Infected cell covered with SARS-CoV-2 viruses. Source: NIAID
Findings from a new study reported in The Lancet has found the risks of developing long COVID are greatly reduced (by ~50% to 75%) as a result of Omicron infection compared to Delta infection.
The study, the first of its kind to report on long COVID risk associated with Omicron, highlights the speed with which app-based health surveillance can provide insights. These have further been shown to be consistent and replicable.
A major strength of the study was the ability to log a wide range of symptoms with the app. Limitations of the self-reported data include no direct testing of infectious variants (here assumed from national data) and no objective measures of illness duration. There was insufficient data to estimate the odds of long COVID in unvaccinated individuals. Finally, to enable swift reporting, the period of assessment of omicron cases was slightly shorter than for the delta variant, and assessment of longer durations of long COVID (eg, >12 weeks) was not possible.
In this case-control observational study, the researchers took self-reported data from the COVID Symptom Study app.
Participants were selected on a positive COVID test after SARS-CoV-2 after vaccination, at least one log per week in the app for at least 28 days after testing positive, and no previous SARS-CoV-2 infections before vaccination. The researchers identified 56 003 UK adults first testing positive between Dec 20, 2021, and March 9, 2022, when the Omicron variant became dominant. Similarly, researchers identified 41 361 UK adult cases first testing positive between June 1, 2021, and Nov 27, 2021, during the period of Delta dominance.
Both symptomatic and asymptomatic infections were considered, and, for the Omicron period, only participants testing positive before Feb 10, 2022, were included, to ensure all participants had at least 28 days for symptom reporting after testing positive. The researchers adjusted for socioeconomic deprivation and other factors.
In both periods, female participation was higher than male participation (55% for Omicron and 59% for Delta cases). Delta and Omicron cases had similar age (mean age 53 years) and prevalence of comorbidities (around 19%). Among Omicron cases, 2501 (4.5%) of 56 003 people experienced long COVID and, among Delta cases, 4469 (10.8%) of 41 361 people experienced long COVID. Cases were stratified by time from vaccination to first infection. They found that, for all vaccine timings, Omicron cases were less likely to experience long COVID, with an odds ratio ranging from 0.24 to 0.50.
However, the researchers noted that the the absolute number of people with long COVID at a certain time depends on the pandemic curve. Considering the UK Omicron peak of more than 350 000 new symptomatic COVID cases per day estimated on March 26, 2022, by the ZOE app model, with 4% of cases being long COVID, future numbers with long COVID will inevitably rise.
