The highly transmissible Delta variant of the coronavirus now accounts for 83% of all sequenced cases in the US, a top federal health official said on Tuesday.

“This is a dramatic increase, up from 50% [in] the week of 4 July,” Rochelle Walensky, director of the Centers of Disease Control and Prevention (CDC), said in Senate testimony.

Walensky also said Covid fatalities had risen by nearly 48% over the past week to an average of 239 a day.

“Each death is tragic and even more heartbreaking when we know that the majority of these deaths could be prevented with a simple, safe available vaccine,” she said.

A cluster of midwestern and southern states have emerged as the new hotspots for Covid-19.

With less than half of the US population fully vaccinated, infection rates in Oklahoma, Missouri, Arkansas, Alabama, Louisiana and Mississippi are among the highest – with vaccination rates among the lowest.

Alabama ranks lowest in vaccination rates, with only 42.4% of its adult population fully vaccinated. Vermont has the highest fully vaccinated adult population, at 77.3%.

In the last two weeks, the rate of infection across the US has increased by 198%. States that had some of the highest increases in that period include Oklahoma, at 387%, and Louisiana and Mississippi at 376% and 308%.

The national vaccination campaign has slowed down significantly. The US is administering 521,000 doses daily, a 85% decrease from a peak in April when 3.38m doses were administered every day.

“This is becoming a pandemic of the unvaccinated,” Walensky said last week. “We are seeing outbreaks of cases in parts of the country that have low vaccination coverage because unvaccinated people are at risk. Communities that are fully vaccinated are generally faring well.”

With the Biden administration having fallen short of its goal of administering at least one shot to 70% of adults by 4 July, the rapid transmission of the Delta variant poses a serious challenge to attempts to control the pandemic.

In addition to prioritizing speed, the administration is focusing on distributing shots equitably to communities hardest hit.

According to Jeff Zients, coordinator of the White House coronavirus team, the administration is working on outreach in communities with high vaccine hesitancy, as well as improving accessibility at doctors’ offices and workplaces.

According to data from the New York Times, the most socially vulnerable counties have a lower average vaccination rate than the least vulnerable. The majority of the most disadvantaged counties are in the south.

“The people who have not gotten vaccinated so far are a very diverse group,” said Dr David Dowdy, an infectious disease expert at Johns Hopkins University.

“It’s easy to portray it as … a political stripe, but vaccination rates are higher, for example, in those who are older, and that’s irrespective of political leaning.”

The most vaccinated and least vulnerable counties are in the midwest and north-east.

There also remains a divide on Capitol Hill surrounding the role that the National Institutes of Health played in funding controversial research in Wuhan, China.

During the Senate hearing on Tuesday, Senator Rand Paul accused Dr Anthony Fauci, the nation’s top infectious disease expert, of previously lying to Congress about his knowledge of how the Wuhan lab used the NIH funds. He cited an academic paper that purportedly shows the lab was conducting research to create “potential pandemic pathogens that exist only in the lab, not in nature”.

Fauci vehemently denied the claims, saying: “Senator Paul, you do not know what you are talking about,” before adding, “I totally resent the lie you are now propagating.” Fauci maintains that it is molecularly impossible that NIH-funded research was responsible for SARS-CoV-2.

Overall, the US remains in significantly better shape than at all previous points of the pandemic, despite the national rise in infections. Death rates have remained near their lowest levels since last spring and hospitalization rates are much lower compared with last winter’s peak.