Experts have credited omicron's swift spread to both biological and social factors, from its greater transmissibility to a relaxation of social precautions.

But new research shows another insidious factor at work.

A new study in JAMA Network Open finds more than half of people infected with the omicron coronavirus variant may not know they have it.

The findings are based on two years of blood samples taken from employees of Cedars-Sinai Medical Center in Southern California as part of a longitudinal study of COVID-19.

Only 44% of those with coronavirus antibodies knew they were infected.

Of the rest, only 10% reported symptoms, which they wrote off as colds or similar illnesses.

Lack of awareness and a resulting absence of precautions can substantially bolster community transmission.