Disease caused by the Omicron variant of coronavirus is on average two days shorter than the Delta variant, according to a large observational study of vaccinated people in the UK.

The research, published in The Lancet journal on April 7, used data from vaccinated people who kept a smartphone log of their COVID-19 symptoms after breakthrough infections.

Eligible participants were aged 16-99 years, and had received at least two doses of COVID-19 vaccine, were symptomatic, and logged a positive symptomatic test for SARS-CoV-2.

"The duration of acute symptoms was shorter during Omicron prevalence than during Delta prevalence, with the average presentation of Omicron being 2 days shorter than that of Delta, the authors of the study noted.

"Furthermore, the third dose of vaccine was associated with a greater reduction in symptom duration in participants infected during Omicron prevalence compared with those infected during Delta prevalence, they said.

Between June 1, 2021, and January 17, 2022, the researchers identified 63,002 participants who tested positive for SARS-CoV-2 and reported symptoms in the ZOE app. These patients were matched 1:1 for age, sex, and vaccination dose, across two periods.

The researchers found that loss of smell was less common in participants infected during Omicron prevalence than during the Delta wave. The sore throat was more common during Omicron prevalence than during Delta prevalence, they said. There was a lower rate of hospital admission during Omicron prevalence than during delta prevalence, according to the researchers.

The prevalence of symptoms that characterise an Omicron infection differs from those of the Delta variant, apparently with less involvement of the lower respiratory tract and reduced probability of hospital admission, they said.

Our data indicate a shorter period of illness and potentially infectiousness which should impact work health policies and public health advice, the researchers said. However, this might not be the case in unvaccinated individuals, they added.