The Delta variant has taken over in the UK, where the most common COVID-19 symptom reported is a headache.
Most people getting COVID-19 in the UK are young and not fully vaccinated.
Health experts are worried that the Delta variant could hit less vaccinated areas of the US hard.
The Delta coronavirus variant, first identified in India, has taken over in the UK, where it's now responsible for over 95% of infections. Delta's also coming for the US fast, already accounting for more than 20% of sequenced cases, with a doubling time of about two weeks.
But Tim Spector, an epidemiologist at King's College London, says don't be surprised if it shows up with COVID-19 symptoms different from those we've come to expect.
His latest research suggests that a headache and a runny nose are now two leading indicators of a COVID-19 infection in the UK, especially among the young and the partially vaccinated.
Spector's research draws on daily illness data from more than a million people in the UK who log into his ZOE app every day and report how they're feeling. ZOE's research suggests that about 19,000 people in the UK catch COVID-19 every day. Most are young, and most are not fully vaccinated.
"It's evolved to be more infectious, which is what many viruses do," Spector said, adding that the Delta variant may be roughly twice as infectious as its early predecessors, with each person who catches it transmitting to about six others.
"There's no hard evidence yet that it's more lethal or fatal, but because of that extra stickiness, it's going to still keep breaking through," he said.
ZOE's latest data indicates that the COVID-19 case rate in the UK is highest among 20- to 29-year-olds.
"We're seeing this mainly in young people who are unvaccinated - they are three-quarters of the cases," Spector said. "There's hardly anyone who's over 60 who's getting it without a vaccine."
Most of the people who test positive for COVID-19 in the UK are weathering the traditional signs of a bad cold - headaches, runny noses, and sore throats - rather than the earlier telltale COVID-19 symptoms like shortness of breath or loss of taste.
It's tough to know, though, whether the Delta variant is causing a milder disease or whether this is just how COVID-19 presents among young people, healthy people, and people with vaccine protection.
"Our hope is it'll get milder," Spector said. "So it will just become like a cold."
The 'most able and fastest and fittest' variant
Disease watchers at the World Health Organization agree with Spector - yet another reason to get more of the world's most vulnerable people vaccinated fast.
"The Delta variant is the most able and fastest and fittest," Mike Ryan, the executive director of WHO's health-emergencies program, said on Monday, stressing that vaccines are still "highly protective against hospitalization and death," even with variants in play.
"This particular Delta variant is faster. It is fitter. It will pick off the more vulnerable more efficiently than previous variants," Ryan said, adding, "We can protect those people now."
In the UK, nearly three-quarters of adults have had both doses of the COVID-19 vaccine, giving them strong armor against severe disease and death. So far, only 16 US states have gotten 70% of adults any vaccine protection.
"This virus isn't going to give up easily," Spector said. "I don't think we can be too complacent, particularly in areas of the US that have high nonvaccination rates."
Dr. Rochelle Walensky, the director of the Centers for Disease Control and Prevention, stressed in a White House briefing on Tuesday that "nearly every death" from COVID-19 in the US is now preventable. But time could be running out to ramp up protection, and the race with increasingly fitter variants is not over.
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