Canberrans infected with the Omicron sub-variant of COVID-19 were more likely to be asymptomatic than those infected in the previous Delta wave, preliminary research data has shown.
An infection of the Delta sub-variant was also more likely to result in hospitalisation, but researchers noted the increased rate of vaccination by the time of the later Omicron wave may have reduced the number of people who needed hospital treatment.
The study found 32 per cent of people infected with Omicron exhibited no symptoms, while just 12 per cent of people who were infected with Delta were asymptomatic.
The study, which gathered information from COVID infections in the ACT between August 12, 2021 and January 21, 2022, found notable differences in the symptoms reported by people who were infected with Omicron compared to Delta, and that an infection with the Delta sub-variant was more likely to result in hospitalisation.
The most commonly reported symptoms for Delta infections were cough, which 62 per cent of people reported; headache, reported by 55 per cent; and fever and runny nose, both reported in 47 per cent of cases.
Runny nose was the most common symptom among people who were infected with the Omicron variant, with 54 per cent reporting the symptom. Fifty-three per cent of cases reported experiencing cough, 41 per cent reported sore throat and 39 per cent reported lethargy.
"We found that symptom characteristics for Omicron and Delta SARS-CoV-2 infections were significantly different. Furthermore, we found that individuals infected with Omicron were more likely to report no symptoms at all, although vaccination status may have played a role in this cohort," the study authors, from the Australian National University and ACT Health, wrote.
The study said lethargy was the only symptom overall that was significantly more common in proportion in Omicron infected people than those who caught the Delta sub-variant. The study found no significant differences between symptom presentation by sex.
The study was published online earlier this month but is yet to be peer reviewed, meaning the results are not final.
Between August 12, 2021 and January 21, 2022, there were 1065 confirmed cases that met inclusion criteria for the study, with 58 per cent confirmed with genomic sequencing to be the Delta variant. The median age was 28, with cases in people aged 0 months to 95 years.
People infected with the Delta sub-variant were generally younger than those infected with the Omicron variant.
Just 19 per cent, or 118 of 613, of the Delta cases were fully vaccinated. Eighty-five per cent of those infected with Omicron were fully vaccinated.
The Delta wave began in the ACT before many Canberrans had had the opportunity to receive their full course of COVID-19 vaccinations. The first Omicron case was recorded in the ACT on December 1, 2021, after the city had reached world-leading vaccination rates.
"The rapid detection and characterisation of new and emerging SARS-CoV-2 variants are imperative in informing public health and control measures," the study's authors wrote.
"Moving forward, the focus will continue in addressing waning immunity through booster vaccine doses, development of multivalent vaccines and active surveillance within high-risk settings. Public health measures such as social distancing and mask use will still need to be in the forefront of the public's mind."
Nearly 214,000 infections of COVID-19 have been reported in the ACT since March 2020, when the pandemic began.
The territory reported 1757 in the week to Friday, with 34 people in hospital on Thursday afternoon with the virus.
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