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 still transmit delta
It’s feasible that a fully vaccinated person can be an asymptomatic carrier of Covid, and potentially transmit the virus, including the more contagious delta variant, Dr. Anthony Fauci said during an interview with NBC’s “Meet the Press” July 4.

″[W]e are looking at situations where you have vaccinated people who have breakthrough infections,” the White House chief medical advisor told Chuck Todd. “Namely, they’re infected despite the fact that they’ve been vaccinated.”

It could be that there are many more breakthrough cases than are being accounted for, because they tend to be asymptomatic and CDC guidance says vaccinated people don’t have to get tested after exposure.

It’s important to note, however, that vaccinated people who get infected have significantly less viral load 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.