Two months after the US Centers for Disease Control and Prevention (CDC) said vaccinated individuals didn't need to wear masks indoors, health experts are calling for people to put them back on as the Delta variant now accounts for 83 percent of coronavirus cases in the nation, the head of the Centers for Disease Control and Prevention said Tuesday.

The percentage of cases from the Delta variant is "even higher" in some parts of the country, "particularly in areas of low vaccination rates", CDC Director Dr Rochelle Walensky told a Senate hearing. COVID-19 fatalities have risen by nearly 48 percent since last week to an average of about 239 per day, she said.

"The message from CDC remains clear: The best way to prevent the spread of COVID-19 variants is to prevent the spread of disease, and vaccination is the most powerful tool we have," Walensky said.

The weekly rolling average for coronavirus cases in the US has nearly tripled in the last month, from fewer than 12,000 on June 19 to almost 35,000 on Monday, according to The Washington Post.

A White House official and a spokesperson for House Speaker Nancy Pelosi, both fully vaccinated, tested positive this week for the coronavirus after attending an event together, White House press secretary Jen Psaki confirmed during a news briefing Tuesday.

Pelosi's office said that an aide to the speaker tested positive after meeting with the Democrats who went to Washington last week amid a battle in the state Legislature over voting rights. Six of the Texas lawmakers have tested positive for the coronavirus, including a former nurse who announced her test result Tuesday.

The individual in Pelosi's office has had no contact with the speaker since exposure, Pelosi spokesperson Drew Hammill told ABC News.

"Yesterday, a fully vaccinated White House official tested positive for COVID-19 off-campus," an anonymous White House official said in a statement. The staffer is self-isolating, according to "rigorous COVID-19 protocols", the official said.

"The White House Medical Unit has conducted contact-tracing interviews and determined no close contacts among White House principals and staff. The individual has mild symptoms," the official said.

Meanwhile, the Delta variant is causing rising numbers of coronavirus cases in all states, and Arkansas, Missouri and New York are weighing mask mandates.

In California, where last week Los Angeles County reinstated an indoor mask mandate, 17 California counties are asking even fully vaccinated individuals to wear face coverings as a precaution while inside places like grocery stores, movie theaters and retail outlets.

The new round of mask advisories means that roughly 56 percent of Californians live in a county that either recommends or requires indoor masking for all individuals, including those vaccinated for COVID-19, according to the Los Angeles Times.

"Universal masking indoors is a way of taking care of each other while we get more people vaccinated," said Barbara Ferrer, director of the Los Angeles County Department of Public Health. "It really doesn't disrupt any business practices. It allows us to remain fully open — while we acknowledge that the delta variant [is] spreading like wildfire here."

The American Academy of Pediatrics (AAP) issued recommendations Monday for the 2021-22 school year that include everyone above age 2 wearing masks, regardless of vaccination status.

The AAP is recommending a "layered approach" to make schools safe for students, teachers and staff alike as the Delta variant continues to drive cases of the novel coronavirus, which causes COVID-19, higher in all 50 states. The guidance contradicts the CDC, which relaxed its masking guidelines earlier this month to say that vaccinated teachers and students don't need to wear face coverings.

The AAP represents 67,000 primary care pediatricians, pediatric medical subspecialists and pediatric surgical specialists from across the US.

Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, suggested on Tuesday that parents follow new COVID-19 guidance for mask-wearing issued by the AAP. He said the CDC is "carefully looking" at its school guidance.

The academy wants to "go the extra mile" to make sure children are protected at school because of the rise in cases blamed on the Delta variant of the coronavirus, Fauci told CBS This Morning.

The nation's former surgeon general warned the nation should brace for a broader return to mask-wearing.

"Instead of vax it OR mask it, the emerging data suggest CDC should be advising to vax it and mask it in areas with (rising) cases and positivity until we see numbers going back down again," former US Surgeon General Dr Jerome Adams said on Twitter.