A health care worker administers a dose of COVID-19 vaccine to a recipient at a new vaccination site in the California Polytechnic State University in Pomona, Los Angeles County, California, the United States, Feb. 5, 2021. (Xinhua)

Nearly 1,200 infection cases of coronavirus variants have been reported in the 50 US states, according to the latest data of the U.S. Centers for Disease Control and Prevention (CDC).

The vast majority of these cases, 1,173, were caused by the variant known as B.1.1.7, which was originally detected in Britain.

There were 17 cases of a new strain initially discovered in South Africa, called B.1.351, and three cases of the P.1 strain first discovered in Brazil.

The number of B.1.1.7 variant cases last week was a 61 percent increase over the previous week, according to the CDC.

These are the three dominant coronavirus variants currently spreading in the United States, according to the CDC.

Numerous SARS-CoV-2 variants have emerged over the last several months, attracting the attention of health and science experts worldwide.

The CDC and partners are increasing the numbers of specimens sequenced in laboratories around the country.

COVID-19 cases, hospitalizations and deaths continue to trend downward in the United States. However, the growing spread of variants increased complexity for containing the pandemic and drew great concern from experts and officials.

The B.1.1.7 and B.1.351 concern scientists because of emerging data suggesting their increased transmissibility, according to a study published in the Journal of the American Medical Association (JAMA).

The B.1.351 variant may be partially or fully resistant to certain SARS-CoV-2 monoclonal antibodies currently authorized for use as therapeutics in the United States, according to the study.

A new SARS-CoV-2 variant, CAL.20C, has been detected in southern California amid a surge in local infections and is spreading through and beyond the United States, according to another research letter published in JAMA.

By Jan. 22, the variant had grown to account for 35 percent of all coronavirus strains in California and 44 percent of all samples in the southern part of the state and had been detected in 26 states as well as other countries, according to the study by researchers at Cedars-Sinai Medical Center in Los Angeles.

The United States has recorded over 27.7 million cases with more than 486,500 related deaths as of Tuesday afternoon, according to the real-time count kept by Johns Hopkins University.