How XBB.1.5, the new Covid-19 variant, is shaping the pandemic

XBB.1.5, the latest emerging version of the virus that causes Covid-19, follows an all-too-familiar pattern: it’s yet another branch of the omicron variant of SARS-CoV-2, and it’s even more transmissible than previous versions, gaining traction as people creep indoors in the winter.

In the United States, XBB.1.5 is growing rapidly and, as of January 6, caused nearly 28 percent of new Covid-19 cases nationwide, according to the Center for Disease Control and Prevention. That’s more than just 4 percent of cases in early December. The new subvariant accounts for the most reported infections in places like New York and New Jersey, and the rest of the country is ready to follow. It has also been found in at least 29 countries.

“It is the most transmissible subvariant detected to date,” said Maria Van Kerkhove, an epidemiologist who serves as the World Health Organization’s technical lead for Covid-19, told reporters this week. “We’re concerned about the growth benefit.”

It is alarming and frustrating that another slippery variant has emerged. But XBB.1.5 is unlikely to lead to a huge increase in hospitalizations and deaths, as seen in previous winters, now that the vast majority of people in the US have some degree of protection against the virus.

Nevertheless, the new sub-variant is still concerning. More than 42,000 people are now hospitalized with Covid-19, and more than 2,700 people die per week on average, according to the CDC. There are probably thousands more infections that go undetected and unreported. These are the highest Covid-19 levels since January 2022 and the numbers are on an upward trend, but they are still well below last year’s massive spike fueled by the original Omicron variant. At the time, more than 17,000 people were dying every week.

What is different now is that there is a lot more immunity to Covid-19 in the population between vaccinations, bivalent boosters and previous waves of infections. There are also effective treatments for Covid-19. Meanwhile, XBB.1.5 isn’t that different from its omicron siblings, so immunity to sub-variants like BA.5 still provides protection.

“We do not expect a large increase in hospitalizations from this variant, as it is similar to the previous one,” said Ali Mokdada professor of health metrics sciences at the Institute for Health Metrics and Evaluation at the University of Washington, in an email.

But Covid-19 is not the only health concern this season, as other infections such as flu and RSV are also filling hospital beds. And as case detection becomes more difficult, it will become more difficult to stay ahead of the next form of SARS-CoV-2. That said, much of the resulting damage from hospitalizations and deaths from Covid-19 can still be avoided if people take steps to prevent infection, get updated vaccines and seek treatment when they are sick.

What makes XBB.1.5 different from other sub-variants

XBB, the predecessor of XBB.1.5, was actually first identified in October last year. It is a recombinant version of the virus, meaning it combines features of two existing variants rather than splitting off from just one. Those characteristics allow it to spread more easily than any other known version of SARS-Cov-2.

Like all viruses, SARS-CoV-2 mutates as it replicates, although most mutations are harmless or harmful to the virus. However, XBB.1.5 does mutations in its spike protein. That is the part of the virus that is most easily recognized by the immune system and the part that attaches to human cells to initiate the infection process. These changes make the virus harder to detect and easier to reproduce. Older adults and people with weakened immune systems are still at the highest risk of dying from SARS-CoV-2.

Since XBB.1.5 uses parts of older versions of the virus, protection against older variants is likely to still protect against the worst effects of this new threat. In particular, the bivalent Covid-19 vaccine boosters contain the tools to address both the original version of SARS-CoV-2 and the BA.4 and BA.5 sub-variants of omicron.

“The vaccines and the booster are still very effective against hospitalizations and deaths,” Mokdad said. Starting this week, 15 percent of Americans over age 5 have received a bivalent booster vaccination against Covid-19.

At the same time, the public health landscape around Covid-19 is changing. Face mask mandates and social distancing rules are all but gone. Fewer people are bothering to get tested at all, and many of those who do are testing at home, so public health officials have less insight into the spread of the disease. The CDC now reports weekly rather than daily cases. According to Mokdad, hospital admissions are now the best indicator of Covid-19’s trajectory. Health officials also use wastewater monitoring to anticipate future infection spikes.

There are effective treatments for Covid-19, such as the antiviral drug Paxlovid. But there are no longer any monoclonal antibodies, formerly an important back-up treatment, that have been approved for use against the new subvariants. The loss of monoclonal antibody treatments is of particular concern to people at high risk of severe Covid-19 and those who cannot take Paxlovid due to its interactions with other drugs.

It is also important to remember that hospitalizations and deaths are the worst consequences of Covid-19, but not the only problems the disease can cause. Many people who went through even a mild course of the disease continue to experience persistent respiratory and neurological symptoms, a condition known as lung Covid. Scientists are still figuring it out long-term effects of the virus. And the more the virus spreads, the more likely it is to mutate in a dangerous way, so it’s worth trying to prevent infections in the first place.

Some parts of the country are now reduce mask requirements. Improving indoor air quality is another important tactic for preventing infection. These tactics have the added benefit of slowing down other respiratory infections. Flu rates have already broken records this winter, with hospitalizations four times normal last month. Fortunately, flu and RSV rates are declining, but they remain high and put pressure on hospitals, According to the CDC.

With Covid-19 on the rise again, patients may still struggle to find relief. Similar patterns could emerge again next season, so it’s important to keep pandemic tools sharp.

“Covid will be with us for a while from now on, just like flu,” Mokdad said. “We will have a bad season and not that bad depending on the variant.”

On its own, XBB.1.5 is probably not the massive disruptor that its ancestors were. But keeping it under control still requires vigilance and effort, which in turn can prevent damage from other diseases.