US coronavirus cases have crossed the 20 million mark as officials seek to speed up vaccinations and a more infectious variant surfaces in Colorado, California and Florida.

The United States has seen a spike in the number of daily COVID-19 fatalities since Thanksgiving with 78,000 lives lost in December.

A total of 345,000 have died of COVID-19, or one out of every 950 US residents, since the virus first emerged in China late in 2019.

To slow the death toll, Senator Mitt Romney on Friday urged the US government to enlist veterinarians and combat medics to give out coronavirus vaccinations.

The US rate of new COVID-19 infections increased in the second half of last year.

An analysis of Reuters data shows it took 200 days to reach the first 5 million cases, 93 days to go from 5 million cases to 10 million, 31 days from 10 million to 15 million cases and only 25 days to go from 15 million to 20 million cases.

California has the most total cases of any state, with about 2.28 million infections followed by Texas with 1.76 million cases and Florida with 1.32 million cases.

The United States is averaging 186,000 cases a day, down from a peak in mid-December of over 218,000 new infections each day.

Health officials have warned that cases will likely spike again after holiday gatherings.

Currently, there are more than 125,000 COVID-19 patients in US hospitals, up 25 per cent in the last month.

While the United States has approved two vaccines, roll-out is going more slowly than the government hoped.

About 2.8 million Americans received a COVID-19 vaccine by December 31, falling far short of a 20 million target.

Despite early setbacks in the vaccine roll-out, leading US infectious disease specialist Dr Anthony Fauci said on Wednesday that he expects Americans to achieve enough collective COVID-19 immunity through vaccinations by autumn 2021.

The government’s goal is 100 million shots in arms by March 1.

-AAP