Updated: March 15, 2022 10:41 AM EDT | Originally published: January 28, 2022 11:00 AM EST

Even as SARS-CoV-2 gets better at spreading and evading some of the immune protection that people have built through infections or vaccination, new variants inevitably arise. That’s been the case with Omicron (also known as BA.1), which has generated several subvariants, including BA.2, which is beginning to surge in parts of Europe and the U.S. as countries loosen mask-wearing requirements and more people attend large public gatherings.

While cases of COVID-19 have been dropping since the beginning of the year in some parts of the world, including the U.S., it’s too early to predict what BA.2 might mean for the course of the COVID-19 pandemic. But detecting and monitoring such variants early will help public health experts better identify which ones pose the most danger to people’s health, and therefore require more stringent mitigation measures. Here’s what we know so far.

What is the BA.2 variant?

BA.2 appears to be a descendant of Omicron, or BA.1. Researchers at the World Health Organization first reported increasing numbers of BA.2 infections in January in Denmark, India, and the U.K. Since then, dozens of countries have reported BA.2 cases, including in nearly every state in the U.S. In about 18 countries, BA.2 is already the dominant SARS-CoV-2 strain responsible for new COVID-19 cases.

Scientists found BA.2 thanks to more widespread genetic sequencing, which is helping them detect changes in SARS-CoV-2 more quickly and determine how those mutations might affect human health. In the U.S., such genetic sequencing shows that BA.2 cases continue to increase, accounting for an estimated 23% of cases in the country.

BA.2’s quick foothold around the world is not surprising, given that studies show that variant may be up to 30% more transmissible than the original Omicron—which means that without mitigation measures such as mask-wearing, handwashing, and social distancing, the latest version of Omicron can spread rapidly in communities.

Is the new BA.2 variant more dangerous than Omicron?

Like Omicron, BA.2 contains numerous mutations, according to researchers at the Massachusetts Consortium on Pathogen Readiness, a collaboration of academic researchers from different institutes in the Boston area who monitor genetic changes in SARS-CoV-2. About 20 mutations have been located in the antibody-binding regions that vaccines target. BA.2 also contains some mutations not found in BA.1, but scientists aren’t sure yet what those changes mean. Some of them may make BA.2 more difficult to detect using current PCR testing methods, which is why some scientists have dubbed it the “stealth variant.”

A recent study in animals from Japanese scientists—which was published on the pre-print server bioRxiv but has not been peer-reviewed—suggests that BA.2 may contain mutations that could lead to more severe disease than Omicron. These mutations may also make the virus resistant to treatments, including monoclonal antibodies, that doctors are currently using to protect vulnerable patients from getting very ill, the research suggests. BA.2 may also escape vaccine protection, but boosters can make severe disease 74% less likely, one Danish study found.

Should you be worried?

Studies from around the world about the severity of BA.2 are mixed. Dr. Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention, says the agency’s scientists are continuing to monitor BA.2’s effects on disease severity.

Will the current COVID-19 vaccines work against BA.2?

The protection from vaccines wanes over time, which is why public health officials have authorized booster shots in most countries. For now, the evidence suggests that vaccines generate both immediate and longer lasting immunity against all variants of SARS-CoV-2. The antibodies that the shots catalyze may only protect against infection for a relatively short amount of time—Moderna reported that even after a booster dose of its vaccine, these virus-neutralizing antibodies tend to wane after six months—but the body’s immune response to vaccination also includes T cells. These are more durable and are aimed at more conserved regions of the virus; T cells seem to provide good and lasting protection against severe disease from every variant so far, even if people do get infected.

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