3 concerning variants have caught scientists' attention since Delta emerged - here's how they compare

• Three coronavirus variants - Lambda, Mu, and C.1.2 - have raised red flags among scientists lately.
• The variants show signs of being highly transmissible, able to circumvent vaccine protection, or both.
• But so far, none is more concerning than Delta, the world's dominant strain.

After the Delta variant took the world by surprise this summer, scientists have been keeping a lookout for the next worrisome coronavirus strain. Three in particular - Lambda, Mu, and C.1.2 - have caught scientists' attention recently, since they share a few key similarities with other variants of concern.

Like Delta, these three variants show signs of being highly transmissible, resistant to the immune defenses generated by vaccines, or both. But none is spreading faster than Delta - an indication that Delta will remain the world's dominant strain for at least the near future.

"I don't lose sleep over new variants," Dr. Ashish Jha, dean at the Brown School of Public Health, wrote Sunday on Twitter. "I worry about people's fatigue with the current one."

Delta represents nearly 60% of coronavirus infections worldwide, according to Scripps Research's Outbreak.info tracker.

The three newer variants represent a much smaller share, but they've each spread to several continents. Here's a rundown of how prevalent the variants are and why they've raised red flags among scientists.

Mu has been spotted in 45 countries so far

The World Health Organization labeled Mu, also known as B.1.621, a "variant of interest" last month. The term applies to variants that may cause significant community transmission or have genetic changes that could alter the way the virus behaves. At the time, WHO scientists were concerned because Mu had spread quickly in South America since May.

But globally, Mu represents just 0.1% of new coronavirus cases, according to data from GISAID, a global database that collects coronavirus genomes. In total, the variant has been detected in 45 countries since it was first identified in Colombia in January.

The prevalence of Mu cases has also declined, both globally and in South America, since July. That's a sign the variant won't become dominant, since variants with a big advantage over Delta would quickly account for a larger and larger share of cases.

"In the US, Mu is unlikely to have a big impact," Alexandre Bolze, a genetics researcher at Helix, wrote on Twitter. The variant makes up less than 0.2% of US coronavirus infections sequenced in the last month.

In Colombia, however, Mu still represents 100% of coronavirus infections detected in the same time period.

So far, researchers have determined that Mu carries several key mutations in the genetic code for the spike protein - the crown-like bumps on the surface of the virus that help it invade our cells. These mutations could make it resistant to antibodies produced by vaccines or a prior infection.

Last month, seven elderly residents of a nursing home in Belgium died of Mu infections, despite being fully vaccinated.

Italian researchers have found evidence, however, that Pfizer's vaccine still neutralizes the Mu variant, even though the vaccine offers better protection against the original version of the virus.

Lambda poses a slight challenge to vaccines

Lambda, also called C.37, has been spotted in 40 countries since it was first identified in Peru last year.

The variant represented around 12% of South America's new coronavirus infections at its peak in mid- to late-June. The WHO designated Lambda a "variant of interest" around that time.

Now, Lambda represents less than 3% of South America's new infections and just 0.1% worldwide. Only 10 countries - including Argentina, Chile, Costa Rica, and Ecuador - have spotted Lambda cases in the last month.

"It really doesn't seem to take off once it is reported in a country," Maria van Kerkhove, the WHO's technical lead on COVID-19, said in August.

But, like Mu, Lamda carries several concerning mutations in the spike protein.

A Japanese study that's still awaiting peer review found that these mutations could make Lambda more infectious than the original coronavirus, and possibly resistant to antibodies from vaccines or a prior infection.

A July study, also not peer-reviewed, found that China's Sinovac vaccine was less effective against Lambda than against the original coronavirus. And another study found that China's Sinopharm shot lowered the risk of coronavirus infections by 50% among healthcare workers in Peru from February through June, when Lambda was spreading widely. Before that, the vaccine was found to be 78% effective at preventing symptomatic COVID-19 in clinical trials.

The Pfizer and Moderna vaccines, meanwhile, still offer good protection against Lambda, according to another recent preprint study. That research also suggested that Regeneron's monoclonal antibody cocktail is just as effective against Lambda.

C.1.2 infections rose quickly in South Africa this summer

Unlike Lambda and Mu, C.1.2 hasn't been labeled a "variant of interest" yet.

It was first detected in South Africa in May, when it accounted for 0.2% of the country's sequenced cases. That figure rose to 1.6% in June, then 2% in July - a pattern similar to the early rise of Delta in South Africa.

But South Africa's C.1.2 cases have declined considerably in the last month, even though the variant has spread to most provinces there, as well as 10 other countries.

Globally, C.1.2 cases seemed to peak in early July, suggesting the variant isn't becoming more prevalent.

According to a preprint from South African researchers, C.1.2 carries "concerning constellations of mutations" that could help the virus spread more easily or evade the immune system's vaccine-induced defenses. Some of these mutations are the same as those found in other variants, including Delta.

Others are unique to C.1.2, which is reason for alarm, the South African researchers wrote, because scientists are most worried about a new variant with properties that would enable it to overtake Delta.

"At the moment the 'best of class' is the Delta variant, so it tends to out-compete other variants," Dr. Michael Ryan, head of the WHO's health emergencies program, said in a briefing on Tuesday.

He added: "Not every variant means the sky is going to fall in."