The WHO warns of the spread of a new variant of omicron 2:24
(CNN) --
The highly contagious BA.2 subvariant of omicron is now the dominant coronavirus strain in the United States, causing more than half of all Covid-19 infections last week, the Centers for Disease Control reported Tuesday. and Disease Prevention (CDC).
The new numbers come from CDC genomic surveillance.
Based on its models, the agency says the BA.2 subvariant caused between 51% and 59% of all new COVID-19 infections in the United States the week ending March 26, versus an estimated 39% of all. new infections the previous week.
The most affected region was the Northeast, where BA.2 caused more than 70% of all cases.
The south and mountainous west of the country saw the fewest cases.
The BA.2 subvariant caused a little more than a third of infections in those regions last week.
Omicron subvariant BA.2 is now the dominant strain of COVID-19 in the United States, according to the CDC
What distinguishes the covid-19 BA.2 subvariant from the omicron?
0:54
"We are not immune to what is happening in Europe"
Although BA.2 is still taking the stage in the United States, it has had notable performances in many other parts of the world, including Southeast Asia and the Western Pacific, and finishing its European tour.
According to the World Health Organization (WHO), BA.2 is also the main cause of covid-19 worldwide, surpassing two other omicron lineages, BA.1 and BA 1.1, to become the dominant strain.
Since its acquisition, the number of international cases, which had decreased since the first week of January, has increased again.
advertising
In the UK, which has a more vaccinated population than the US, a combination of lifted restrictions, waning immunity and an even more contagious version of the virus have created a new wave of BA.2.
Covid-19 cases, hospitalizations and deaths have been trending upwards since late February, and the weekly average of new cases is now roughly where it was at the end of January.
However, BA.2 infections have not reached the peaks seen with BA.1.
The case count appears to be leveling off in the UK, although hospitalizations and deaths continue to rise.
As BA.2 grows in the US, experts look to other countries to predict its impact
This is what we know about BA.2, new omicron subvariant 0:41
Throughout the pandemic, the United States has followed the United Kingdom for about three weeks, so when cases began to rise there, local health officials took notice.
In the United States, BA.2 has been gaining strength since the end of January and the number of cases has stagnated.
However, that flattening hides regional differences.
In 13 states, the weekly average of new cases is rising and has stopped falling in 14 others, according to data compiled by Johns Hopkins University.
It is not yet clear what this subvariant will do in the United States;
even the experts don't know exactly what to expect.
“We are not immune to what is happening in Europe,” said former CDC director Dr. Tom Frieden, who is now president and CEO of the nonprofit Resolve to Save Lives.
"In Europe, you see BA.2 becoming mainstream and fueling a resurgence, and the probability of that not happening in the US is pretty low, actually," Frieden said.
"I think part of the reason we're stalling is that we're about to start going up again."
Frieden doesn't think it's a coincidence that the Food and Drug Administration (FDA) authorized additional booster doses for Americans over 50, on the same day that the CDC estimated BA.2 to be dominant.
But anyone can guess how high cases will be, whether many people will need hospital care, and whether the nation will continue to see a staggering death toll.
Facing "a family uncertainty"
Most of the predictions about the BA.2 subvariant in the United States have not been dire.
Covid-19 projections from the Institute for Health Metrics and Evaluation at the University of Washington, updated last week, predict that BA.2 will not drive another surge in the United States.
But they say we may see something like what happened in South Africa, where BA.2 quietly replaced its cousin BA.1 as the leading cause of Covid-19 infections, with no increase in cases or deaths.
Instead, he prolonged the omicron's descent, causing a long tail.
Michael Osterholm, who directs the Center for Infectious Disease Research and Policy at the University of Minnesota, called the new coronavirus waiting game we play every few months "a familiar uncertainty."
He dedicated his latest podcast to "all of us who are still confused about what the immediate or intermediate future looks like with covid."
Should we worry about the increase in cases in China?
