Disease caused by the Omicron variant of the coronavirus is on average around two days shorter than the Delta variant, a large study of vaccinated Britons who kept a smartphone log of their COVID-19 symptoms after breakthrough infections has shown.

The study was published in medical journal The Lancet on Thursday and will be presented at the European Congress of Clinical Microbiology and Infectious Diseases later this month.

"The shorter presentation of symptoms suggests – pending confirmation from viral load studies – that the period of infectiousness might be shorter, which would in turn impact workplace health policies and public health guidance," the study authors wrote.

Based on the Zoe COVID app, which collects data on self-reported symptoms, the study also found that a symptomatic Omicron infection was 25 percent less likely to result in hospital admission than in a case of Delta.

While Omicron's lesser severity has been known, the study is unique in its detailed analysis and in that it corrected for any distortions caused by differences in vaccination status by looking at vaccinated volunteers only.

The researchers at King's College London analysed two sets of data from June 1 to November 27, 2021, when the Delta variant accounted for more than 70 percent of cases, and from December 20, 2021, to January 17, 2022, when Omicron was more than 70 percent prevalent.

The patients – close to 5,000 in each group – were matched and compared 1:1 with a person of the same age, sex, and vaccination dose in the other group.

Omicron's shorter symptom duration, relative to Delta, was more pronounced in those with three vaccine doses. Symptoms lasted 7.7 days on average during the Delta-dominated period, and only 4.4 days, or 3.3 days less, during the Omicron period.

Among those with only two vaccine doses, symptoms from Delta lasted for 9.6 days and 8.3 days from Omicron, a difference of just 1.3 days.

The Zoe COVID Study application, previously known as the COVID Symptoms Study App, collects data on self-reported symptoms.

The company ZOE Ltd was initially founded to offer customized nutritional advice based on test kits. Its app is a not-for-profit initiative in collaboration with King's College London and funded by the Department of Health and Social Care.