delta variant spreads throughout the United States

On Monday, CDC Director Rochelle Walensky told governors that the delta variant, which was first identified in India, accounted for about 10% of Covid-19 cases in the United States in June, up from 2.7% in May. Research indicates the delta variant is between 40% and 80% more transmissible than previous variants, and that people infected with the delta variant were nearly twice as likely as those infected with the alpha variant—first identified in Britain—to require hospitalization.

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According to Walensky, the delta variant "probably" will become the dominant strain of the virus in the coming months.

"We saw [the alpha variant] quickly become the dominant strain in a period of one or two months," she said. "I anticipate that is going to be what happens with the delta strain here."

Separately, Andrew Pekosz, a virologist at the Johns Hopkins Bloomberg School of Public Health, said the growing prevalence of the delta variant in the United States "very much mirrors what happened in the United Kingdom about a month ago." He added, "And so I fully expect that sometime in the next three or four weeks, the delta variant will be the dominant SARS-CoV-2 lineage in the U.S."

However, research has shown that vaccines are effective against the delta variant when fully administered. For instance, one study from Public Health England concluded that two doses of the Pfizer-BioNTech Covid-19 vaccine was 88% effective at preventing symptomatic Covid-19 infection by the delta variant—an efficacy level that dropped to 33% among people who had received just one part of the two-dose regimen.

And while Surgeon General Vivek Murthy said there's insufficient data to determine how effective Johnson & Johnson's single-dose Covid-19 vaccine is against the delta variant, data shows it does help reduce hospitalization and fatality risk with other variants.

Why experts are worried variations in vaccination rates

Experts have said that one potential defense against the delta variant would be to quickly boost overall vaccination rates. However, even as the delta variant gains a foothold in America, the nation's vaccination rates have slowed.

Currently, according to CDC, 53.3% of the U.S. population has received at least one dose of a Covid-19 vaccine while 45.1% have been fully vaccinated. However, the number of Americans getting their first dose of a vaccine has dropped from around 500,000 a day in early June to about 200,000 a day.

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In addition, polling indicates that of those who have yet to get vaccinated, a significant portion don't plan to. A poll released last month by the Kaiser Family Foundation found that 20% of Americans said they definitely will not get vaccinated or will only get vaccinated if they have to. And a poll from the AP-NORC Center for Public Affairs Research found that a full 46% of unvaccinated Americans said they "will definitely not get a vaccine."

Vaccination rates vary significantly across the United States. For example, in 16 states and Washington, D.C., around 50% of their population is fully vaccinated. But in states such as Alabama, Arkansas, Louisiana, Mississippi, Tennessee, and Wyoming, fewer than 35% of people are fully vaccinated.

"When we look across the United States, we see wide variance in terms of vaccination rates," former FDA Commissioner Scott Gottlieb said.

Citing modeling projections for the delta variant's spread, he added, "Connecticut, for example where I am, shows no upsurge of infection, but Mississippi, Alabama, Arkansas, Missouri, show very substantial upsurges of infection. That's based entirely on how much population-wide immunity you have based on vaccination."

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This variance has led experts to worry that "two Americas" will form—one where most people are vaccinated and largely protected from the delta variant, and another where few are vaccinated and vulnerable to Covid-19 surges.

For instance, Steve Edwards, CEO of the Missouri-based CoxHealth, said the low vaccination rates in his state, coupled with the spread of the delta variant, has accounted for much of the surge in Covid-19 cases his hospital has seen recently.

"I think the delta variant is what's fueling this," Edwards said. "Much of the South, Midwest, much of the places that have low vaccination rates—if confronted with the delta variant, will see a similar kind of surge of patients as we're beginning to see right now."

Looking ahead, Bob Wachter, chair of the department of medicine at the University of California-San Francisco, said, "I think there's a decent chance that come winter or fall there will be significant surges and they will almost exclusively strike people who are unvaccinated and strike in regions that have low rates of vaccine uptake."

He noted in particular the speed with which the delta variant spreads, and the comparatively low protection afforded by just one dose of a two-dose vaccine. "Some of those people [who decide to delay getting vaccinated] are going to get a nasty surprise," he said (Leonard, Politico, 6/18; Stolberg/Weiland, New York Times, 6/20; Aldhous, Buzzfeed News, 6/17; Holcombe, CNN, 6/21; Owens, Axios, 6/21; Reuter, Business Insider, 6/18; Reed, "Vitals," Axios, 6/18; Whalen, Washington Post, 6/20; Zhang, The Atlantic, 6/18).