In a recent study published in The Lancet, researchers at King's College London and ZOE Limited assessed the risk of long coronavirus disease (COVID) associated with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Delta and Omicron variants in the United Kingdom (UK).
Studies have reported that Omicron causes acute SARS-CoV-2 infections of lesser severity compared to those by previously circulating variants, especially among vaccinated individuals; however, a large population could develop long COVID symptoms. Data on the relative odds of long COVID among those infected with relatively recent variants such as Delta and Omicron could inform health authorities and policy-makers for more appropriate allocation of health resources.
Correspondance: Risk of long COVID associated with delta versus omicron variants of SARS-CoV-2. Image Credit: Donkeyworx / Shutterstock
About the study
In the present case-control study, researchers evaluated the risks of long COVID during the periods of Delta predominance and Omicron predominance among UK citizens.
The COVID Symptom Study ZOE application was used to obtain data reported by individuals with real-time-polymerase chain reaction (PCR)-confirmed or lateral flow antigen test-confirmed COVID 2019 (COVID-19) diagnoses post-vaccination. The study participants did not have pre-vaccination COVID-19 history and had ≥1 log of reported symptoms weekly in the ZOE application for ≥28 days.
For individuals testing positive during Omicron predominance, only those who tested SARS-CoV-2-positive before 10 February 2022 were included. For both groups of individuals, the index of multiple deprivations (IMD) for local areas was calculated to estimate the relative local deprivation. For the index, a score of 1 denoted the most deprived individuals and a score of 10 denoted the least deprived individuals.
The study exposure was the period of infection and the study outcome was the development of long COVID. To determine the association between the outcome and the exposure, logistic regression modeling was used with data adjustments for long COVID risk factors such as IMD, sex, age, comorbidities, body mass index (BMI), and the status of vaccination (single, double, or triple vaccine doses received). The period between the most recent vaccination and development of SARS-CoV-2 infection was stratified into three groups which were: three months, three to six months, and >6 months.
A total of 56,003 Omicron cases (adults who initially tested SARS-CoV-2-positive between 20 December 2021 and 9 March 2022) and 41,361 Delta cases (adults who initially tested SARS-CoV-2-positive between 1 June 2021 and 27 November 2021) were identified. The proportion of women identified with Delta (59%) and Omicron (55%) infections was more significant than men.
However, both the groups had similarly aged (average age was 53 years) participants with a similar presence of comorbid conditions (19%). IMD indices showed that Omicron variant cases were prevalent in regions of marginally lesser deprivation compared to Delta variant cases (17% versus 17.5% with IMD scores 1 to 3).
Among Delta and Omicron cases, 2501 individuals (4.5%) and 4469 individuals (10.8%) developed long COVID symptoms. The relative odds of long COVID were lower with Omicron infections compared to Delta infections for all vaccination timings (odds ratio range between 0.2 and 0.5). Similar results were obtained by performing an age-stratified analysis.
Overall, the study findings showed that long COVID risk was lower for individuals with Omicron infections compared to those with Delta infections; however, the absolute counts of long COVID cases during a particular period are dependent on the amplitude and shape of the curve representing the pandemic. Considering the peak in Omicron cases of >350,000 new and symptomatic COVID-19 cases daily estimated to have occurred on 26 March 2022, and a mere 4.5 percent of which being long COVID cases as estimated by the ZOE app, the case numbers for long COVID would only increase in the future.
The authors believe that the present study is the first of its kind to be peer-reviewed in which the risk of long COVID among Omicron cases has been assessed. The findings highlight the use of smartphone applications in COVID-19 surveillance. Further studies must be conducted, however, in order to determine if unvaccinated children and adults have a greater risk of long-term COVID.
The study limitations include the use of data that was reported by the SARS-CoV-2-positive individuals themselves without any direct SARS-CoV-2 testing for variant identification and the absence of objective outcome measures for COVID-19 duration. In addition, the period for Omicron case assessment was shorter (although marginally) compared to that for Delta cases. Furthermore, individuals could not be assessed for long COVID risks for more extended periods, e.g., for more than 12 weeks.