A new COVID-19 variant, XBB.1.5, is spreading rapidly throughout the US.
In December 2022, the proportion of new COVID-19 infections due to this Omicron offshoot has increased from 4 percent to 18 percent, according to a January 6 release from the US Centers for Disease Control and Prevention, and is projected to rise further still. In some parts of the country, it constitutes more than half of all new infections.
According to the World Health Organization, XBB.1.5 is the most transmissible form of Omicron yet.
What should people know about XBB.1.5? Do vaccines and treatments work against it? Can tests pick it up? Will hospitals become overwhelmed again? Should children wear masks to school again? And could there be even more worrisome variants that emerge in the future?
To guide us through these questions, I spoke with CNN medical analyst Leana Wen, an emergency physician, public health expert and professor of health policy and management at the George Washington University Milken Institute School of Public Health. She is also the author of Lifelines: A Doctor's Journey in the Fight for Public Health.
CNN: What should people know about the latest COVID-19 variant, XBB.1.5?
Dr Wen: People should not be surprised that there is a new variant. The more viruses replicate, the more they mutate.
Most mutations do not confer evolutionary advantage and won't spread further but some do.
There are three key questions to ask about new variants. First, is it more contagious? Second, does it cause more serious disease? And third, is it more immune-evasive, meaning it undercuts the protection of existing vaccines and treatments?
The mutations XBB.1.5 has acquired have made it more contagious.
A more transmissible strain has the evolutionary advantage that it will spread faster than others and, therefore, could displace other strains. This is a trend seen throughout the coronavirus pandemic - new, even more, transmissible strains replacing their predecessors and becoming dominant.
The good news is, thus far, this strain does not appear to cause more severe disease. Like other Omicron descendants, it probably causes milder illness compared with the Delta variants that predated Omicron.
There are some studies that suggest XBB.1.5 is more immune-evasive compared with previously dominant Omicron strains. Further research is underway to identify the degree of immune protection afforded by existing vaccines; the White House's COVID-19 response coordinator Ashish Jha said "data suggests that if you've been vaccinated, if you've gotten that updated bivalent booster, you're still going to have a good amount of protection", during an interview Friday with CNN's Kate Bolduan.
But even if it turns out these vaccines don't hold up as well against infection with XBB.1.5, they will probably protect well against severe illness - which underscores the need for people to receive the updated booster if they are eligible.
CNN: Can tests pick up this new variant?
Dr Wen: PCR tests definitely can and there's no reason to think that this variant won't be picked up by rapid home antigen tests. If you have symptoms or are exposed to someone with the coronavirus, you should certainly get tested.
The tests won't show you which strain you picked up but they should detect circulating variants.
CNN: Do existing treatments work against XBB.1.5?
Dr Wen: Antiviral treatments like Paxlovid should work against XBB.1.5. Unfortunately, monoclonal antibody treatments probably don't.
In November, the US Food and Drug Administration withdrew their authorisation of the last remaining monoclonal antibody because of its lack of efficacy against new variants. And on January 6, the agency issued a statement that the preventive antibody Evusheld may be ineffective against XBB.1.5.
On a policy level, it's critical there are urgent investments into better treatments. There are many people vulnerable to severe outcomes due to COVID-19 and we need to have a wider range of effective treatments available for them.
CNN: Could hospitals become overwhelmed again?
Dr Wen: COVID-19 infections could rise in the coming weeks due to a combination of this new variant and the fact that many people will have traveled and gathered over the holidays. I don't think the surge will be nearly as bad as the initial Omicron wave in early 2022, though, because of the large proportion of Americans who have by this point already contracted COVID-19 and have some baseline immunity to it.
Increasing booster rates, particularly among the elderly, will help blunt the rise in hospitalisations. It's a major problem that only about a third of Americans ages 65 and older have received the updated bivalent booster, which has been shown in a recent study to reduce hospitalization by 73 percent in this age group.
CNN: How much should people worry about XBB.1.5?
Dr Wen: It depends on the individual.
There are many people who are not concerned about contracting COVID-19. They may be young and healthy and unlikely to become severely ill due to the coronavirus. Maybe they have just recovered from a previous infection and are protected against serious illness for several months. Or maybe the downside of continuing precautions is significant to them.
I don't think it's wrong for people to proceed with their pre-pandemic routines, considering that XBB.1.5 is not likely to be the last variant of concern we see - and that it doesn't appear to cause more severe disease.
On the other hand, there are many people who are worried about becoming severely ill from COVID-19. People who are elderly or who have underlying health conditions should speak with their physician about their risk of severe illness due to COVID-19.
If they are at high risk even after getting the bivalent booster, they should consider additional precautions to avoid infection while this highly transmissible variant is circulating. That includes asking others to take a rapid test prior to socialising and wearing a high-quality N95 or equivalent mask while in crowded indoor places.
CNN: Some school districts are bringing back mask mandates. Should children wear masks to schools again?
Dr Wen: This will depend on the family. If everyone is generally healthy, the parents or caregivers are going to work without a mask and all members are socialising freely with others outside of school, then it wouldn't add much more protection to masks in the classroom.
On the other hand, families that are still taking many precautions because of, for example, a severely immunocompromised household member might decide to all masks while in crowded indoor spaces.
My children have not been masking in school since the beginning of this school year and I don't currently plan for this to change. We would reconsider if a new variant emerges that causes much more severe disease, but that does not appear to be the case with XBB.1.5.
CNN: Could there be even more worrisome variants that emerge in the future?
Dr Wen: Yes. This is the reason why genomic surveillance is so important. We need to identify and study new variants as they emerge.
This is part of our "new normal" - there will be new variants that, from time to time, lead to surges of infections.
The key is to make sure people are still protected against severe disease and to keep hospitals from becoming overwhelmed. And we must make sure everyone makes use of the tools we have available, including vaccines.
