Dr Birx, who served as Barack Obama’s global Aids coordinator in the previous administration, urged Americans to wear face coverings to help stop transmission of the disease.
“What we are seeing today is different from March and April,” Dr Birx told CNN on Sunday. “It’s into the rural as equal urban areas”.
“To everybody who lives in a rural area, you are not immune or protected from this virus. This epidemic right now is different and it’s more widespread.”
Dr Birx’s comments came as the US edged towards 5 million infections – the highest reported by any country in the world by a considerable distance.
As of Monday morning, US officials had recorded 154,860 deaths while just over 2.3 million have recovered after contracting the respiratory disease, according to Johns Hopkins figures.
Brazil has the world’s second-highest number of infections at 2.7 million. Health authorities have recorded 94,104 deaths in the South American country.
The pandemic has been surging in the US in the summer months. Several states in the south and west of the US – including Florida, Texas and California – have experienced a spike in new infections.
California, the country’s most populous state, overtook New York in July as the area worst hit by the pandemic. The state has reported 493,140 infections since the pandemic first took hold.
Meanwhile, Texas logged 9,539 new cases on 1 August alone, with clusters of infections recorded in rural areas as well as in major cities such as Dallas, Houston and El Paso.
Dr Birx warned that tourists travelling to holiday hot spots in the US could be accelerating the spread of the disease.
“As I travelled around the country, I saw all of America moving,” she said. “If you have chosen to go on vacation into a hot spot, you really need to come back and protect those with co-morbidities and assume you’re infected.”
President Trump, who has been languishing well behind Joe Biden in the polls, has sought to take advantage of the crisis by calling for November’s election to be delayed, citing unfounded claims over voting fraud.
But it is not within the president’s power to change the date of the US election. Any delay to the vote would require an act of Congress, and many Republicans have spoken out against Trump over the move.