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A pair of conflicting reports released this week raised questions about whether the Covid-19 monoclonal antibody sotrovimab — the one such treatment that has continued to work against the Omicron variant — maintains its power against a sister form of the virus, BA.2.

A study posted to a preprint server Wednesday (meaning it has not been peer-reviewed) indicated that sotrovimab failed to neutralize BA.2 in lab experiments. “This new finding shows that no presently approved or authorized monoclonal antibody therapy could adequately cover” BA.2, the researchers wrote.

But Vir Biotechnology, which developed sotrovimab with GlaxoSmithKline, said its research indicated that BA.2 was not resistant to the therapy. The company said Thursday it was sharing its results with regulatory agencies and governments. The company’s press release did not include details of its study, but Vir said it would post the findings on a preprint server “in the coming week.”

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In a statement, Vir CEO George Scangos said the company was aware of the other research, but that “we believe that the 500 mg dose of sotrovimab is sufficient to retain activity against the BA.2 variant, just as it has against all other variants of concern and interest.”

Losing sotrovimab as an effective therapy would be another blow for doctors trying to keep Covid-19 patients out of hospitals — and for drug developers’ efforts to keep up with the evolving SARS-CoV-2 virus. Commonly used monoclonal antibodies from Lilly and Regeneron stopped working when confronted with Omicron, and the government halted distribution of the treatments as the newer variant overtook the previously dominant Delta variant.

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BA.2 is a sublineage of the Omicron variant. The most common version of the variant is technically called BA.1, but it’s broadly been referred to as Omicron because it is the iteration that raced around the world starting late last year and ignited new surges.

But research increasingly shows that BA.2 is in fact more transmissible than its sister. It’s been building up around the world and is dominant in countries including South Africa, the Philippines, Denmark, and India. It only accounts for a few percent of sequenced cases in the United States — the rest of infections are almost universally BA.1 — but it’s increasing.

There are other therapies beyond sotrovimab that can counter Omicron, including the oral drug Paxlovid from Pfizer and the intravenous antiviral remdesivir from Gilead. All have shown they can cut the risk of hospitalization from Covid-19 dramatically. But because of supply constraints of sotrovimab and Paxlovid, and some of the logistical challenges of delivering remdesivir, hospitals and doctors have been restricting their use to patients who are at high risk of developing severe disease — including unvaccinated or undervaccinated people with other health conditions and some vaccinated patients whose preexisting health status still leaves them vulnerable.

There is another oral antiviral authorized, Merck’s molnupiravir, but because of safety concerns and lower effectiveness data, experts say the other treatments are preferred.

A separate monoclonal antibody, AstraZeneca’s Evusheld, is authorized for reducing the risk of Covid-19 infection in people with compromised immune systems, not for treating people after they contract the virus.

One piece of good news is that new Covid-19 infection counts continue to fall in the United States — meaning that if the pattern holds, there will be less demand on doctors to try to find treatments for patients. BA.2 appears to be just somewhat more transmissible than BA.1, and many experts think that while it might slow the decline in infections in the United States, it shouldn’t lead to another spike in cases. 

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