A more contagious coronavirus variant first identified in the United Kingdom continues to crop up across the U.S. and around the globe, threatening to further strain overburdened health care systems just as vaccines are rolling out worldwide.

At least five U.S. states and 33 countries have identified the new variant, known as B.1.1.7. Several nations have also identified an additional variant, first identified in South Africa, that also appears to infect people more easily.

"Because the variants spread more rapidly, they could lead to more cases and put even more strain on our heavily burdened health care systems," said Dr. Henry Walke, incident manager for the Centers for Disease Control and Prevention's COVID-19 response.

"We need to be even more vigilant in our prevention measures to slow the spread of COVID-19."

Here's what we know about B.1.1.7.

How much more contagious is the new strain?
The strain first identified in the U.K. spreads more easily and quickly than other strains, according to the CDC. The strain was first spotted in September in southeastern England and accounted for a quarter of cases in London by November. By the week of Dec. 9, it was responsible for 60% of cases in the city.

Scientists in the U.K. estimate that the new variant is 40-70% more infectious based on analysis of affected populations in Britain.

What makes the new strain more contagious?
SARS-CoV-2, the virus that causes the disease COVID-19, acquires about one new mutation in its genome every two weeks, according to the CDC. The U.K. variant has several mutations that affect the "spike protein" on the virus surface that attaches to human cells.

"It’s able to bind to the receptors on cells better, and therefore is transmitted better," Dr. Anthony Fauci, the nation's leading infectious disease expert, said at the end of December.

Is the new strain more lethal?
There is no evidence that B.1.1.7 causes more severe illness or increased risk of death, according to the CDC.

Is the vaccine effective for the new variant?
Researchers believe current COVID-19 vaccines will likely protect against B.1.1.7, but data is needed. The virus would "likely need to accumulate multiple mutations in the spike protein to evade immunity induced by vaccines or by natural infection," according to the CDC.

"From what we know from experience with this mutation and other mutations, it's unlikely to have a large impact on vaccine-induced immunity, or existing immunity from previous strains," said Dr. Greg Armstrong, director of the CDC's Office of Advanced Molecular Detection. Armstrong said it is unclear how the variant may respond to COVID-19 treatments, such as monoclonal antibody treatments.

How long has the variant been in the US?
Researches first identified the B.1.1.7 variant in the U.S. in Colorado on Dec. 28 in a COVID-19 patient with no reported travel history, suggesting that the virus was spreading from person to person in the community. It's unclear how widespread the variant has become, experts say.

The U.S. has sequenced viruses from only about 51,000 U.S. cases, according to the CDC. The nation now has more than 20 million confirmed coronavirus cases. "Given the small fraction of U.S. infections that have been sequenced, the variant could already be in the United States without having been detected," the CDC said last month.

The CDC said it plans to launch a national strain surveillance program this month that requires each state to submit at least 10 samples biweekly for sequencing.

"The new strain is estimated to represent about 1% of all infections (in the U.S.) at this moment but because of its increased contagiousness, the best estimates are that it will become a majority of all new infections by March," Dr. Ashish Jha, dean of Brown University's School of Public Health, wrote in a blog post Tuesday.

Where has the new strain been detected?
The B.1.1.7 strain has been identified in California, Colorado, Florida, New York and Georgia, and public health experts expect it to be identified in more states in the coming days.

Colorado identified the first known case of the variant in the U.S. on Dec. 28 and was investigating a second possible case – both in National Guard soldiers who were deployed to support staffing at a nursing home in Simla, outside Denver, state health officials said.

California identified the second confirmed U.S. case on Dec. 30 in a 30-year-old San Diego County man with no travel history. Less than a week later, the county reported 32 cases of the new strain have been detected.

Florida reported its first case Dec. 31 in a Martin County man in his 20s with no travel history.

New York reported its first case on Jan. 4, a man in his 60s who is associated with a jewelry store in Saratoga Springs, north of Albany.

Georgia reported its first case on Jan. 5, an 18-year-old man with no travel history.

The strain has been detected in at least 33 countries, including Australia, Belgium, Brazil, Canada, Chile, China, Denmark, Finland, France, Germany, Iceland, India, Ireland, Israel, Italy, Japan, Jordan, Lebanon, Malta, The Netherlands, Norway, Pakistan, Portugal, Singapore, South Korea, Spain, Sweden, Switzerland, Taiwan, Turkey, the United Arab Emirates, the United Kingdom and the United States.

South Africa has also identified a strain similar to B.1.1.7, but it emerged in October independently of B.1.1.7 and is not related to it, according to the CDC. Like B.1.1.7, the South Africa variant (B.1.351) appears to spread more easily and quickly but is not more severe. U.S. health officials said last week they did not know if the South Africa strain was also circulating in the U.S.

A third variant also emerged this fall and has been detected in Nigeria, but there is no evidence that it is more severe or more transmissible, according to the CDC.