Although the novel coronavirus has been mutating since the pandemic's start, three newly detected variants that may make the virus more contagious and deadly have experts concerned. But why are these troublesome variants emerging now—and what do they mean for America's response?

When will the Covid-19 epidemic end? Here are the good, bad, and ugly scenarios.

4 reasons why these concerning variants are emerging now

To understand why these concerning new coronavirus variants—one that was first detected in the United Kingdom, another first detected in Brazil, and a third first detected in South Africa—are emerging now, Vox's Brian Resnick spoke with experts in epidemiology and biology. According to Resnick, there are four main reasons why we're likely seeing these new variants at this point in the pandemic:

1. As a pandemic goes on, a virus's genetic diversity increases

The first reason is based in how viruses spread—and for how long. To spread, viruses—including the novel coronavirus—constantly make copies of themselves. Some of those copies can be imperfect, which leads to mutations. And, given enough time, millions of mutations can emerge, Resnick writes. So ultimately, the longer the new coronavirus pandemic goes on, the more opportunities the virus has to mutate and evolve, Resnick notes.

Sarah Cobey, an epidemiologist studying viral evolution at the University of Chicago, explained to Resnick that there's "no evidence that the underlying mutation rate [of the novel coronavirus] is changing." However, because the virus has been copying itself over and over for more than a year now, it's had many chances to mutate.

2. The virus is so widespread that rare occurrences are happening

Along those same lines, the more a virus is able to spread, the more opportunities there are for rare mutations to occur, Resnick writes. And that might be the case with the new variant that was first detected in the United Kingdom and has since been found in countries throughout the world, including the United States, Resnick notes.

According to Resnick, scientists think the U.K. variant may have first emerged in an immunocompromised patient who was infected with the novel coronavirus. Emma Hodcroft, a molecular epidemiologist at the University of Bern, explained that in the majority of people, the immune system will eliminate the virus in a couple of weeks.

However, "[i]n people [who] have compromised immune systems, … there's a very different dynamic," she said. For example, "the virus could be in them for months instead of weeks," Hodcroft said, which gives the virus more time to mutate.

"These are all like 'super-edge cases,'" Hodcroft said. But with persistently high rates of novel coronavirus cases throughout the world, "you increase the chance that sooner or later, you're going to hit that jackpot ... we keep rolling the die when we keep the cases up so high," she said.

3. The virus could be responding to increased human immunity

Some mutations have no effect on how the novel coronavirus spreads or infects people, and those mutations can die out over time. However, other mutations give the virus an advantage over its original version, as is the case with the new variants that were first discovered in Brazil, South Africa, and the United Kingdom. Data suggests those new variants are more transmissible than the original version of the novel coronavirus, and they may be resistant to some Covid-19 vaccines and treatments.

"Some of those [genetic] substitutions are actually helping the virus replicate better," Cobey said. Further, the variants discovered in Brazil and South Africa both have a mutation known as E484K, which "might allow reinfection," according to Hodcroft.

Typically, mutations such as these that give a virus an advantage over its original form will outperform other variants and, eventually, they can become the dominant variants circulating in a population, Resnick writes.

Hodcroft said she believes the variants originally discovered in Brazil and South Africa—which have now been identified in the United States and other countries—evolved in response to increased immunity among humans, which stemmed from more humans becoming infected with and developing immune responses to the original version of the novel coronavirus. As more humans became immune to the original version of the virus, mutations that helped it evade that immunity began to emerge and overtake other variants, she said.

However, Hodcroft noted, "I want to be really clear: We aren't 100% sure, scientifically, that this is what happened." She continued, "But these are the kinds of things to think about when we think about why we might be seeing different variants now. We've changed the rules of the game."

4. Some treatments for Covid-19 may have led to some mutations

Lastly, some of the newly identified coronavirus variants might have emerged in response to certain Covid-19 treatments, Resnick writes.

For instance, some of the emerging variants "may have something to do with the use of convalescent plasma," according to Michael Worobey, head of the department of ecology and evolutionary biology at the University of Arizona.

Convalescent plasma treatments involve a patient who has Covid-19 receiving a blood plasma transfusion from a patient who has recovered from the disease. According to Resnick, in some people, that plasma could create an environment that allows a stronger variant of the novel coronavirus to emerge.

"So there are cases where the identical [mutations] that characterize the U.K. variant have also evolved in patients who are chronically infected with the virus and were then given convalescent plasma," Worobey said. "It's a perfect storm."

Worobey qualified that it's not for certain that this is how the U.K. variant emerged, but that it's possible, Resnick notes.

How do we respond?

While the emergence of these new, more transmissible variants is concerning, experts say there are ways countries can respond to curb their spread.

First, countries should focus on vaccinating as many people as possible against Covid-19 as quickly as possible, so that the virus doesn't have a chance to evolve in a way that allows it to completely bypass vaccine immunity—which doesn't seem to be the case with the variants that emerged in Brazil, South Africa, and the United Kingdom.

"What we don't want is for there to be high levels of virus circulating and spending a lot of time with a partially vaccinated population," Hodcroft said. "We want to keep case numbers while we're vaccinating as low as we can."

And to help keep case numbers as low as possible until vaccines are widespread, the public needs to double down on wearing masks, adhering to physical distancing measures, and limiting their exposure by staying home when possible, experts say—even as case rates start to fall.

"It is true things are coming down, but we are at a very high level," Ashish Jha, dean of the Brown University School of Public Health said in regard to new coronavirus case rates in the United States. "This is the time to hunker down for what is likely to be a very difficult two or three months."

"What we need to do right now is to plan for the worst-case scenario," Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, said. "And when I say 'worst-case,' I'm potentially talking about the most likely case. Let's not wait until we wrap the car around the tree to start pumping the brakes."

Lastly, William Hanage, an epidemiologist and professor at Harvard University, said the United States should perform more genomic sequencing to better track the variants as they move through the country—something that's largely been lacking so far. "As it is, people like me cobble together partnerships with places and try and beg them" for coronavirus samples, he said (Resnick, Vox, 1/27; Klein, New York Times, 1/28; Walsh, Axios, 1/27; Barry-Jester, Kaiser Health News, 1/28).