However, available antibody therapies — typically given intravenously in hospitals — are substantially less effective against Omicron compared to earlier variants of the virus, the researchers said.
The lab tests also showed that some antibodies have entirely lost their ability to neutralise Omicron at realistic dosages, they said.
“The bottom line is we have countermeasures to treat Omicron. That’s good news,” said study lead author Yoshihiro Kawaoka, from the University of Wisconsin–Madison in the US.
“However, this is all in laboratory studies. Whether this translates into humans, we don’t know yet,” Kawaoka said.
The findings, published in the New England Journal of Medicine on Wednesday, corroborate other studies that show most available antibody treatments are less effective against Omicron.
The clinically available pills and antibodies were designed and tested before researchers identified the Omicron variant, which differs significantly from earlier versions of the virus.
When Omicron was identified, scientists feared that these differences, caused by mutations in the viral genome, might reduce the effectiveness of drugs designed to treat the original version of the virus.
In lab experiments using non-human primate cells, Kawaoka and colleagues at the National Institute of Infectious Diseases in Japan tested a suite of antibody and antiviral therapies against the original strain of the COVID-19 virus and its prominent variants.
They found that US pharmaceutical company Merck’s pill molnupiravir and the intravenous drug Remdesivir were just as effective against Omicron as they were against earlier viral strains.
Instead of testing Pfizer‘s Paxlovid pill, which is designed to be taken orally, the team tested a related drug by the company that is given intravenously.
The two drugs disrupt the same part of the viral machinery.
The researchers found that the intravenous form of the drug retained its effectiveness against Omicron, and this version is currently in clinical trials.
All four antibody treatments the researchers tested were less effective against Omicron than against earlier strains of the virus.
Two treatments, Sotrovimab by GlaxoSmithKline and Evusheld by AstraZeneca, retained some ability to neutralise the virus, according to the researchers.
However, they required anywhere from 3 to 100 times more of the drugs to neutralise Omicron compared to earlier versions, they said.
The study also showed that two antibody treatments by Lilly and Regeneron were unable to neutralise Omicron at common dosages.
The researchers said these findings are expected given how the Omicron variant differs from earlier strains of SARS-CoV-2 virus.
Omicron has dozens of mutations in the spike protein, which the virus uses to enter and infect cells.
Most antibodies were designed to bind to and neutralise the original spike protein and major changes to the protein can make antibodies less likely to attach to it.
In contrast, the antiviral pills target the molecular machinery the virus uses to make copies of itself inside cells, the researchers said.
The Omicron variant only has a few changes to this machinery, which makes it more likely that drugs will retain their ability to disrupt this replication process, they added.