1:21
How strong is our immune wall?
There are many reasons why it's hard to know what you can do about BA.2.
The CDC estimates that 37 million Americans, about 1 in 11, contracted covid-19 during the winter, during the first wave of the omicron.
Many more have immunity from vaccination and boosters.
So based on random blood samples, the CDC says that 95% of Americans may now have some degree of immunity against covid-19.
Dr. Jorge Salinas, an infectious disease expert at Stanford University, calls this an immune wall.
"That gives us some protection against future waves," he said.
"However, that wall deteriorates over time. The more that happens after a wave or after vaccination, the more the wall breaks down."
Studies have determined that the BA.2 subvariant evades our vaccines as well as the original omicron did, so boosters are needed to restore protections against these variants.
But less than half of the US population over the age of 12 has received a third recommended dose.
The biggest concern is adults over the age of 65 because they are more likely to get seriously ill from covid-19.
One in three people over the age of 65 in the United States has not received a critical third dose of vaccine.
"The real problem is that so much of our population is older people who are not vaccinated," Frieden said.
"That's our Achilles heel."
Doubts about reinfection
The other variable is found in the virus itself.
The omicron variant threw our immune defenses for a loop.
It was so different from previous coronavirus strains that many people who had gotten sick with delta or other early strains got infected again.
BA.2 has about 40 amino acid changes from omicron's BA.1, making it as different from its cousin as alpha, beta, and delta were from each other.
Some have wondered if BA.2 could re-infect people who had COVID-19 from BA.1.
A large study from Denmark suggests that such reinfections are possible, but rare.
Research into more than 1.8 million infections found just 1,739 cases where people tested positive for Covid-19 twice in a two-month period.
Of these, 47 were BA.1 infections followed by BA.2.
When the researchers took a closer look, they found that these types of reinfections tended to occur in young, unvaccinated people, mostly children.
And their symptoms tended to be mild.
The study was published as a preprint, meaning it has not yet been reviewed by outside experts or published in a medical journal.
Studies point to high viral loads
The BA.2 subvariant is extremely contagious.
Some epidemiologists have said its basic reproduction number can be as high as 12, meaning each sick person infects an average of 12 others. That would put it on par with measles, which is also spread through the air.
The basic reproduction number for the BA.1 is estimated to be around 8.
In a preliminary study from Sweden, researchers measured viral levels in swabs from the back of the nasal cavity.
They found almost twice as much viral RNA in samples from patients with BA.2 than in those who tested positive for BA.1, "pointing to a substantial difference in viral load."
Viral loads were about the same for delta and BA.1 infections, they said, "while the increase in viral load in BA.2 cases was striking."
Another preliminary study from Qatar also found this difference.
Laith Abu-Raddad, Professor of Population Sciences at Weill Cornell Medicine-Qatar, has been studying the efficacy of vaccines and boosters against BA.1 and BA.2.
One important difference between the two infections is a person's viral load, he said.
"It's definitely much higher" with BA.2 over BA.1, he said.
Her study found she was "nearly 10 times older."
Instead of penetrating deep into the lungs, as the delta variant did, omicron strains seem much more focused on the upper respiratory tract, where the nose meets the back of the throat, Abu-Raddad said.
He thinks that because the infection is concentrated there, that also helps it to spread efficiently when people talk, cough or sneeze.
A bright spot?
Perhaps a bright spot in the BA.2 picture is severity.
Although animal studies have suggested that BA.2 infection was not entirely mild, data on human infections from the UK, Denmark, and South Africa show that BA.2 is no more likely to result in hospitalization than with the BA.1.
This week, the UK Health Security Agency updated its data on the efficacy of the covid-19 vaccine in cases of the BA.2 variant.
Up to 14 weeks, boosters were still 90% effective in preventing serious illness in people over 65, pointing to an important way to make sure BA.2 doesn't bring us down.
Covid-19