Why Are the New COVID-19 Strains Spreading More Quickly?

The CDC recently announced that the COVID variant from the UK is now "the most common" strain currently circulating in the U.S. Here are some theories as to why.

As health officials warn of another possible surge of COVID infections across the U.S., there's been some hopeful news: Roughly 110 million people (and counting) in the country have received at least one dose of the COVID-19 vaccine. (ICYMI: How Effective Is the COVID-19 Vaccine?)

But, there's a bit of a curveball. A more infectious COVID variant that was first identified in the UK has now become "the most common" COVID strain currently circulating in the U.S., Rochelle Walensky, M.D., director of the Centers for Disease Control and Prevention (CDC), said at a White House press briefing on April 7.

The variant, known as B.1.1.7, began spreading in the UK back in December 2020 and has since made its way to Europe as well. While researchers are still learning more about the variant, current estimates suggest the strain may be about 60 percent more contagious than the original form of the virus.

But B.1.1.7 isn't the only COVID-19 variant experts are tracking. Among the other CDC variants of concern are B.1.351 — which was first discovered in South Africa and has an estimated 50 percent increased risk of transmission — and P.1, a strain identified in Brazil that's also suspected to be more contagious, not to mention potentially resistant to antibodies in people who should have immunity because of a previous infection with an earlier strain of the virus. The CDC also lists two strains that were first identified in California, B.1.427 and B.1.429, as COVID variants of concern due to their potential risk for increased ability to spread.

It's a lot to digest, but, deep breaths. Here's what you need to know about these new COVID variants, plus why they're thought to be more infectious than "regular" COVID-19.

First, why do viruses mutate in general?

All viruses mutate. It happens as a virus replicates — something the virus needs to do to survive. "Viruses multiply billions of times, so many small mutations occur," explains William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine. "The vast majority of these mutations are harmless, useless, and sometimes even detrimental to the virus so they don't go anywhere."

But sometimes mutations can make a virus stronger — and that seems to be the case with these new COVID variations. "In the instance of the UK and South African variants, a series of mutations took place that has made these viruses more contagious," says Dr. Schaffner.

Why are the new COVID-19 strains more contagious?

B.1.1.7 (the UK strain) has several mutations in its genome (aka genetic makeup), some of which are in the virus's spike protein, an area that the virus uses to bind to receptors in your cells to infect you with COVID-19, explains infectious disease expert Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Center for Health Security. "One series of mutations, in particular, seems to make the virus able to bind to your cells in a more efficient manner," he says.

The CDC explains it this way: The B.1.1.7 variant has a mutation in the receptor-binding domain of the spike protein. That mutation causes an amino acid called asparagine — which is normally present in the "regular" SARS-CoV-2 virus — to be replaced with a different amino acid called tyrosine.

That's a little technical but, essentially, this amino-acid swap makes the virus's spike protein stronger and, as a result, appears to improve the virus's ability to infiltrate someone's cells and infect them with COVID-19.

It's also worth noting that B.1.351 (the COVID-19 variant that was first spotted in South Africa), P.1 (the strain first detected in Brazil), and the two COVID variants discovered in California similarly have multiple mutations in the spike protein that seem to allow the virus to spread more readily and infect human cells, according to the CDC. "The more viruses can get into cells more quickly, the more the virus replicates and the more it sheds, potentially infecting others," explains Dr. Schaffner.

As of now, experts estimate that a person infected with the COVID variant from the UK may spread the virus to, on average, 1.5 people, while someone with the "standard" COVID strain may pass the virus to about 1.1 people, said Maria Van Kerkhove, a technical lead at the WHO, according to U.S. News & World Report. Researchers are still studying just how quickly the COVID strains from South Africa, Brazil, and California can spread compared to previous variants.

But Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York, says it's important to note that, at this point, the data does not definitively say that B.1.1.7 (or the other new COVID strains, for that matter) is more contagious.

"B.1.1.7 is believed to be more infectious because it was first recognized in the UK at the end of September and, by December, the proportion of cases of COVID-19 due to this strain had significantly increased," explains Dr. Russo. "That suggests the new strain was outcompeting the current strain and was, therefore, more infectious."

While Dr. Russo says it's "likely the case" that the genetic makeup of these new COVID variants does make them more contagious, he also points out the possibility that a phenomenon called the "founder effect" could be playing a role in the spread. In other words, it may be the case that someone who was infected with B.1.1.7 became a superspreader in the UK (i.e. perhaps they weren't social distancing or wearing a mask in public), and the strain became more widespread from there, he explains. The same theory might explain the spread of the other variants that were first found in South Africa, Brazil, and California, respectively, but more data is needed to be certain.

So, while Dr. Russo says these new strains are "likely more infectious," it's not a "slam dunk" of a conclusion just yet.

Are the new COVID-19 strains more deadly?

On the bright side, there are currently no definitive data to suggest that these variants could be more deadly or dangerous than the existing dominant strain of SARS-CoV-2, says Dr. Schaffner. "It's just more spreadable, more contagious," he adds. (

But there could be an indirect effect here. "The more contagious it is, the more widely it will spread, the more people will become infected, the more people with underlying health conditions will get it, and the more people will die," explains Dr. Schaffner. "But not because the virus is more virulent."

What does this mean for COVID-19 vaccines?

Right now, it appears that the Pfizer, Moderna, and Johnson & Johnson vaccines — the first vaccines to receive emergency use authorization (EUA) from the FDA — are still effective in protecting the body from a COVID-19 infection involving any of these new mutations.

Still, considering how quickly these variants have developed (and spread), you might be wondering if the COVID-19 vaccine will eventually become an annual injection that needs to be updated each year to account for these new mutations, similar to the flu vaccine. Dr. Adalja says that's unlikely at this point. (

At some point in the future, there could be a "need for an update" to one or more of the COVID-19 vaccines, says Dr. Adalja. But he doesn't anticipate it being at the level of annual flu vaccination. (

Overall, Dr. Schaffner recommends taking a slow, socially-distanced breath, and following the COVID-19 precautions you've memorized by this point. "Wearing masks and social distancing becomes even more important now that these strains are around," he says. "They will help protect against these new strains as well as the standard strains."

The information in this story is accurate as of press time. As updates about coronavirus COVID-19 continue to evolve, it's possible that some information and recommendations in this story have changed since initial publication. We encourage you to check in regularly with resources such as the CDC, the WHO, and your local public health department for the most up-to-date data and recommendations.

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