Most US doctors have no way to determine which variant of the coronavirus a patient is carrying, a distinction that could mean the difference between life and death.

High-risk patients carrying the Delta variant could benefit greatly from two particular monoclonal antibody treatments shown to reduce hospitalisation and death. But those medications would most likely do nothing for patients with omicron, who would only respond to a third antibody treatment that is in very short supply.

While US officials have endorsed using a workaround test that can identify omicron’s genetic signature, experts say it’s not feasible for large health systems facing a crush of patients to employ in each case.

That makes treating patients challenging in places like Maryland, where cases are spiking and omicron accounts for roughly 58 per cent of them. The Delta variant is also holding strong in the Great Plains and swathes of the West, including California.

While there is no approved test to determine each individual’s variant, a national network of state and other labs use genome-sequencing tests to track variants broadly in communities. Health systems then use those regional estimates or their own data to decide which antibody treatments to use in their clinics and hospitals.

Many of them concluded that a community of largely Delta patients would benefit most from the antibody drugs made by Regeneron and Eli Lilly, while communities where omicron patients are predominant would benefit from antibodies from GlaxoSmithKline and Vir Biotechnology.

Federal officials have dabbled with making the decision for the nation. On December 23, they stopped shipments of antibody treatments by Eli Lilly and Regeneron after the Centers for Disease Control and Prevention said 73 per cent of US Covid cases were omicron.

An outcry followed from Republican political leaders, who argued that some people in their states were still infected with Delta. And on Tuesday, the CDC slashed its estimate of national Omicron cases to 59 per cent. On December 31, federal officials resumed national shipping all of the antibody treatments.

For the next few weeks, as the country grapples with this uneven mix of both variants, tailoring treatments to each patient will be “extraordinarily difficult”, said Dr Alex Greninger, assistant director of the clinical virology laboratories at the University of Washington Medical Center.