XBB.1.5 fastest spreading variant since first Omicron wave

A new variant of SARS-CoV-2, XBB.1.5, has exploded across the United States, increasing from just 3% to more than 40% in a matter of weeks, according to projections from the Centers for Disease Control and Prevention (CDC). It is estimated that The mutant is believed to have emerged in New York, and researchers claim that early work shows it can bind to human cells more effectively than previous mutants.

As the world celebrates the start of a new year and enters the fourth year of the pandemic, another twist has emerged in the COVID story with the emergence of XBB.1.5. America. This variant appears to be the first recombination mutation to have a relevant impact on the pandemic trajectory.

A recombinant is a virus with a mixture of genomes of various variants. They can emerge when a person is simultaneously infected with two or more subspecies.

Researcher Emma Hodcroft says, “When a virus copies itself, it can do a ‘template switch’ between different genomes in the air.” There is no difference! But when different strains co-exist, we can find the offspring of these ‘chimeras’, i.e. recombinants!”

A number of SARS-CoV-2 recombinants have emerged recently, often making headlines heralding the threat of the emergence of ‘deltaclone’ strains. However, until very recently, SARS-CoV-2 recombinants have not spread large enough to generate associated waves of infection.

The first full-fledged recombinant appeared a few months ago and was called XBB. The unique variant is a combination of BJ.1 (aka BA.2.10.1.1) and BM.1.1.1 (aka BA.2.75.3.1.1.1).

XBB was first confirmed in India in August and Singapore was the first country to experience a wave of XBB in October. That wave peaked relatively quickly without causing major problems for the country’s healthcare system. This suggested that XBB might not cause a major problem globally, leading many researchers to breathe a sigh of relief.

But last year has shown that SARS-CoV-2 variants don’t stay the same for long, quickly picking up new mutations as XBB travels around the world. Enter XBB.1.5. This is probably the first concerned variant to emerge from the United States.

The first reported XBB.1.5 sample dates back to October 22nd in New York. A week later, variants were detected in Rhode Island and Washington. Sporadic detections were then tracked in India and Indonesia in early November, but by mid-month many European countries had detected the new subspecies.

Until last week, CDC’s variant tracking model had not separated XBB.1.5 from XBB. However, just before the new year began, a new forecast was released that suddenly included XBB.1.5, predicted to account for over 40% of all US cases.

Due to the slow pace of genome sampling, the CDC uses a modeling system called Nowcast to predict variant changes in real time. Current Nowcast models estimate that XBB.1.5 cases have doubled in the United States in the past week, with the northeastern part of the country making up more than 75% of all confirmed cases for him.

As always, the million-dollar question is whether XBB.1.5 causes more serious illness than previous Omicron variants. Eric Topol of the Scripps Research Institute said New York City he can be considered a pioneer of XBB.1.5, and that the region has seen a worrying rise in hospitalizations in recent weeks. According to Topol, the surge in hospitalizations may not be solely due to the rise of XBB.1.5, but the new variant is almost certainly playing a role.

“Of course, other factors may have contributed, such as weakened immunity, indoor/holiday gatherings, cold weather, lack of mitigation,” Topol explained in his blog. It’s worth noting that Covid hospitalization rates are the highest since late January (and are above the summer delta wave of 2021, but there’s ambiguity about how hospitalizations were classified then and now). .”

In addition to these epidemiological observations, new experimental data by researcher Yunlong Cao suggest that key mutations in XBB.1.5 may explain how this mutant is able to bind human cells more effectively. is showing. The mutation, named F486P, enhances the virus’ ability to bind to specific receptors on human cells. While other mutations help this particular mutant evade immune antibodies developed from previous infections, F486P means the virus can enter human cells more efficiently.

About a year ago, Jesse Bloom, a researcher who tracks virus evolution, warned that 486 was a key genomic site for future problematic SARS-CoV-2 mutations. Current vaccines and inducing antibodies specifically rely on the spots to effectively neutralize the virus.

As such, most mutations at site 486 lead to more immune-evading variants, but that is evolutionary compensation. was doing. This means that previous SARS-CoV-2 variants sacrificed cell binding for antibody evasion.

A major difference between XBB.1.5 and its parental lines (XBB and XBB.1) is the replacement of the F486S mutation with F486P, the only 486 site mutation that straddles the best of both worlds, sacrificing efficiency Avoids antibodies without Human Receptor Affinity.

“Thus, XBB.1.5 is not expected to shed more antibodies than XBB.1 (which already had the F486 mutation), but should have a higher affinity for ACE2.” Bloom recently explained“This means that it has a higher affinity for ACE2 (and possibly RBD [receptor binding domain] stability) makes XBB.1.5 communicable and proliferate. “

Yunlong Cao also speculates that the unique cell-binding affinity of XBB.1.5 may make it easier to generate additional new mutations in the future. So, of course, XBB.1.5 is unlikely to be the end of the road for SARS-CoV-2 variants.

“Another important observation is that the hACE2-binding affinity of XBB.1.5 is nearly comparable to that of BA.2.75. You may get more mutations.” Cao Cao proposed on Twitter“XBB.1.5 just hasn’t felt much immune pressure yet.”

This does not mean that XBB.1.5 necessarily causes more severe illness than previous variants, but XBB.1.5 infects more people, more easily, and subsequently to COVID-19. It means that we may be able to track the most vulnerable people: the elderly, the unvaccinated, and the immunocompromised. While XBB.1.5 will never bring us back to the drawing board, it certainly has the potential to prolong the pandemic, spike infections, and put pressure on the healthcare system once again.

Another potential concern of XBB.1.5 is its long-term COVID impact. Akiko Iwasaki, a researcher at Yale University, said the increased cell-binding affinity for this new mutant could lead to a new wave of long-lasting COVID infections in her, due to the virus’ enhanced ability to infect human cells. pointed out that there is

“I am concerned because XBB.1.5 presumably enhances the ability to infect cell types that express even lower levels of ACE2.” Iwasaki recently tweeted“This may lead to increased tropism and increased persistence in long-lived cell types.”

Vaccine scientist Peter Hotez said XBB.1.5 raises many new questions that need answers, but one should not be afraid or overreact. He points to recently published studies showing that new bivalent COVID vaccines are still effective against new variants such as his XBB. Therefore, although XBB1.5 may have increased cell-binding affinity, our vaccine may still help reduce disease severity.

Should we consider adding new targets to update the vaccine in 2023, Hotez asks. As the new year progresses, SARS-CoV-2 continues to morph around our defenses.

“I thought 2022 would be better than 2021,” Hotes wrote on Twitter. “Maybe a little bit, but not a lot…I hope 2023 does better than 2022.”