Scientists have shed light on why the Omicron strain of the coronavirus caused such havoc, sweeping through populations after first appearing in autumn 2021.
Researchers from Cedars-Sinai Medical Centre, in Los Angeles, found that 56 per cent of people were unaware they were infected with virus that causes Covid-19, the study published in JAMA Network Open noted.
Compared to other Covid-19 variants, Omicron generally had less severe symptoms, such as fatigue, cough, headache, sore throat and a runny nose.
“More than one in every two people who were infected with Omicron didn’t know they had it,” said Dr Susan Cheng, from Cedars-Sinai and corresponding author of the study.
The UK this week authorised a Moderna vaccine that tackles both Omicron and the original versions of the disease.
The European Medicines Agency has started a review of a Pfizer-BioNTech Covid-19 vaccine also adapted to target two strains of the virus.
As part of newly published research into the effect of vaccines, the study began testing the blood of health workers in 2020. By the time the Omicron wave hit, they were also testing Covid-19 patients.
“Our study findings add to evidence that undiagnosed infections can increase transmission of the virus,” said Sandy Joung, first author of the study. "A low level of infection awareness has likely contributed to the fast spread of Omicron."
Of the 56 per cent unaware of their contagion, only 10 per cent reported any symptoms.
Previous research has estimated between 25 per cent and 80 per cent of people infected with Covid-19 may experience no symptoms.
“Awareness will be key for allowing us to move beyond this pandemic,” Dr Cheng said.
“We hope people will read these findings and think, 'I was just at a gathering where someone tested positive' or, 'I just started to feel a little under the weather. Maybe I should get a quick test.'”
“The better we understand our own risks, the better we will be at protecting the health of the public as well as ourselves.”
Dr Cheng and colleagues are also studying patterns and predictors of Covid-19 reinfection.