Efficacy of vaccination against SARS-CoV-2 Omicron BA.1 and BA.2 sublineages in Sweden
In a recently published study to medRxiv* preprint server, researchers compared the vaccine’s effectiveness (VE) against infections of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant of concern (VOC) Omicron BA.1 and BA.2 sublineages in Sweden.
Studies have reported that Omicron BA.2 sublineage is more transmissible than BA.1, with lower protection provided by vaccines against BA.2-associated hospitalizations and deaths. However, some studies have reported similar protection for both sublineages. Therefore, robust evidence for the comparative VEs against BA.1 and BA.2 sublineages is lacking.
About the study
In the current cohort study, researchers compared VE with severe coronavirus disease 2019 (COVID-19) caused by BA.1 and BA.2 sublineages of Omicron VOC between 27 December 2021 and 15 March 2022, among residents of Skåne County in southern Sweden. The subjects were followed up longitudinally for positive covid-19 reports and hospital stays and assessed for covid-19 severity.
Data for all Skåne residents were obtained from routine sample sequencing of COVID-19 cases for comparative RE assessment over three periods: (i) BA.1 overweight (60%) in week 52, 2021 and week 1, 2022, (ii ) transition period during weeks 2 and 3, 2022 (BA.1 47%, BA.2 49%) and (iii) Omicron BA.2 dominance (82%), weeks 4 to 11 2022.
Information on all residents was obtained from regional and national registers by entering the residents’ social security numbers. Updates were received weekly on vaccination type, dates and doses from the national vaccination register.
Data on severe covid-19 patients have been provided by SMINet, an electronic system for the Swedish Public Health Agency. In addition, regional health registries were used as sources of supplementary data to quickly provide data on covid-19 tests and to evaluate the effect of comorbidities on covid-19 outcomes. The co-morbid conditions that were assessed included diabetes, obesity and disorders of the kidney, liver, lung, neurological systems, cancer, immunosuppressive conditions and genetic diseases such as Down’s syndrome, thalassemia and sickle cell anemia.
Severe covid-19 cases were those with at least one day of hospital stay five days before and up to two weeks after positive covid-19 reports with oxygen supplementation requirement of ≥ 5 l / minute or intensive care unit (ICU).
The continuous density case-control sampling technique was used to match cases and controls for age and sex and make data adjustments for differences in comorbidities and covid-19 history. Logistic regression modeling and 95% confidence interval (CI) were used to compare VE for severe SARS-CoV-2 infections.
For each severe COVID-19 case, 10 subjects with SARS-CoV-2 negative reports received 90 days before or within the same time frame as for SARS-CoV-2 positive cases were randomly selected as controls. Only vaccinations administered one week before the date of diagnosis of COVID-19 were selected for the analyzes.
Results
Over 590 serious covid-19 cases were detected during the follow-up assessments, which corresponded weekly to 65 BA.1 predominant cases, 78 BA.1 to BA.2 transition cases and 56 BA.2 cases. Although most (83%) of the study participants were vaccinated, only 57% of booster vaccine uptake was observed. Most (77%) of the study participants were vaccinated with BNT16b2 messenger ribonucleic acid (mRNA) vaccine. The severe BA.2 cases were more common in the elderly with a more uniform prevalence among both sexes compared with the BA.1 cases.
Prior to follow-up, VE against severe SARS-CoV-2 infections was high (mean VE 89%) between March and November 2021 and was stable during BA.1 obesity in December 2021. After two doses, however, VE decreased remarkably from 90% (95 % CI 78% to 95%) under BA.1 consideration to 54% (95% CI 13% to 75%) under BA.2 consideration. In addition, vaccine protection from previous covid-19 history was also lower after the transition to BA.2. This decline in VE was consistent across all ages, comorbidities, and gender. However, among participants who received three vaccine doses, VE remained stable after the transition from BA.1 to BA.2.
In summary, the study results emphasized the importance of booster vaccination to alleviate COVID-19 and that the immunosuppressive Omicron BA.2 sublineage was more likely to cause severe SARS-CoV-2 infections compared to BA.1 sublineage. However, further studies for continued monitoring of VE versus BA.2 are required.
Study restrictions
Data on the causative viral variants for each case for each period were missing. Thus, the real changes in RE during the transition from BA.1 to BA.2 could have been underestimated. In addition, the protection from the history of Omicron infections could not be evaluated as the follow-up period with Omicron obesity is short.
In addition, as routine testing is no longer recommended, VE against infections could not be thoroughly examined. In addition, the study results had high statistical uncertainty, which is indicated by broad CI among the subgroups.
*Important message
medRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered as crucial, guide clinical practice / health-related behavior or be treated as established information.