Other versions of Omicron are gaining ground in the United States, according to CDC estimates

The Omicron sub-variants known as BA.4 and BA.5 now account for 13% of new coronavirus cases in the United States, up from 7.5% a week ago and 1% in early May. according to new estimates by the Centers for Disease Control and Prevention.

The spread of sub-variants adds further uncertainty to the trajectory of the pandemic in the United States, where the current case count is likely to be a significant underestimate. But it’s unclear whether it leads to a major new wave of infections or spikes in hospitalizations and deaths, scientists warn.

The new figures, released Tuesday, are based on CDC models and estimates they missed the mark before. But the general trend suggests that BA.4 and BA.5 could outperform the competition the other two sub-variants of Omicron, BA.2 Other BA.2.12.1, which together account for the majority of cases in the United StatesDenis Nash said, an epidemiologist at the CUNY Graduate School of Public Health & Health Policy.

“This could happen very quickly,” said Dr. Nash said.

Data on BA.4 and BA.5, first collected in South Africa in early 2022, remain limited. But these variants appear to spread more rapidly than previous versions of Omicron, such as BA.2, and may be more effective in avoiding the immune system’s defenses. So far, there isn’t much evidence that they cause more serious diseases, although more studies are needed.

“How analog is it for us?” He said. “I think it’s possible we might see another wave” powered by BA.4 and BA.5. But, if the US follows South Africa’s lead, which isn’t a guarantee, the surge could be more modest than previous waves and cause a less pronounced increase in hospitalizations and deaths, he said.

In recent weeks, the United States has been averaging about 100,000 new cases a dayaccording to a New York Times database, up from fewer than 30,000 at the end of March.

And while the hospitalizations they began to decline in the Northeast, they are climbing elsewhere. Trends in the number of new deaths, which have fluctuated between around 250 and 400 per day in the past month, are less clear, although the metric is on average much lower than Omicron’s winter peak. (Delays in reporting data during Memorial Day holidays make recent statistics less reliable.)

Globally, the most recent data suggests that BA.4 and BA.5 still represent a relatively small share of cases, but that may change in the coming weeks. In a recent reportthe UK Health Safety Agency observed that in many countries the two sub-variants were replacing BA.2 as quickly as BA.2 replaced the original version of Omicron.

In the United States, the new sub-variants are gaining ground. During the week ending June 4, BA.5 accounted for approximately 7.6% of cases and BA.4 accounted for 5.4% of cases, compared to 4.2% and 3.3% for the week, respectively. previous one.

Subvariants have become particularly common in parts of the southern United States. In the region that includes Arkansas, Louisiana, New Mexico, Oklahoma, and Texas, BA.4 and BA.5 account for more than one in five infections, according to the new data.

The data on waste water also show the diffusion of sub-variants. In Missouri in early May, for example, BA.4 and BA.5 showed up in a small number of state sewers or in geographic areas where wastewater flows into a single treatment plant or other collection point. . Subvariants are now detectable in more than half of them, said Marc Johnson, a University of Missouri virologist who analyzed the state’s wastewater.

Preliminary evidence from laboratory researches suggests that unvaccinated people who were infected with the original version of Omicron, known as BA.1, could easily be reinfected by BA.4 or BA.5. (Vaccinated people are likely to fare a little better, the study suggests.)

This immune evasion, in addition to the natural loss of protection against infections over timecould explain why the more recent sub-variants have been able to spread so rapidly.

Sarah Cahalan contributed.

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