● New variants threaten progress made against coronavirus, health officials say.

Health officials are urging Americans to not let their guards down against COVID-19 as researchers discover new variants that already may be more transmissible and could be somewhat resistant to vaccines.

“At this level of cases, with variants spreading, we stand to completely lose the hard-earned ground we have gained,” Centers for Disease Control and Prevention director Dr. Rochelle Walensky said in a White House briefing Monday. “These variants are a very real threat to our people and our progress.”

Though experts have been following variants first identified in the United Kingdom and South Africa, they’re also seeing red flags in other variants discovered closer to home in Brazil, New York and California.

In Monday’s briefing, Dr. Anthony Fauci said a recent study found some COVID-19 treatments may not be as effective against the new variant discovered in New York.

“It’s a theoretical, but it can be a real issue,” the nation’s top infectious diseases expert said.

Here’s what’s known about the newer emerging variants:

One variant identified in Brazil

A variant that originated in Brazil – called P.1 – was first reported in late January by the National Institute of Infectious Diseases in Japan after four travelers from Brazil were screened at an airport outside of Tokyo.

The variant has 17 unique mutations, including three in the virus’s spiked protein, according to the CDC. As of Sunday, the CDC reported 10 cases of the P.1 variant in five states: Alaska, Florida, Oklahoma, Minnesota and Maryland. The agency has designated it a “variant of concern.”

A study published in January reported on a cluster of cases in Manaus, Brazil, the largest city in the Amazon region. It found the P.1 variant was identified in 42% of COVID-19 cases sequenced from late December.

Many of the cases occurred in people who already had recovered from the coronavirus, intensifying concerns about reinfection, said Dr. Cindy Prins, an epidemiologist at the College of Public Health and Health Professions at the University of Florida, who is unaffiliated with the study.

“They had a high level of herd immunity there, and what they found now is that they’re seeing new cases of COVID-19 and reinfections,” she said. Researchers discovered that reinfection was caused primarily by the P.1 variant.

Another identified in New York

Another mutated version of the coronavirus – called B.1.526 – popped up in New York City in late November and has since cropped up in neighboring states.

By Jan. 2021, the variant represented about 3% of samples analyzed by researchers, rising to 12.3% by midFebruary, according to a study by scientists at Columbia University Vagelos College of Physicians and Surgeons that has yet to be peer-reviewed.

As of Feb. 25, more than 80 cases of the variant have been identified in patients across the tri-state area, including Connecticut and Westchester County, New York.

“It’s not just one cluster, which means the lineage is probably spreading widely through the region,” Dr. Anne-Catrin Uhlemann, study author and associate professor of medicine at Columbia University’s Division of Infectious Diseases, said in a statement.

Experts say the effect of the new B.1.526 variant on transmissibility, disease severity and risk of reinfection is not known.

Health experts speculate the COVID-19 vaccine may be less effective against these emerging variants because of a mutation to the coronavirus’s spike protein, called the E484K mutation, which is present in variants first identified in South Africa, Brazil and New York.

Another recent study that has yet to be peer-reviewed found antibodies from vaccinated people are less effective against the B.1.351 virus, the variant first identified in South Africa, because of this mutation.

Variants with the E484K mutation may also be the cause for the drop in effectiveness of certain monoclonal treatments. Researchers also say this mutation may cause reinfection in people who were had recovered from earlier variants of the coronavirus.

The California variant

The variant that popped up in regions of California – called B.1.427 or B.1.429 – does not contain the E484K mutation, but researchers say it’s worth watching after a study suggested it’s associated with increased risk of severe disease and death. Study authors predicted the variant may have emerged in May 2020 and increased in prevalence until it made up more than half of COVID-19 cases throughout California by January 2021.

Contributing: The Associated Press Health and patient safety coverage at USA TODAY is made possible in part by a grant from the Masimo Foundation for Ethics, Innovation and Competition in Healthcare. The Masimo Foundation does not provide editorial input.