COVID Vaccine Effectiveness, Available Treatments: What to Know About XBB.1.5

The latest version of COVID-19 to raise concern is XBB.1.5 -- an omicron subvariant that now accounts for the biggest portion of cases in the US, according to the latest sequencing estimate from the US Centers for Disease Control and Prevention. 

While scientists believe XBB.1.5 appears to be the most transmissible version of the virus yet, vaccines and treatments are expected to continue to protect against severe disease. The spike protein in the BA.5 bivalent vaccine (the booster currently on the market) more closely resembles XXB.1.5 than the original version of the virus, according to a Jan. 11 analysis by Dr. Eric Topol, professor of molecular medicine at Scripps Research.

More data is needed to analyze XBB.1.5, including real-world data on vaccine effectiveness. A rapid risk assessment published Jan. 11 by the World Health Organization found that XBB.1.5 doesn't appear to carry mutations proven to make disease more or less severe. However, XBB.1.5 does appear to be more immune evasive. 

As with all new versions of COVID-19, we'll need more time to see how XBB.1.5 plays out in the US population. Hospitalization rates from COVID-19 have risen in recent weeks, though in much lower numbers compared with earlier spikes of the disease. According to The New York Times data tracker, there are also regional differences in how the virus is affecting the US. Hospitalization and case rates are high on the East Coast (XBB.1.5 was first sampled in New York), while they're much lower or falling out West.

Like with other versions of the virus that causes COVID-19 (especially omicron subvariants), researchers need more time to analyze real-world information on how the virus impacts different populations. The antiviral Paxlovid remains available for people at higher risk of getting severely ill, and adults in their 50s or up and people with chronic health conditions should talk to a health care provider about getting a prescription if they test positive or think they have COVID-19.

Here's what we know in the meantime.

What is XBB.1.5?

XBB.1.5 is part of the same family as the "original" omicron variant, which keeps mutating into more contagious versions of itself. XBB.1.5 comes from the XBB family, more specifically, and has mutations that may make it more able to evade immunity, according to a Jan. 6 post from the Johns Hopkins Bloomberg School of Public Health.

For roughly a year, the omicron variant has been dominant. Omicron, delta, alpha and earlier strains of COVID-19 were given their own Greek alphabet label and separate distinction as variants of concern because they proved troublesome enough to scientists in the way they evade vaccine immunity, cause more severe illness or create more serious problems. By contrast, although BA.5 was extremely contagious and caused more reinfections in people who already had COVID-19, it didn't necessitate a big enough change in our public health response to get its own name from the WHO, according to classification logic.

Will vaccines and treatments work against new subvariants? 

While XBB.1.5 is highly transmissible, we have reason to be optimistic about our vaccines and antiviral treatments remaining effective at preventing severe disease.

According to the Jan. 6 post from the Johns Hopkins Bloomberg School of Public Health, which referenced information from virologist Andy Pekosz, someone who was infected with another version of omicron may be susceptible to reinfection with XXB.1.5 because it's more immune evasive. 

But so far, our vaccines have continued to be protective against severe disease as the virus mutates, including the bivalent boosters currently available that were modeled after the BA.4/BA.5 subvariants. Lab studies have found that new boosters neutralize XBB.1.5 as well as BA.5, according to Topol's report, and that XBB.1.5's and BA.5's spike proteins have more in common.

BQ.1 and BQ.1.1, however -- both of which are still causing COVID-19 cases in the US -- have outflanked COVID-19 monoclonal antibody treatments 

Monoclonal antibody treatments, including Evusheld for immunocompromised patients, will likely no longer work against BQ.1 or BQ.1.1, according to a Dec. 28 update from the National Institutes of Health. Bebtelovimab, which is the monoclonal antibody made by Eli Lilly, also doesn't work against the more recent dominant subvariants of COVID-19, and the FDA revoked its authorization in the US on Nov. 30. Other monoclonal antibody treatments were removed from the US market earlier in the pandemic as the virus rendered them ineffective.

There's no indication that Paxlovid, an antiviral that higher risk patients can take in the first few days of their illness, won't work against the newer forms of omicron. If you believe you're at higher risk of getting severely ill from COVID-19, reach out to your doctor or pharmacist immediately after testing positive for COVID-19 or experiencing symptoms to determine your treatment options.