What we know about BA.2 – now the leading cause of Covid-19 in the US
(CNN) – The highly contagious Omicron BA.2 subvariant is now the dominant coronavirus strain in the United States, causing more than half of all Covid-19 infections last week, the Centers said Tuesday. for Disease Control and Prevention of the United States.
The new numbers come from the CDC genomic surveillance. Based on its models, the agency says BA.2 caused between 51% and 59% of all new Covid-19 infections in the United States in the week ending March 26, compared to about 39% of all new infections the previous week.
The hardest hit region was the Northeast, where BA.2 caused more than 70% of all cases. The south and mountain west saw the fewest cases. BA.2 caused just over a third of infections in these regions last week.
“We are not immune to what is happening in Europe”
Although BA.2 is only in its early stages in the United States, it has had extensive tours in many other parts of the world, including Southeast Asia and the Western Pacific, and is completing its tour European.
According to the World Health Organization, BA.2 is also the leading cause of Covid-19 worldwide, surpassing two other Omicron lines, BA.1 and BA 1.1, to become the dominant strain. Since its takeover, the number of international cases – which had been falling since the first week of January – has risen again.
In the UK, which has a more vaccinated population than the US, a combination of lifting restrictions, waning immunity and an even more contagious version of the virus has created a new BA.2 wave. Covid-19 cases, hospitalizations and deaths have been trending up since late February, and now the weekly average for new cases is roughly where it was at the end of January.
However, BA.2 infections did not reach the peaks seen with BA.1. Number of cases appear to be stabilizing in the UK, although hospitalizations and deaths continue to rise.
Throughout the pandemic, the US trailed the UK by around three weeks, so when cases started to rise there, health officials here took notice.
In America, BA.2 has been gaining momentum since late January and the number of cases has plateaued. However, this flattening hides regional differences. In 13 states, the average weekly number of new cases is increasing and it has stopped falling in 14 others, according to data collected by Johns Hopkins University.
It’s still unclear what this sub-variant 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 predominant and leading to a resurgence, and the likelihood of that not happening in the United States is pretty low, really,” Frieden said. “I think part of the reason we’re plateauing is that we’re about to start going up again.”
Frieden doesn’t think it’s a coincidence that the U.S. Food and Drug Administration on Tuesday authorized additional booster shots for Americans 50 and older, the same day the CDC deemed BA.2 to be dominant. .
But anyone can guess how much the cases will rise, how many people will need hospital care and whether the nation will continue to see a staggering number of deaths.
Faced with “a familiar uncertainty”
Most predictions for BA.2 in the United States have not been disastrous.
The Institute for Health Metrics and Evaluation at the University of Washington Covid-19 Forecastupdated last week, predict that BA.2 will not cause a new surge in the United States.
But they say we could see something like what happened in South Africa, where BA.2 has quietly replaced its cousin BA.1 as the main cause of Covid-19 infections – with no increase in cases or deaths. . Instead, he extended 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 coronavirus waiting game we play every few months “a familiar uncertainty.” He dedicated his latest podcast to “all of us who remain confused as to what the immediate or interim future looks like with Covid”.
How strong is our immune wall?
There are a multitude of reasons why it is difficult to know what BA.2 can do. The CDC estimates that 37 million Americans – about 1 in 11 – contracted Covid-19 over the winter, during the first Omicron wave. Many others are immune to vaccination and boosters. So, based on random blood samples, the CDC says 95% of Americans may now have some degree of immunity to Covid-19.
Dr. Jorge Salinas, an infectious disease expert at Stanford University, calls it an immunological wall.
“It protects us against future surges,” he said. “However, this wall deteriorates over time. The longer it lasts after a wave or after a vaccination, the more the wall breaks down.”
Studies have determined determined that BA.2 escapes our vaccinations about as well as the original Omicron, so boosters are needed to restore protections against these variants. But less than half of the US population ages 12 and older has received a recommended third dose.
Adults over 65 are of greatest concern, as they are mostly at risk of becoming seriously ill with Covid-19. One in three people over the age of 65 in the United States have not received a critical third dose of vaccine.
“The real problem is that a large portion of our population is made up of under-vaccinated seniors,” Frieden said. “It’s our Achilles heel.”
Questions about reinfection
The other variable is in the virus itself.
Omicron has thrown our immune defenses into a loop. It was so different from the strains of coronavirus that preceded it that many people who got sick with Delta or other early strains found themselves infected again.
BA.2 has about 40 amino acid changes from Omicron’s BA.1, making it about as different from its cousin as Alpha, Beta and Delta were any of the other. Some have wondered if BA.2 could re-infect people who had had BA.1.
A great study from Denmark suggests that these types of reinfections are possible but rare.
Searching more than 1.8 million infections found just 1,739 cases in which people tested positive for Covid-19 twice in a two-month window. Of these, 47 were BA.1 infections that were followed by BA.2.
When the researchers took a closer look, they found that these types of reinfections tended to affect young, unvaccinated people, mostly children. And their symptoms tended to be mild.
The study was published in preprint form, which means it has not yet been reviewed by outside experts and published in a medical journal.
Studies indicate high viral loads
BA.2 is extremely contagious. Some epidemiologists said its basic reproduction number can reach 12, which means that each sick person infects an average of 12 others. That would put it on par with measles, which also spreads through the air. The base reproduction number for BA.1 is estimated to be around 8.
In a pre-printed study in Sweden, the researchers measured viral levels in swabs from the back of the nasal cavity. They found nearly twice as much viral RNA in samples from BA.2 patients as in those who tested positive for BA.1, “indicating 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 surprising.”
Another one Qatar pre-printed study picked up this difference, too.
Laith Abu-Raddad, professor of population science at Weill Cornell Medicine-Qatar, has studied the effectiveness of vaccines and boosters against BA.1 and BA.2. A major 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. His study found it to be “almost 10 times higher”.
Instead of going deep into the lungs, as Delta did, Omicron strains seem much more concentrated in the upper respiratory tract, where the nose meets the back of the throat, Abu-Raddad said.
He thinks that because the infection concentrates there, it also helps it spread effectively when people talk, cough or sneeze.
A bright spot?
Perhaps a bright spot in the BA.2 picture can be gravity.
Although animal studies have suggested that BA.2 infection was not entirely benign, data from human infections in the UK, Denmark and South Africa show that BA.2 is not is no more likely to result in hospitalization than BA.1.
This week, the UK Health Security Agency updated its data on the efficacy of the vaccine against BA.2. Up to 14 weeks, boosters were still 90% effective in preventing serious illness in people over 65, indicating an important way to ensure BA.2 doesn’t bring us down.
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