In late 2022 and early 2023, an Omicron subvariant called XBB.1.5 spread rapidly across the United States, especially in the Northeast. Scientists described it as one of the most immune-evasive and transmissible versions of the virus at the time. Looking at what made it spread so well helps explain why COVID-19 vaccines continue to be updated.
How fast it spread
Within weeks of being identified, XBB.1.5 became a major share of U.S. COVID-19 cases. By the end of December 2022, CDC estimates suggested it was causing around 40% of new cases nationally and roughly three-quarters of cases in some northeastern states, including New York and New Jersey and parts of New England.
Two traits that gave it an edge
- Immune evasion. Research led by Dr. David Ho at Columbia University, published in the journal Cell, found that antibodies from vaccination and prior infection were far less able to neutralize the closely related XBB.1 subvariant than earlier Omicron strains, dozens of times less effective in lab tests. Dr. Ho called it “alarming” that such subvariants could lead to more breakthrough infections and reinfections.
- Better binding. XBB.1.5 also carried a mutation that helped it attach more tightly to ACE2, the receptor the virus uses to enter human cells. Computational virologist Jesse Bloom of the Fred Hutchinson Cancer Center noted that this change was clearly helping it spread.
Together, these traits meant XBB.1.5 could both slip past existing antibodies and infect cells efficiently.
Why variants with these traits take over
When most people have some immunity, a variant that can sidestep it has a big advantage: it can infect people who would have been protected against older versions. If it also enters cells more efficiently, each infected person can pass it to more people. Variants with both advantages tend to outcompete others quickly, which is exactly the pattern XBB.1.5 showed. Scientists track these traits closely so they can anticipate which variants are likely to spread next.
What immune evasion does and doesn’t mean
When a variant evades antibodies, people are more likely to catch it even if they’ve been vaccinated or infected before. But antibodies are only part of the immune system’s defenses. Other parts, such as T cells, tend to keep recognizing new variants, which is why vaccines continued to provide important protection against severe illness, hospitalization and death even as infections rose.
Why vaccines get updated
As the virus evolves, vaccine makers and public health agencies periodically update COVID-19 vaccines to better match the strains that are circulating. In 2023, updated vaccines were formulated to target an XBB-family subvariant for exactly this reason. This is similar to how flu vaccines are updated each year.
What this means for you
- Stay up to date with recommended vaccines. Updated shots are designed to match newer variants and restore protection that fades over time.
- Don’t assume past infection protects you. Immune-evasive variants can cause reinfection.
- Test when you have symptoms and stay home when you’re sick.
- Ask about treatment early if you’re at higher risk. Antiviral medicines remained effective against XBB-family variants.
- Use extra precautions during surges, such as better ventilation and a well-fitting mask in crowded indoor spaces.
The bottom line
XBB.1.5 spread quickly because it could evade antibodies and bind tightly to human cells. Its rise is a clear example of why COVID-19 vaccines are updated and why staying current with them still matters.
This article is for general information and isn’t medical advice. Talk with your healthcare provider about your individual needs.
