How concerned should I be about the new XBB.1.5 variant?

COVID Variants

XBB.1.5 has a transmission advantage, but this doesn’t make it a game-changing “super variant.” Keep calm, get boosted (and ramp-up precautions), and carry-on.

At this stage of the pandemic, new variants are *guaranteed* to come and go. Variants arise from random errors as viruses make copies of itself. Most of these errors will be neutral (or even harmful) to the survival of the virus. But if you roll the dice millions of times, you’re bound to hit a winner eventually. A mutation that gives the virus an advantage will spread and create more copies of itself, crowding out other less “fit” variants.

This is what we are currently seeing with XBB.1.5 in the U.S.

XBB.1.5 made up an estimated 40.5% of US cases in the week ending Dec 31st (up from 20% the previous week) and up to 75% of confirmed cases in the Northeast.

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❓Why is XBB.1.5 becoming dominant?

XBB.1.5 seems to have stumbled across two advantages that are powerful in combination.

➡️ Immune evasion: Antibodies from previous infection or vaccination don’t attach as well to the virus to block infection. Given the high levels of population immunity, most new variants today need to evolve a degree of immune evasion to spread.

➡️ Receptor binding: The variant is better at getting into our cells. SARS-CoV-2 famously uses the ACE2 receptor to gain entry to our cells. XBB.1.5 has a mutation that helps it bind more easily to ACE2 receptors and enter our cells, where it makes copies of itself.

Recent variants have often had one of these two advantages, but not both. This likely explains why XBB.1.5. is outcompeting other variants.

❓Is XBB.1.5 more severe?

So far there is no evidence that XBB.1.5 causes more severe disease, but it’s something we always keep an eye on. There are very few “immune naïve” people who have not been infected, vaccinated, or both. While new variants can evade existing immunity enough to infect people and spread, we still have significant protection against severe disease compared to when we had no immunity.

❓Do the boosters work against this variant?

Yes. A recent study in the New England Journal of Medicine (linked below) showed that blood from people who had received the Omicron-updated (or bivalent) booster neutralized all Omicron subvariants (including XBB, the parent of XBB.1.5) significantly better than those who only received one or two doses of the original vaccine. That means boosted people have more antibodies that can bind to XBB and prevent infection. Only 37.5 % of Americans aged 65+ have gotten their bivalent booster (and only 15% overall), so there is LOTS of room for improvement.

❓What can I do?

➡️ Stay up to date on your vaccines, including the bivalent booster. This is the most important thing you can do.

➡️ Masking, improving ventilation, rapid testing, and staying home when sick can all limit the opportunity of XBB.1.5 to spread

(Fun fact–this works for other viruses too!)


While they deserve a watchful eye, there will ALWAYS be new variants. Right now, it is XBB.1.5.

Don’t panic– but let’s use the tools we have to limit the spread and be less hospitable hosts.


Those Nerdy Girls


Additional Links:

“Neutralization against BA.2.75.2, BQ.1.1, and XBB from mRNA Bivalent Booster”

Twitter thread from Bloom Lab

Twitter thread from virologist Dr. Angie Rasmussen

“What we know and don’t know about the XBB.1.5 COVID subvariant”

CDC variant tracker

Link to original Those Nerdy Girls Facebook post