A case-control observational study published in "Lancet" reported that compared to delta cases, omicron cases were less likely to encounter long COVID (having novel or ongoing symptoms four weeks or greater following commencement of acute COVID-19). Researchers aimed to recognize the relative odds of long-COVID during omicron period when compared to delta period. From COVID Symptom Study app, self-reported data was used.

Data extraction and pre-processing were done. The inclusion criteria for both periods were a coronavirus real-time polymerase chain reaction (RT-PCR) or lateral flow antigen test that was positive after vaccination, at least one log per week in the app for at least 28 days after testing positive, and no past coronavirus infections. Between 20 December 2021 and 9 March 2022, a total of 56,003 United Kingdom (UK) adults were detected who met the inclusion requirements. These instances were designated as "omicron cases" since the omicron variant was responsible for greater than 70% of cases in the UK.

Overall, 41,361 UK adult cases were discovered that initially tested positive between 1 June 2021 and 27 November 2021 using the same selection criteria. These cases were known as delta cases since more than 70% of them were elicited by the delta variant. Both asymptomatic and symptomatic infections were taken into account. For omicron period, only those candidates were included who tested positive prior to 10th  February 2022 to ascertain that everyone had at least twenty-eight days to report symptoms following testing.

Compared to men participation, women participation was more in both periods (55% for omicron cases, 59% for delta cases). The average age of the delta and omicron cases was 53 years old, and comorbidities were common (around 19%). Omicron cases were dispersed in areas with marginally less deprivation than delta cases (16.7% vs. 17.5% for IMD 1-3), according to local area Index of Multiple Deprivation (IMD). This score ranged from 1 (most impoverished) to 10 (least disadvantaged).

Using univariate logistic regression model adjusted by gender, body-mass index, comorbidities, IMD, age, and vaccination status (1, 2, or 3 doses), all of which were connected with risk of long COVID, the relationship between long COVID (outcome) and infection period (exposure) was examined. In order to account for any potential waning of vaccine protection, the study was segregated into 3 groups based on how long had passed between the infection and most recent vaccination: 3 months, 3-6 months, and more than 6 months.

Long COVID was experienced by 2501 (4.5% of omicron cases) of 56,003 individuals and 4469 (10.8% of delta cases) of 41,361 individuals. For all the vaccine schedules, Omicron cases exhibited lower probability to develop protracted COVID, with odds ratios ranging from 0.24 (0.20-0.32) to 0.50 (0.43-0.59). When data stratification was done as per age groups, similar findings were supported.

This study highlighted that healthcare vigilance using smartphone apps is accurate, can produce rapid insights, and can be subsequently replicated. Prospective symptom logging of a broad range of symptoms was the major strength of the study in relation to long COVID. Depending on age and the length of time since immunization, the likelihood of protracted COVID decreased by 0·24–0·50 with the omicron variation when compared to the delta variant. But, the absolute number of individuals witnessing long COVID at a given time is dependent on pandemic curve's amplitude and shape.

The data displayed consistency with UK Office for National Statistics' estimate that number of subjects encountering long COVID actually escalated from 1 million in January 2022 to 1 million in March 2022, given the high numbers of subjects infected with omicron in the UK from December 2021 to February 2022. Future numbers of cases with long COVID will certainly increase given  UK omicron peak of over 3,50 000 novel symptomatic coronavirus cases per day predicted on 26 March 2022 by ZOE app model and 4% of cases of long COVID.