A substrain of omicron is responsible for a growing number of infections.

BA.2.12.1, a subvariant of the COVID-19 Omicron strain, is now the cause of approximately 1 in 5 infections in the United States. Most U.S. infections are still linked to BA.2, the parent strain, which is responsible for 75% of domestic outbreaks.

"SARS-CoV-2, the virus that causes COVID-19, is constantly changing and accumulating mutations in its genetic code over time," the CDC warned in a recent report. "New variants of SARS-CoV-2 are expected to continue to emerge. Some variants will emerge and disappear, while others will emerge and continue to spread and may replace previous variants."

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The U.S. has seen a major deescalation of COVID-19 regulation as the rate of vaccination has increased and the amount of deaths caused by the novel disease has steadily fallen.

London workers walk over the London Bridge

FILE - Workers walk over London Bridge towards the City of London financial district during the morning rush hour, in London, Monday, Jan. 24, 2022. For many in the U.K., the pandemic may as well be over. Mask requirements have been dropped everywhere and free mass testing is a thing of the past. The sense of freedom is widespread even as infections soared to record levels in Britain in March, driven by the milder but more transmissible Omicron BA.2 variant that’s rapidly spreading around Europe, the U.S. and elsewhere.  ((AP Photo/Matt Dunham, File))

The Biden administration has been leaning on Congress to approve additional pandemic relief funding, saying money for fourth COVID-19 vaccine shots is gone

Dr. Ashish Jha, the new White House coronavirus response coordinator, told "CBS Mornings" on Monday that if the coronavirus situation in America becomes "substantially worse," U.S. health officials could return to looking at things like masking.

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The U.K. Health Security Agency said last week that work was underway to "precisely define the phylogeny" of the new variants BA.4 and BA.5.

A worker in protective gear walks past residents lined up for the first round of mass COVID testing in the Jingan district of western Shanghai, China, Friday, April 1, 2022. As residents of western Shanghai start a four day lockdown for mass testing, some in eastern Shanghai about to end their lock down are being told they will be confined to their homes for at least 10 more days. It was the latest wrinkle in the lockdown of China's largest city as it struggles to eliminate an omicron-driven coronavirus outbreak under China's zero-COVID policy. (AP Photo/Chen Si)

A worker in protective gear walks past residents lined up for the first round of mass COVID testing in the Jingan district of western Shanghai, China, Friday, April 1, 2022. As residents of western Shanghai start a four day lockdown for mass testing, some in eastern Shanghai about to end their lock down are being told they will be confined to their homes for at least 10 more days. It was the latest wrinkle in the lockdown of China's largest city as it struggles to eliminate an omicron-driven coronavirus outbreak under China's zero-COVID policy. (AP Photo/Chen Si) (AP Photo/Chen Si)

In an April 8 update, the organization wrote that BA.4 had been found in South Africa, Denmark, Botswana, England and Scotland. 

All BA.5 cases were in South Africa, but Botswana's ministry of health reportedly said it had identified four cases of both BA.4 and BA.5 among individuals aged 30 to 50 years old who were fully vaccinated and experiencing mild symptoms. 

Fox News's Julia Musto contributed to this report.