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Since the delta variant was first detected in the United States in March, it quickly overtook other variants of the virus. FREDERIC J. BROWN/AFP via Getty Images
  • The delta variant of the coronavirus is now the dominant variant in the United States, federal officials estimate.
  • The alpha variant, which once made up more than two-thirds of new COVID-19 cases in the United States, now accounts for less than one-third of cases.
  • The latest CDC estimate shows the delta variant accounted for 51.7 percent of COVID-19 cases during the 2 weeks ending July 3.

The highly transmissible delta variant of the coronavirus now makes up the majority of COVID-19 cases in the United States, according to a recent estimate from the Centers for Disease Control and Prevention (CDC).

Since the delta variant was first detected in the United States in March, it quickly overtook other variants of the virus.

During the 2 weeks ending June 5, it accounted for 10.1 percent of COVID-19 cases, rising to 30.4 percent of cases by June 19.

The latest CDC estimate shows the delta variant accounted for 51.7 percent of cases during the 2 weeks ending July 3.

The alpha variant, which once made up more than two-thirds of new COVID-19 cases in the United States, now accounts for less than one-third of cases.

The rapid spread of the delta variant, also known as B.1.617.2, is not unexpected.

In the United Kingdom, which has a similar fully vaccinated rate as the United States, the delta variant quickly replaced the alpha variant. By mid-June, it accounted for around 90 percent of infections, a government study showed.

“Although we expected the delta variant to become the dominant strain in the United States, this rapid rise is troubling,” CDC Director Dr. Rochelle Walensky said in a White House COVID-19 briefing July 8.

“We know the delta variant has increased transmissibility, and it’s currently surging in pockets of the country with low vaccination rates,” she said.

In parts of the Midwest and upper Mountain States, the delta variant accounts for around 75 to 80 percent of cases, CDC data shows.

Even as the delta variant gains ground, the overall COVID-19 cases, hospitalizations, and deaths in the United States are at levels far lower than the peaks seen earlier in the pandemic.

However, “we are starting to see some new and concerning trends,” Walensky said. “Simply put, in areas of low [COVID-19] vaccination coverage, cases and hospitalizations are up.”

These increases are being seen in many states, including Nevada, Iowa, Arkansas, Alaska, and Mississippi, according to data tracked by The New York Times.

Missouri is currently a leading COVID-19 hot spot. A surge of COVID-19 cases in the southern part of the state has forced some hospitals to transfer patients to other facilities, according to local reports.

In low vaccination areas, clusters of COVID-19 outbreaks related to the delta variant have also occurred at summer camps and recreational facilities.

Dr. Ashley Lipps, an infectious disease physician at The Ohio State University Wexner Medical Center, emphasized that vaccination is the best protection against the delta variant.

“The COVID-19 vaccines appear to retain high levels of efficacy against the delta variant,” she said, “so the risk is a lot lower for those who are fully vaccinated compared with those who are not.”

While some research has reportedly suggested that the delta variant may be more likely to cause breakthrough infections in fully vaccinated people, the vaccines still protect against severe illness, hospitalization, and death.

However, newer research shows that for two-dose vaccines such as the Pfizer-BioNTech vaccine and Moderna vaccine, getting the second dose is crucial for full protection.

Only 47.8 percent of the total U.S. population is fully vaccinated, according to the CDC. In many states in the South and some in the West, the rates are even lower.

This has created two nations: one emerging from the pandemic, and the other still at risk of severe COVID-19.

While older adults and people with existing health conditions such as diabetes, heart disease, and obesity are at higher risk of severe illness from COVID-19, people under age 40 can still end up in the hospital.

The rapid, but uneven, rollout of the COVID-19 vaccines in the United States has made one thing clear: Severe COVID-19 is now largely a disease in unvaccinated people.

“Preliminary data from several states over the last few months suggest that 99.5 percent of deaths from COVID-19 in the United States were in unvaccinated people,” said Walensky at the White House briefing. “Those deaths were preventable with a single, safe shot.”

Brandon Brown, PhD, an associate professor in the School of Medicine at the University of California, Riverside, said the ability of the delta variant to spread more easily has implications for everyone, regardless of their vaccination status.

“Infections and hospitalizations may continue to rise,” he said, “and it is possible for fully vaccinated people to transmit the virus to others who are unvaccinated.”

Fully vaccinated people are much less likely to contract an infection, which greatly reduces their ability to transmit the virus.

But scientists are still trying to determine exactly how often fully vaccinated people who contract an infection transmit the virus to others.

In addition, large numbers of COVID-19 cases in an area can disrupt healthcare systems, which can lead to delayed screenings and medical treatments for all people in that area.

Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, reiterated at the July 8 White House briefing that even with the rapid spread of the delta variant, the CDC’s mask recommendation remains unchanged.

“If you are vaccinated, you have a very high degree of protection,” he said, “and therefore you do not need to wear a mask.”

But some health officials say masks add an additional level of protection against this highly transmissible variant.

“With so many unknowns, it makes sense to wear face coverings when indoors in public spaces or outside in crowded areas,” said Brown, something that he continues to do.

However, “everyone has their own personal preferences for increased physical distancing and social interaction,” he said, “which may extend beyond local regulations.”

The COVID-19 vaccines have not yet been approved in the United States for children under 12 years old.

On July 9, the CDC updated its guidance for K-12 schools, emphasizing getting as many older children vaccinated before the fall as possible.

For younger children, mask wearing, physical distancing, increased ventilation, and other measures will be needed to reduce the risk of coronavirus transmission in schools.

Lipps said it’s important to remember that while the COVID-19 vaccines are highly effective, like all vaccines, they don’t offer complete protection.

“It is important to consider additional precautions in certain circumstances that may be higher risk. For instance, when gathering with large groups of people indoors, particularly if you have underlying health conditions,” she said.

These types of situations are riskier in parts of the country with high case numbers and low vaccination rates.

For people who are unvaccinated or partially vaccinated, masking, physical distancing, and other measures remain key ways to protect themselves from the coronavirus, including the delta variant.

But vaccination can add even greater protection.

“People who are not vaccinated are at greatest risk of infection and illness,” Lipps said. “If you have not been vaccinated yet, there is still time to do so.”