DENVER (BLOOMBERG) - Colorado public-health authorities have identified a second possible person with the British variant of Covid-19 in the state, one day after reporting the first known case of the faster-moving form of the pathogen in the US.

Both patients were members of a National Guard unit sent on Dec 23 to Good Samaritan Society Nursing Home in Simla, Colorado, as the facility struggled with staff shortages caused by a coronavirus outbreak.

Routine testing conducted the day after they arrived showed telltale signs of the recently emerged variant that has been spreading widely in Britain.

The first case, found in a man in his 20s, was confirmed on Tuesday.

The second is "highly suspicious" but still under evaluation, with results likely within a week, according to state scientific director Emily Travanty.

Genetic sequencing of 22 additional samples has not turned up any signs of the variant, known as B.1.1.7. Another 12 are awaiting more detailed tests.

"There is a lot we don't know about this variant," Colorado Governor Jared Polis said.

"But if it does transmit more quickly, more people will get it and more people will be hospitalised."

The US Centres for Disease Control and Prevention is looking into whether the mutations will make existing treatments, including convalescent plasma and monoclonal antibodies, less effective, Henry Walke, the agency's Covid-19 incident manager, said on a call with reporters on Wednesday.

He said there was no reason measures like wearing a mask and social distancing would be less effective in preventing transmission of the new strain.

"We firmly believe our mitigation measures will work," Walke said.

Colorado recorded 451 new cases of Covid-19 on Wednesday, Polis said at the news briefing.

Just over 1,000 people are hospitalised with the virus in the state, and 3,901 Coloradans have died because of Covid-19.

Health officials believe the vaccine will remain effective against the new strain. Colorado will also allow all residents over the age of 70 to be vaccinated, subject to availability. So far, 84,000 doses have been administered in the state.

All 26 residents of the nursing home in Simla tested positive for Covid-19, as did 20 of the facility's 34 regular staff members, said Rachel Herlihy, Colorado's lead epidemiologist. The National Guard did not arrive until long after most of the cases associated with the outbreak had surfaced, with preliminary results yielding little evidence the variant virus was circulating in the facility, she said.

Public-health officials are investigating close contacts for both patients to determine whether they became infected through other work or outside activities, Herlihy said.

Neither travelled internationally and both are isolating. All of their close contacts will also be asked to quarantine for the next two weeks to try to avert further spread of the virus, she said.

Missing gene

The state is committed to doing full genetic sequencing on any samples that show signs of the British variant, Travanty said.

Samples are flagged when only two of the three genes targeted by the gold-standard PCR tests used by the state are found, indicating a mutation has occurred in the third - the critical Spike protein that helps give the virus its name.

The missing gene is known as the "S dropout," Travanty said. It is present but rendered undetectable by the test because of the mutation, making it a signature of the variant, she said. When laboratories find that red flag, it indicates more investigation is needed.

"We are sequencing all samples with the S dropout to see how widespread this is in the state," she said. "We are also increasing our surveillance across the state with CDC sequencing, in addition to the sequencing that we're doing here at the state lab."

The CDC launched a national surveillance system to detect novel strains of Sars-CoV-2 in November, and is scaling up now so that it can sequence 750 samples each week, said Greg Armstrong, director of the CDC's advanced molecular detection programme, and the leader of the team monitoring the coronavirus variants at the agency.