The latest dominant COVID-19 strain in the United States appears to be far better at evading antibodies than its predecessors, scientists say.

Data from the Centers for Disease Control and Prevention this week showed BA.5, an omicron subvariant, now accounts for an estimated 54% of cases in the U.S.

In a recent blog post, Dr. Eric Topol, a professor of Molecular Medicine at The Scripps Research Institute, called BA.5 “the worst version of the virus that we’ve seen.” 

“It takes immune escape, already extensive, to the next level, and, as a function of that, enhanced transmissibility, well beyond Omicron (BA.1) and other Omicron family variants that we’ve seen,” he wrote.

The BA.5 subvariant, as well as BA.4, which accounts for 17% of infections today, has a spike protein mutation that allows it to escape some antibodies from vaccination and previous infection, including from omicron.

A study conducted by researchers at Columbia University, published Tuesday in the scientific journal Nature, analyzed individuals who received at least three doses of an mRNA vaccine or received two shots and then were infected with omicron. They found the BA.4 and BA.5 subvariants are at least four times more resistant to protection than the other versions of omicron.

“The virus is continuing to evolve, as expected, and it is not surprising that these new, more transmissible subvariants are becoming more dominant around the world,” Dr. David Ho, the professor who led the study, said in a news release. “Understanding how currently available vaccines and antibody treatments stand up to the new subvariants is critical to developing strategies to prevent severe disease, hospitalizations, and deaths—if not infection.”

Ho’s team also tested 19 monoclonal antibody treatment and found that only one — designed by Eli Lilly and targeted specifically at omicron — remained highly effective against BA.2.12.1, BA.4 and BA.5. 

The study, however, suggests vaccines still offer good protection against severe disease, and health officials say the oral treatment Paxlovid also helps prevent serious illness.

The CDC says there is no evidence BA.4 and BA.5 lead to more severe illness than other omicron subvariants, and researchers in South Africa recently reported that a spring surge there fueled by those strains did not appear to lead to significantly higher rates of severe illness than the earlier omicron wave.

While those subvariants might not be more virulent, they can still lead to an increase in hospitalizations by infecting a larger volume in people. 

COVID-19 hospitalizations in the U.S. have been rising for nearly three months. Nearly 30,000 people are hospitalized in the U.S. with COVID, according to the CDC. That number was below 10,000 in early April.

Meanwhile, official case and death counts have remained relatively steady. As of Tuesday, the seven-day average for infections was 106,178 and deaths were averaging 267 a day.

But the case numbers are believed to be a vast undercount due to the popularity of at-home tests that are not reported to health departments, the closure of many public testing sites run by state and local governments, and the growing number of states that have stopped providing daily updates on cases, hospitalizations and deaths.

Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security at the Bloomberg School of Public Health, told The New York Times earlier this week he estimates there are actually about 1 million infections per day.

The CDC currently deems 19.5% of counties to be high-risk areas and 35.5% to be medium risk. The agency recommends people in high-risk counties wear masks indoors in public. 

Thirty-five states have seen COVID cases rise over the past two weeks, according to data compiled by The New York Times. Utah, Oklahoma and Minnesota have seen cases double or nearly double in that time. Alaska, California, Florida and Hawaii have the highest per-capita infection rates.