One chart shows how well COVID-19 vaccines work against the 3 most worrisome coronavirus variants

• Some mutated coronavirus strains are more transmissible and can evade vaccine-induced antibodies.
• Pfizer's and Moderna's vaccines seem to be less effective against the variant found in South Africa.
• The charts below compare how well leading vaccines work against worrisome variants.

More than 33 million Americans have been fully vaccinated against COVID-19 - more than the number of US cases reported during the pandemic. But the proliferation of pernicious variants, some of which can partially evade vaccines, means we're not out of the woods.

New research suggests that Moderna's and Pfizer's shots are significantly less effective against the variant first found in South Africa, called B.1.351. That study used blood samples from vaccinated people to compare antibody responses to the original virus and to the B.1.351 variant. Those who got the shots had far fewer antibodies that could neutralize B.1.351.

The study's authors said both vaccines also would likely be less effective against P.1, a variant first found in Brazil.

The chart below summarizes what we know so far about how well five vaccines work to protect people from three of the most worrisome coronavirus variants.

3 variants, 3 stories

Countless versions of the coronavirus circulate worldwide, each separated by a small number of genetic mutations.

Once a slew of mutations makes a particular strain better at infecting people, deadlier, or more able to evade the antibodies generated from a vaccination or a previous infection, geneticists label it a variant of concern.

There are three of these: B.1.1.7, the variant initially spotted in the UK in September; P.1, which was discovered in December; and B.1.351, which was detected in samples from South Africa dating back to October.

They share a mutation that affects the shape of the virus' spike protein, which it uses to invade cells. That may be why these variants are more transmissible.

Studies have found that the B.1.1.7 variant - which has been reported in 94 countries, including the US - is 50% to 70% more contagious than its viral predecessors. Recent evidence suggests that people infected with this variant may face a higher risk of death than those who get other strains.

The B.1.351 variant has been reported in 48 countries and 23 US states. But studies have not found it to be more lethal than the original virus.

The same goes for the P.1 variant, which a March study suggested was 40% to 120% more transmissible than earlier versions of the virus. P.1 has been detected in 26 countries and 10 US states.

Research suggests that existing vaccines work to protect people from B.1.1.7 but are less effective against B.1.351 and P.1. That's likely because those two variants share a mutation that can prevent the antibodies generated in response to the original virus from recognizing them.

This genetic tweak is mostly missing in the B.1.1.7 variant, though UK researchers did find 11 cases of B.1.1.7 with that mutation in a set of more than 200,000 samples.

B.1.351 and P.1 also have a mutation that may help the virus bind more tightly to cells.

What these variants mean for you

Both Pfizer and Moderna said in January that they planned to develop and test booster shots to tackle the B.1.351 variant. That means vaccinated people may need to get a follow-up shot. Until then, people who've been vaccinated or previously got COVID-19 could still be at risk of infection from the B.1.351 and P.1 variants.

However, vaccines probably still confer some degree of protection against these variants, even if they're less effective.

"The more vaccines we can get into the arms of people, the fewer numbers of overall infections," Kristian Andersen, an immunologist the Scripps Research Institute in California, tweeted in January. That includes infections with the B.1.351 and P.1 variants, he added.

What's more, the variants jump between hosts the same way as the original virus, so social distancing and using personal protective equipment should still help to stop their spread. That's why those mitigation measures remain critical - especially given that if you get exposed to a more transmissible variant, you're more likely to get infected.

"Let's get our genomic surveillance in place, better masks, more masks, much more widespread testing and screening - and avoid gatherings and crowded settings," Andersen wrote. "Combine that with accelerated vaccine distribution and an accelerated plan for updated boosters."

Dr. Catherine Schuster-Bruce contributed reporting to this story.