The delta variant's "rapid rise is troubling," CDC director Dr. Rochelle Walensky said of the dangerous Covid strain in a White House press briefing Thursday.

The more transmissible delta variant is now the most dominant strain of the coronavirus in the United States, representing over 50% of cases across the country, according to data released by the Centers for Disease Control and Prevention Wednesday.

Pfizer and BioNTech announced Thursday that they are developing a booster shot to target the delta variant. The authorized Covid vaccines appear to work well at preventing severe disease, hospitalization and death from the delta variant. But pockets of the country remain unvaccinated and therefore at risk.

Delta's speed and high transmissibility makes it able to "pick off the more vulnerable more efficiently than previous variants," Dr. Mike Ryan, executive director of the WHO's health emergencies program, said during a news conference on June 21.

Getting fully vaccinated is the best way to protect yourself from the delta variant.

Here's what else you need to know about the strain:

Vaccinated people can get breakthrough infections

In theory, vaccinated people can still get a breakthrough infection. However, White House chief medical advisor Dr. Anthony Fauci said it's unlikely that a vaccinated person could transmit the virus because the amount of virus in their system would be so low.

"[W]e are looking at situations where you have vaccinated people who have breakthrough infections," the White House chief medical advisor told Chuck Todd during an interview with NBC's "Meet the Press" on July 4. "Namely, they're infected despite the fact that they've been vaccinated."

It's important to note, however, that vaccinated people who get infected have significantly less virus in their nasopharynx, Fauci said.

"When you look at the level of virus to be lower, that would mean you could make a reasonable assumption that those individuals would be less likely to transmit the infection to someone else," he said.

Delta is already causing Covid spikes in parts of the U.S. and could cause 'major outbreaks' this fall

To date, 47.6% of the U.S. population is fully vaccinated. But delta is surging in pockets of the country where vaccine rates are concerningly low such as the South and Midwest. In some parts of the Midwest and upper mountain states, delta accounts for 80% of Covid cases, Walensky said in the press briefing.

Virtually all new Covid deaths and hospitalizations are among unvaccinated people.

"[T]here will likely continue to be an increase in cases among unvaccinated Americans and in communities with low vaccination rates, particularly given the spread of the more transmissible Delta variant," White House coronavirus response coordinator Jeff Zients said at a press briefing on Thursday.

Lawrence Gostin, director of the World Health Organization's Collaborating Center on National and Global Health Law, told CNBC that there could be "major outbreaks" in the fall, just as many people are going back to offices and schools.

That said, a nationwide spike like the U.S. witnessed last fall and winter is unlikely given the proportion of people who are now vaccinated, Fauci said.

Delta could bring back masks and social distancing

Mitigation measures, such as wearing masks and occupancy limits, will likely return by fall in certain places where there is a high level of transmission and low vaccination rates, according to experts.

Ultimately, returning to in-person work comes with risks that individuals and businesses will have to navigate.

Fauci said that in the meantime, some people who are vaccinated and are "very risk averse" might opt to start wearing a mask now in places where there are surges, during the White House briefing.

"Even if you are vaccinated, for example, if you are an elderly person, or if you have a person with an underlying disease, you might want to go the extra mile of protection of wearing a mask if you were indoors, in an environment with a high degree of infection in the community, and a low level of vaccinations," he said.

The CDC and the White House have said that they will leave it to individual communities and states to make decisions about safety measures depending upon the level of spread and vaccination in a community.

Delta could affect kids' return to school

The delta variant "poses a particular threat to our young people," Zients said in the press briefing.

Covid vaccines are not yet authorized for children under 12 years old, which puts many school-aged children at higher risk of infection. In the United Kingdom, 90% of Covid cases in June were attributed to the delta variant, with the highest prevalence among children ages 5 to 12.  

"Pediatricians worry that if [the delta variant] spreads further, it could lead to schools not reopening, sports and camps being closed, and all of that having an adverse effect on the mental health of children," says Dr. Steven Abelowitz, a board-certified pediatrician and medical director and president of Coastal Kids pediatric medical group, told Parents.

Dr. Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia, told Yahoo! Finance that he thinks safety measures such as masking and social distancing might be necessary in schools come fall and winter. (The CDC recommends that unvaccinated children ages two and up wear masks in public settings.)

Correction: This article has been corrected to reflect that Dr. Anthony Fauci said it's unlikely that a vaccinated person could transmit the virus because the amount of virus in their system would be so low.

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