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An observational study of 210 adults in California with detectable SARS-CoV-2 antibodies during an Omicron variant wave shows that 56% didn't know they had been infected, fueling concerns about asymptomatic transmission, according to a report today in JAMA Network Open.

Researchers at Cedars-Sinai Medical Center led the study of adult employees and patients of the hospital in Los Angeles County who had at least two SARS-CoV-2 antibody tests 1 month or more apart. The first test occurred after the Delta variant surge on Sep 15, 2021, and the second was conducted amid the Omicron wave from Dec 15, 2021, to May 4, 2022. Participants also completed surveys on vaccinations, viral exposures, and symptoms at baseline and throughout the study.

The investigators called the study the first to assess Omicron infection awareness, which is key to timely self-isolation, in a diverse urban area in the United States.

Among the 210 participants, median age was 51 years, 65% were women, 44% knew they had a recent Omicron infection, and 56% said they didn't know they were infected. Of the 118 participants who were unaware of their infection, 12 (10%) said they had symptoms but had attributed them to a common cold or other non-COVID infection.

A multivariable analyses showed that overall awareness was low but that center employees were more likely than patients to know they were infected (adjusted odds ratio [aOR], 2.46).

"This finding may be explained not only by the differences in general health awareness and literacy between employees and nonemployees but also by the mandatory daily screening protocols for employees that are supported by COVID-19 sick pay policies, which were broadened at the institution during the Omicron variant surge," the study authors wrote.

Among the 92 participants who were aware of their infection, 67 (73%) had a positive polymerase chain reaction (PCR) result recorded in their electronic health record, self-reported the result, or both.

An unadjusted analysis showed that participants who knew they were infected were more likely to be younger than those who were unaware (47 vs 53 years). Men were also more likely than women to know they were infected (OR, 1.87).

"If these results are found to be generalizable, they could be associated with the tendency of certain individuals to develop a greater symptomatic response or more severe forms of Omicron variant infection," the authors wrote. "The extent to which self-reported awareness of Omicron variant or symptoms of infection appeared lower in older participants could be associated with worse recall in survey assessments."

Of the 92 participants who knew about their Omicron infection, 62 (67%) indicated their awareness on a survey, with 56 (90%) reporting COVID-attributable symptoms and 6 (10%) reporting no symptoms.

Before SARS-CoV-2 antibodies were detected, 94% of participants had received a COVID-19 vaccine, and 93% of previously vaccinated participants had completed a series with at least one mRNA vaccine.

The researchers noted that while general access to COVID-19 testing may have improved over time, Omicron has been linked to milder illness than that caused by previous SARS-CoV-2 strains. The milder nature of the infection may have led infected people to unknowingly spread the virus.

"Lack of awareness of Omicron variant infection, either owing to the relative absence of symptoms or lack of timely testing, likely had a role in rapid transmission within the communities in Los Angeles County," they wrote.

They added that antibody testing can complement rapid antigen testing and household and wastewater surveillance programs in understanding trends in vulnerability to emerging SARS-CoV-2 strains.

"The greater availability and use of rapid antigen testing could be associated with augmented awareness and, in turn, reduced rates of person-to-person transmission of the Omicron variant," the researchers concluded. "Further studies are needed to more completely understand the factors associated with unawareness of infectious status, particularly factors that could be mitigated by available interventions."

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