What it actually means that a new coronavirus strain is more transmissible - and how that changes your chances of getting sick

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Masked travelers on a London Underground platform, September 24, 2020. Getty

A new, more transmissible strain of coronavirus that was first identified in the UK has spread to more than 32 other countries, including the US.

The strain, called B.1.1.7., has several mutations in the genetic code for its spike protein, which the virus uses to invade cells. These tweaks may make it easier for the variant to infect people.

A preliminary study suggests the strain is about 56% more contagious than the original virus that emerged in China. UK government officials previously estimated that the variant could be up to 70% more transmissible. Either way, experts say, it can be thwarted in the same way as the original virus: strict mask wearing and social distancing.

"Human behavior has a very large effect on transmission - probably much larger than any biological differences in SARS-CoV-2 variants," Paul Bieniasz, a virologist at the Howard Hughes Medical Institute, told Business Insider. "Americans should be doing everything they can to limit the spread of SARS-CoV-2, regardless of whether there has been a biological change in the transmissibility of circulating strains."

This variant spreads more easily, but we don't know exactly how

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A waiter delivers food to a table at Chelsea Square Restaurant in New York City on October 1, 2020. BRYAN R. SMITH/AFP via Getty Images

A more transmissible strain, by definition, spreads more easily from person to person. The World Health Organization announced last month that the new variant has a reproductive, or R0, value of 1.5 rather than 1.1. The measure refers to the average number of people one sick person infects, so a difference of 0.4 means 100 sick people will infect another 150, not 110, on average.

"The working hypothesis is that the variant increases the chances of infection upon exposure," Nathan Grubaugh, an epidemiologist at the Yale School of Medicine, told Business Insider.

But precisely how it does that isn't known yet. Given its spike-protein mutations, it's possible the strain is better at binding to human cells. Or perhaps once it makes its way into our bodies, the variant is more proficient at evading our immune system.

Yet another possibility is that the new variant's infectious dose is lower, meaning it takes fewer viral particles to make you sick than the original virus would.

Grubaugh thinks the explanation could also simply be that people infected with the new strain have higher viral loads, meaning they produce more viral particles when they're infected. Early research suggests this is the case. The more viral particles a person spews, the more likely they are to infect others.

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ljubaphoto/Getty Images

So far, though, the variant doesn't appear to jump between people any differently than the original virus, and there's not yet any indication that existing personal protective equipment would be insufficient.

"Distancing, masks, and ventilation should still work," Grubaugh said.

The new strain's ability to spread rapidly may, however, necessitate extra precautions like being even stricter about mask-wearing and avoiding all indoor social gatherings.

"Maybe this is a simple way to think about it: Let's say a person infected with the non-variant virus is in a room with 20 other people - all not wearing masks," Grubaugh said. "The person infects 10 people. With the more transmissible strain, maybe the person infects 12 to 17 maskless people instead. If everyone was wearing a mask, maybe no one would get infected. That part shouldn't change."

Treat this strain like any other, experts say

In the US, eight states have all confirmed cases of the new variant.

Charles Chiu, a virologist at the University of California, San Francisco helped identify the cases in California. But he said people in the US can likely protect themselves by following the same public-health guidance experts have already suggested in response to the current surge in cases.

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Respiratory therapist Andrew Hoyt cares for a COVID-19 patient in the intensive care unit at Sharp Chula Vista Medical Center in Chula Vista, California, on December 21, 2020. Mario Tama/Getty Images

"In San Francisco, we have roughly three to four times the number of patients we've ever had in the hospital at any one time," Chiu told Business Insider, adding, "this really is a kind of a danger period for us, where we really have to adhere to public-health guidance on social distancing, on mask wearing."

Of the nearly 385,000 people in the US who have died from COVID-19 since the start of the pandemic, nearly one-third - more than 145,000 deaths - have happened in the last three months. More than 130,000 Americans are hospitalized with coronavirus, a record high.

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A woman wearing a mask moves her shopping cart in a Trader Joe's supermarket in New York City, December 3, 2020. Robert Nickelsberg/Getty Images

However, Chiu cautioned that experts' recommendations related to the new strain could change as we learn more, especially if it overtakes the original, as it did in parts of the UK. By the week of December 9, 62% of new cases in London were the new strain, up from 28% three weeks prior.

"We might have a really different conversation in like, four weeks from now," Chiu said.

A more contagious strain could cause more total deaths than a deadlier strain

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A crowded London street as seen on October 18, 2020. Matthew Chattle/Barcroft Media via Getty Images

There's no evidence yet that the new strain is deadlier than the original, or that existing vaccines wouldn't work. But because people who get this strain will, on average, infect more people, that could lead to a higher death toll in total.

Adam Kucharski, an epidemiologist at the London School of Hygiene and Tropical Medicine, compared three hypothetical scenarios on Twitter to demonstrate this. Assume the original strain of the coronavirus has an R0 of 1.1 and kills 0.8 people out of every 100 who get sick. In a population of 10,000 people, that virus would kill 129 people in a month.

But then say two new strains emerged, one of which is 50% deadlier, while the other is 50% more transmissible. The strain that's 50% more lethal would kill about 193 of the 10,000 people in a month. But the strain that's 50% more transmissible would wind up causing 978 of the 10,000 people to die in that time because of its quick spread.

"I think increasingly there's consensus that something unusual is going on here," Kucharski told Business Insider. "And if it's genuinely that much more transmissible, we've got a real problem."

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