XBB.1.5, the latest rising version of the virus that causes Covid-19, is following an all-too-familiar pattern: It’s yet another branch off the omicron variant of SARS-CoV-2, and it’s even more transmissible than past versions, gaining traction as people huddle indoors for the winter.

In the United States, XBB.1.5 is rapidly growing, causing almost 28 percent of new Covid-19 cases nationwide as of January 6, according to the Centers for Disease Control and Prevention. That’s up from just 4 percent of cases at the beginning of December. The new subvariant is driving the majority of reported infections in places like New York and New Jersey, and the rest of the country is poised to follow. It’s also been found in at least 29 countries.

“It is the most transmissible subvariant that has been detected yet,” Maria Van Kerkhove, an epidemiologist who serves as the World Health Organization’s technical lead for Covid-19, told reporters this week. “We are concerned about its growth advantage.”

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

Nonetheless, the new subvariant is still concerning. More than 42,000 people are hospitalized with Covid-19 now and more than 2,700 people are dying per week on average, according to the CDC. There are likely thousands more infections going undetected and unreported. These are the highest Covid-19 levels since January 2022 and the numbers are trending upward, but they’re still far, far below that massive spike last year fueled by the original omicron variant. Back then, more than 17,000 people were dying per week.

What’s different now is that there is much more immunity to Covid-19 across the population between vaccinations, bivalent boosters, and prior waves of infections. There are also effective treatments for Covid-19. Meanwhile, XBB.1.5 isn’t all that different from its omicron siblings, so immunity to subvariants like BA.5 still provides protection.

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

But Covid-19 isn’t the only health worry this season, as other infections like influenza and RSV are also filling hospital beds. And as tracking cases gets more difficult, it will be harder to stay ahead of the next form of SARS-CoV-2. That said, much of the ensuing harm of hospitalizations and deaths from Covid-19 can still be avoided if people take steps to prevent infection, get updated vaccines, and seek treatment if ill.

What makes XBB.1.5 different from other subvariants

XBB, the predecessor of XBB.1.5, was actually first identified in October last year. It’s a recombinant version of the virus, meaning it blends traits from two existing variants rather than branching off of just one. Those attributes 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, though most mutations are harmless or detrimental to the virus. XBB.1.5, however, has mutations in its spike protein. That’s the part of the virus most readily recognized by the immune system and the part that attaches to human cells to begin the infection process. These changes make the virus harder to detect and better at reproducing. Older adults and people who are immunocompromised remain at highest risk of dying from SARS-CoV-2.

Since XBB.1.5 draws on parts of older versions of the virus, protection against older variants likely still provide shielding against the worst effects of this new threat. In particular, the bivalent Covid-19 vaccine boosters contain the tools to target the original version of SARS-CoV-2 as well as the BA.4 and BA.5 subvariants of omicron.

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

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 is now reporting cases on a weekly rather than daily basis. Hospitalizations now are the best indicator of Covid-19’s trajectory, according to Mokdad. Health officials are also using wastewater surveillance to anticipate future infection surges.

There are effective treatments for Covid-19, like the antiviral drug Paxlovid. But there are no more monoclonal antibodies, previously a key backup treatment, that are authorized for use against the new subvariants. The loss of monoclonal antibody treatments is especially concerning for people at high risk of severe Covid-19 and people who can’t take Paxlovid because of its interactions with other medicines.

It’s also important to remember that hospitalizations and deaths are the worst Covid-19 outcomes, but not the only problems the disease can cause. Many people who faced even a mild course of the illness continue to suffer lasting respiratory and neurological symptoms, a condition known as long Covid. Scientists are still sorting out the 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 preventing infections in the first place.

Some parts of the country are now bringing back mask requirements. Improving indoor air quality is another key tactic for preventing infection. These tactics have the added advantage of slowing other respiratory infections. Influenza rates have already broken records this winter, with hospitalizations at four times the typical levels last month. Fortunately, influenza and RSV rates are declining, but they still remain high and are stressing hospitals, according to the CDC.

With Covid-19 rising again, patients could still have a hard time finding 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, like flu, from now on,” Mokdad said. “We will have a bad season and not so bad, depending on the variant.”

On its own, XBB.1.5 is unlikely to be the massive disruptor its ancestors were. But keeping it in check still demands vigilance and effort, which in turn can prevent harm from other diseases.