A COVID-19 variant discovered in California has been dubbed "the devil" by one of the scientists studying it.
And he fears if its spread isn't stopped, it could one day meet the highly infectious UK variant and swap genes, creating a "nightmare scenario".
The Californian mutant strain of the virus, formally known as B.1.427/B.1.42, was first detected in the state last year ahead of a massive surge in infections over the northern hemisphere winter.
According to new research, B.1.427/B.1.42:
- is four times less susceptible than the original coronavirus to neutralising antibodies
- is two times less susceptible to antibodies from the blood of people vaccinated with the Pfizer or Moderna vaccines
- has a case-fatality ratio five to six times higher than normal COVID-19
- spread six times faster in an outbreak at a nursing home than previous strains
- had viral loads twice as high in the nasopharynx than people infected with other strains
- has mutations not yet seen in other coronavirus variants, including one - L452R - that makes it 40 percent better at infecting the lungs
- is perhaps three times as infectious (considerably more than the dreaded UK variant, estimated at between 40 and 80 percent more infectious).
It's not clear yet if the California variant makes people sicker or those infected are more at risk of hospitalisation or death, but the data suggests it's likely.
"This variant is concerning because our data shows that it is more contagious, more likely to be associated with severe illness, and at least partially resistant to neutralising antibodies," said senior author Charles Chiu, an infectious diseases physician at the University of California, San Francisco.
The UK and South African variants have been detected in California, but Dr Chiu says they'll likely be outcompeted by the homegrown strain, which already accounts for half of all infections.
"The devil is already here," he told the Los Angeles Times. "I wish it were different. But the science is the science."
The research also found that none of the California virus' mutations appear to have weakened it - usually viruses that spread faster aren't as deadly, because killing your host isn't beneficial.
Anthony Fauci - the US' top infectious diseases expert - warned if the outbreaks weren't snuffed out, the different variants would evolve to compete with each other in a survival-of-the-fittest contest.
"Together these two are going to likely make old strain extinct," added epidemiologist Eric Feigl-Ding, tweeting about the coming battle between the California and UK variants.
"The thing is that the pandemic is likely diverging. The old less infectious 1.0 Wuhan and derivative common strains are slowly disappearing and being replaced. There is an underbelly that is hidden. Please double mask."
Even scarier could be the instantaneous creation of a new strain, a "nightmare scenario" in Dr Chiu's words. If a person is infected with two different strains, and the competing viruses infect the same cell, their genetic material can get jumbled up - vastly accelerating the process of evolution in a process known as recombination.
Some studies have suggested that recombination could be how the coronavirus came to be in the first place.
The research is yet to be published, the Los Angeles Times reporting it's expected to come out later this week. Some experts have played down its significance, saying the research was done on too few people to come to any solid conclusions.
"If I were a reviewer, I would want to see more data from more infected people to substantiate this very provocative claim," University of Wisconsin virus expert David O'Connor told Science.
Of 61 patients looked at with the California variant, 13 percent ended up in ICU, compared to 2.9 percent of others; and 11.3 percent died, compared to 2 percent with normal COVID-19.
Other experts said the findings made sense, even with the small sample size.
"The biology of having a higher level of virus … would certainly fit the thesis that people would not do as well," Robert Schooley, a virologist at UC San Diego, told Science, adding that it would explain why many California patients have recently been spending longer than expected in ICU.
The number of daily new cases in California has dropped rapidly in recent weeks, falling from 50,000 a day in mid-January to 6000 a day this week. The number of active cases has remained stubbornly high though, falling from 1.7 million to 1.65 million, and the death toll hasn't fallen as far either, hundreds still losing their lives every day.
The US passed 500,000 deaths from the pandemic earlier this week, California accounting for about 10 percent of them, slightly less than its share of the population.