A year into the pandemic, it’s clear the coronavirus has meaningfully changed. 

While mutations are normal and most don’t change a virus’s behavior, sometimes these genetic errors give the virus an evolutionary leg-up against its hosts — mutations that make it more infectious, deadlier, or perhaps able to evade the protective immunity we accrue from vaccines or prior infections.

Since the fall, researchers have identified worrisome mutated strains spreading worldwide, many of which are nicknamed after where they were first detected. Many of these variants — including those from the UK, South Africa, and Brazil — share similar mutations in their spike proteins, which is what the virus uses to invade human cells. Research suggests those genetic tweaks may make it easier for the strains to infect people; some may decrease the efficacy of vaccines.

“We can see that we are going to be challenged” by the variant from South Africa, the US’s leading infectious disease expert Dr. Anthony Fauci said in a press call on Friday. 

Here’s what to know about four coronavirus variants, where they’ve been spotted, and what they mean for global vaccination efforts.  

B.1.1.7, the variant first spotted in the UK

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Researchers spotted the first variant of concern, named B.1.1.7, in the southeastern part of the UK on September 20. By mid-December, it had overtaken all other versions of the virus in the country. B.1.1.7 is now present in 70 countries; US officials have spotted 315 cases in 28 states.

Studies have shown B.1.1.7 is between 56% and 70% more contagious than its viral predecessors. While initial evidence suggested the strain wasn’t deadlier, UK officials said last week people infected with B.1.1.7 may face a higher risk of death than those who get other strains. The World Health Organization and other experts, however, cautioned more data was needed to confirm whether the variant was associated with higher mortality.

What makes B.1.1.7 so pernicious is that people infected with B.1.1.7 have higher viral loads, meaning they produce more viral particles when they’re infected. Higher viral loads, multiple studies show, are associated with a higher risk of death and more severe disease.

The variant has at least 17 mutations. But two are among the most concerning because they affect its spike protein shape and may increase transmissibility: the swapping of two amino acids in the 501st position of its genome, or N501Y, and a deletion of amino acids in its 69th and 70th positions, known as the 69/70 deletion.

Virologists think vaccines will still be effective against B.1.1.7, and recent research from Pfizer-BioNTech and Moderna found that the companies’ shots held up well against that variant.

Novavax, too, said Thursday its vaccine was 89% effective at preventing cases of COVID-19 based on results from a UK trial, which suggests it works against B.1.1.7.

B.1.351, the variant first found in South Africa

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Experts agree the variant first detected in South Africa in October, known as B.1.351 (or 501.Y.V2), is more worrisome than the variant first found in the UK.

This variant also has the N501Y mutation, though it lacks the worrisome 69/70 deletion. But what makes this strain troublesome is two spike mutations — K417T and E484K — that are missing in B.1.1.7. E484K, in particular, has been shown to reduce how well antibodies — proteins that identify and help destroy pathogens — can recognize the coronavirus.

B.1.351 has spread to 31 countries. The US reported its first two cases of the variant, which is 50% more transmissible, Thursday.

While there’s no evidence it is deadlier or causes more severe COVID-19, preliminary research published last week suggested the variant can partially evade the defenses existing vaccines build in our bodies’ immune systems.

Vaccine developers Novavax and Johnson & Johnson announced Friday that their shots didn’t work as well against B.1.351 than other strains. Novavax’s vaccine was 89% effective at preventing COVID-19 cases in the UK, but only 49% effective in South Africa, where B.1351 is the dominant version of the virus. J&J found its shot was, on average, 66% effective; that efficacy dropped to 57% in South Africa.

Though even with the variant concerns, J&J said its vaccine was 85% effective at preventing severe COVID-19 across all geographic regions — the UK, US, Latin America, and South Africa — in which it conducted clinical trials.

Following lab research, Pfizer and Moderna, both say their vaccines will work again B.1.351. Yet recent preliminary data from Moderna showed blood samples taken from vaccinated people that were infected with B.1.351 developed six times fewer virus-neutralizing antibodies than samples exposed to other variants.

This suggests vaccines won’t provide as much protection against B.1.351, and people who have already been infected with a different version of the virus could get reinfected with the new strain. (However, this lab testing doesn’t take into account other elements of immunity beyond antibodies, like virus-destroying T cells.)

P.1, the variant from Brazil

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A variant named P.1, with a similar genetic profile to B.1.351, is surging in Brazil. In mid-December, 42% of coronavirus cases in the Amazon region involved P.1.

Researchers first detected this strain in four travelers from Brazil who had traveled to Japan’s Haneda airport outside Tokyo. Data show it’s 50% more transmissible that the original version of the virus, and has the same trio of concerning spike mutations as B.1.351: N501Y, K417T, and E484K, so may also be able to evade antibodies generated from previous infections or a vaccine.

A study from earlier in January pinpointed P.1 as a variant of concerning after finding that cases had started spiking in Manaus, Brazil in December. Three-quarters of Manaus residents had already had COVID-19 by then, so the city should have been close to herd immunity from the virus. According to the CDC, the Manaus surge suggests P.1 increases the risk of reinfection in people who’ve already had COVID-19.

P.1 has been reported in eight countries; the US spotted its first case in Minnesota on Monday. 

CAL.20C, the variant first detected in California

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The US has its own homegrown variant to contend with, too. 

Researchers at Cedars-Sinai Medical Center in Los Angeles first spotted a case involving a new strain, which they called CAL.20C, in July. The variant didn’t appear in Southern California again until October. At the time, it didn’t seem to be spreading widely in the community, but two months later the variant represented nearly one-quarter of all samples from Southern California.

The researchers think the strain may have contributed to LA’s record coronavirus case surge in the late fall. 

CAL.20C does have mutations in its spike protein, though it lacks the trio of mutations characterizing B.1.351 and P.1, so additional studies are needed to find out whether it spreads more easily than other existing strains, or if it can evade antibodies.

There is no sign yet that CAL.20C is deadlier than its viral counterparts, the researchers added. 

Northern California, New York, Washington DC, and island countries in the Pacific Ocean have also reported cases involving CAL.20C.

Booster shots could combat variants

The spread of B.1.351, in particular, has prompted vaccine developers to start working on shots to combat virus variants.

“It’s really a wake-up call for us to be nimble and to be able to adjust,” Fauci said.

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Pfizer CEO Albert Bourla said Tuesday that his company is working on boosters to combat coronavirus variants. BioNTech, Pfizer’s partner, said it could produce a new vaccine for a COVID-19 variant in about six weeks. Bourla said at Bloomberg’s The Year Ahead conference on Monday that the company is working on a process to create booster shots. 

Moderna, too, will soon start clinical trials to test whether giving people another booster shot of its current vaccine, or a booster shot tailor-made to neutralize B.1.351, will improve antibody responses.

“I think our technology is very well suited to actually rapidly deploy a vaccine based on the new variant,” Tal Zaks, Moderna’s chief medical officer, said in a recent presentation at the J.P. Morgan Healthcare Conference.

Hilary Brueck and Dr. Catherine Schuster-Bruce contributed reporting to this story.