More than half of people infected with omicron may not know it

Out of 210 people who had antibodies suggesting they had recently had the covid-19 omicron variant, more than half were unaware they were carrying the virus

By Carissa Wong

More than half of people infected with the omicron covid-19 variant may be unaware they are carrying the virus, raising the risk they could inadvertently spread the infection.

“Having most people with covid-19 being unaware of their infection status, especially while actively transmissible, makes it likely a major driver of the ongoing pandemic,” says Susan Cheng at the Cedars-Sinai Medical Center in California.

Cheng and her colleagues took blood samples from 2479 people who worked at or were registered at a medical centre in Los Angeles. At least two samples were taken per participant. The first was drawn before 15 December 2021, shortly before the area experienced a surge in covid-19 infections driven by the omicron variant.

Subsequent samples were taken between 15 December 2021 and 4 May 2022, during which time omicron became the dominant variant and several subvariants, such as BA.4 and BA.5, emerged.

Of the participants, 210 were found to have had covid-19 between the first time their blood was taken and subsequent samples, based on levels of coronavirus-specific antibodies in their blood.

To ensure this antibody response wasn’t induced via vaccination – 94 per cent of the 210 participants had received at least one vaccine dose – the researchers specifically looked at the participants’ IgG-N levels. IgG-N is an antibody to a structural protein on SARS-CoV-2 that becomes elevated when someone is naturally exposed to the coronavirus, but stays low post-vaccination.

At the time the participants’ blood was sampled for a second or subsequent time, more than half (56 per cent) of the 210 participants who had recently caught covid-19 – based on their IgG-N levels – were unaware they had been infected.

Of the participants who didn’t know they had omicron, one in 10 (10 per cent) said they had experienced mild symptoms but attributed them to other infections, such as the common cold.

Cheng says the findings highlight the importance of each individual working to reduce the likelihood of them transmitting the coronavirus, even if they think they aren’t infected.

“Being thoughtful with self-testing and taking precautions especially after being knowingly exposed to covid or developing even mild symptoms that one might assume are not likely due to covid… these are actions that everyone can be empowered to take on and can make a difference in curbing spread of the virus,” she says.

More work is needed to establish whether these findings apply beyond the single centre studied in Los Angeles.

“It would be helpful to see if other centres, especially in other parts of the world, with ideally larger numbers, could look at similar data and see if they might find similar or different results,” says Cheng.

Journal reference: JAMA Network Open, DOI: doi:10.1001/jamanetworkopen.2022.27241