When the coronavirus runs rampant, mutations and new strains are more likely, experts say. That may be what happened in the UK.

• A new coronavirus strain that seems to be more infectious is spreading in the UK.
• All viruses change over time. The more people a virus infects, the more chances it has to mutate into a new variant.
• So countries that let the coronavirus spread widely are more likely to see problems like this, experts say.
• Genetic data suggest the UK strain is already in other European countries.
• Visit Business Insider's homepage for more stories.

The surest way to boost the chances of a worrisome coronavirus mutation: let it spread unchecked.

"More infected people means more opportunity for the development of mutations. More movement of people ... means new variants can spread faster," Dr. Shira Doron, an infectious-disease specialist at Tufts Medical Center, told Business Insider.  

When viruses infect a body, they replicate. More spread means more replication, which raises the likelihood of genetic errors. So in places where transmission is high, we are more likely to see a harmful variant emerge, experts say.

That may be what happened in the UK.

Earlier this month, UK Health Secretary Matt Hancock announced that a new coronavirus strain was responsible for an uptick in cases in the south of England. The variant quickly overtook all other versions of the virus in the country, he said. By December 9, six out of every 10 coronavirus cases in the UK were the new variant, which government leaders suggested may be 70% more infectious than its predecessors. 

UK researchers first detected the variant three months ago, after a second wave of infections started on the heels of a summer of indoor dining, drinking, and exercising. The variant was found just week's after the UK reported 2,988 cases on September 6 - its highest daily record since late May.

Until just last week, London was in the second-lightest tier of coronavirus lockdowns, which allowed pubs to remain open and limited spectators at sporting events and performances. 

"Virus mutations can only accumulate if the virus is allowed to be transmitted. So the longer that we allow uncontrolled transmission to occur, the more chances that the virus will have to adapt to human transmission," Nathan Grubaugh, an epidemiologist at the Yale School of Medicine, told Business Insider.

Why the UK coronavirus strain is concerning

Countless versions of the coronavirus are circulating, each separated by a handful of tiny changes in its genetic code. The virus typically accumulates two mutations a month, many of which "have no real public-health impact," Grubaugh said.

Most of the mutations don't affect the virus's infectiousness or deadliness, according to Lucy van Dorp, a researcher at University College London's Genetics Institute.

But every so often, she told Business Insider, "a mutation, or combination of mutations, can arise which confers an advantage to the virus in some way."

That may be the case with the new UK strain, which geneticists have named B.1.1.7. It collected at least 17 mutations at once. Experts believe the strain could have emerged in a patient who was infected for a long time, allowing the virus to mutate in their body, Science magazine reported.

Some of the strain's mutations affect the virus' spike protein, which it uses to invade cells. That could make it easier for the virus to infect people.

Not all scientists agree that the new strain is 70% more infectious, though Grubaugh said existing data suggest it "is associated with increased transmission."

Indeed, 236,275 people tested positive for COVID-19 in the UK between December 17 and 23 - a 61% increase from the week before. The number of daily new cases has doubled in the last two weeks, to almost 40,000 per day. A couple of hotspots have more than 1,100 cases per 100,000 residents. Between mid-November and December 9, the variant jumped from being responsible for 28% of London's cases to 62%.

South Africa, too, is dealing with a new coronavirus strain that appears to be more transmissible and shares one mutation related to the spike protein with the UK strain.

Neither of these strains, however, seem to be more deadly. And virologists don't think the mutations threaten the efficacy of vaccines - they should still work.

'The more replication, the more opportunities for evolution'

From June to September, the entire UK was put in the middle tier of its COVID alert level system. Restrictions were eased: Bars and gyms opened in July, and more than 100 million discounted meals were served to indoor diners in August via the government's Eat Out to Help Out plan to stimulate the restaurant sector.

But that reopening may have happened too quickly. One researcher estimated Eat Out to Help Out was linked to almost 20% of all new UK infection clusters in August. London, where new daily cases were in the dozens in July, saw that figure spike to more than 1,000 by the start of October. Summer days with record lows of just 400 to 500 new cases in the UK gave way to daily fall totals of 4,000 to 5,000 new infections.

"The more replication, the more opportunities for evolution and adaptation," Richard Neher, an epidemiologist tracking coronavirus strains with the Nextstrain project, told Business Insider.

Neher added, though, that since it's hard to predict what prompts a virus to evolve and when particular mutations will arise, "the chance of this happening might not be exactly proportional to the number of cases."

The new variant shouldn't be blamed for all the spread

The new variant has an increased reproductive, or R0, value - the average number of people one sick person infects - of 1.5 rather than 1.1, the World Health Organization announced Monday. That difference of 0.4 means 100 sick people will infect another 150, not 110, on average. 

"However, that does not mean that it is responsible for the rising cases in London and the surrounding area," Doron said. She and Grubaugh both emphasized that human behavior and mitigation measures play a big role, too. 

"If we have a higher proportion of the population that distances and wears masks, it will stop the virus, variant or not," Grubaugh said.

Indeed, CDC noted that although a variant may dominate a geographic area, "that fact alone does not mean that the variant is more infectious." A strain could just get lucky, arising by chance in populous areas at the time when a government relaxes restrictions.

These same questions emerged after a different variant of the virus was detected earlier in the pandemic. That strain also has a mutation affecting the spike protein. It's predominantly the version that spread in Europe and North America last winter, and it's now more prevalent worldwide than the original virus that emerged in China. Some evidence suggests the strain is more infectious, but it might also have just hit the US and Europe at a moment when testing was limited and lockdowns hadn't yet been implemented.

'Variants rarely stay a local problem'

Prime Minister Boris Johnson placed 16 million people in southeastern England under a tier 4 lockdown, the UK's strictest, on Saturday. Soon after, at least 27 countries have blocked travel from the UK.

But it's already too late.

"Variants rarely stay a local problem," Neher said.

Genetics data reveal the strain is present in Scotland and Wales, as well as Denmark, Belgium, Iceland, and the Netherlands. There are a handful of cases in Italy, Australia, Gibraltar, and Singapore, too. Grubaugh said it hasn't been found in the US yet.

But that may be because US keeps tabs on the genetics of far fewer coronavirus samples than the UK and South Africa do. Only 51,000 of the 17 million US cases have been genetically sequenced, so "the mutation could be more widespread and we just don't know it," Grubaugh said.

Van Dorp thinks that's likely, as does Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. Fauci told PBS NewsHour on Monday that he "would not be surprised" if the UK strain is already in the US.

"When we start to look for it, we're going to find it," he said.

That reality leaves only one real option, Grubaugh said: "If a country is worried about the new variant being introduced and causing increased local transmission, a more effective plan is to put measures in place to decrease local transmission."

He added that his won't be the last time we learn of a new, potentially more infectious strain.

"Have there been others? Possibly, and maybe they didn't perpetuate for some reason," he said. "Will there be others in the future? Likely."