A highly contagious variant of the novel coronavirus that was initially identified in India now accounts for about 58% of all COVID-19 cases in the United States, according to the Centers for Disease Control and Prevention.
Data updated by the CDC on Tuesday evening shows the so-called delta variant, also known as B.1.617.2, was estimated to be responsible for 57.6% of newly confirmed cases nationwide from June 20 through July 3. The proportion was estimated at just 31.1% for the two weeks prior.
In late May, the delta variant was estimated to account for approximately 3% of new cases in the U.S, according to CDC data.
After being initially identified in India in October, the delta variant has since been reported in at least 104 countries around the globe and is expected to soon be the dominant coronavirus variant circulating worldwide, according to the World Health Organization. The variant was first detected in the U.S. in March and is now present in all 50 states.
"The delta variant is ripping around the world at a scorching pace, driving a new spike in cases and death," WHO Director-General Tedros Adhanom Gebreyesus said during a COVID-19 press briefing on Monday.
Last week marked the fourth consecutive week that the number of newly confirmed COVID-19 cases has increased globally. Deaths are also on the rise again after 10 weeks of decline, according to Tedros, who noted that the WHO has received reports from all regions of the world about hospitals reaching capacity.
"In places with high vaccination coverage, Delta is spreading quickly; especially infecting unprotected and vulnerable people and steadily putting pressure back on health systems," he said. "In countries with low vaccine coverage, the situation is particularly bad."
The WHO declared delta a "variant of concern" in May, and the CDC upgraded its classification of the strain in June from "variant of interest" to a "variant of concern." Both the WHO and the CDC say that variants of concern have shown to be both more infectious and more virulent than other strains.
The delta variant has shown to be particularly dangerous to those who are unvaccinated or partially vaccinated against COVID-19, and preliminary data indicates it may increase the risk of hospitalization.
However, current evidence suggests that the full dosage of a COVID-19 vaccine is highly effective in preventing hospitalizations and serious illness. Instances where a fully vaccinated individual becomes infected with COVID-19, known as breakthrough cases, are exceedingly rare. And while early laboratory studies indicated the vaccines seem to produce far fewer virus-fighting antibodies against some of the newer variants, real-world experience tells a different story as researchers across the globe learn that the vaccines still mostly work -- even when those antibodies fail to show up in great numbers -- thanks to other crucial parts of the body's immune system.
Still, there is very little known about the mutating virus and it remains unclear exactly how long immunity from the vaccines will last and whether booster shots will be needed to maintain protection.
As delta and other highly transmissible variants spur "catastrophic waves" of COVID-19 infections, the WHO director-general is urging vaccine manufacturers to prioritize supplying doses to poorer countries with low vaccination rates rather than giving booster shots to wealthier nations with relatively high coverage.
"The global gap in vaccine supply is hugely uneven and inequitable," Tedros said Monday. "Some countries and regions are actually ordering millions of booster doses, before other countries have had supplies to vaccinate their health workers and most vulnerable."
Since the start of the pandemic, the U.S. has reported more than 33.9 million confirmed cases of COVID-19 and over 607,000 deaths from the disease, according to data collected by Johns Hopkins University. The U.S. has recorded the highest death toll, while India has the highest total case count.
More than 184 million people in the U.S. have received at least one dose of a COVID-19 vaccine, including over 159 million -- 48.1% of the population -- who are fully vaccinated, according to CDC data.
ABC News' Brian Hartman, Arielle Mitropoulos and Dr. Onyema Okolo contributed to this report.