COVID-19 vaccine effectiveness against severe disease from the Omicron BA.1 and BA.2 subvariants: surveillance results from southern Sweden, December 2021 to March 2022

We compared vaccine effectiveness (VE) against severe COVID-19 during calendar periods from December 2021 to March 2022 when Omicron BA.1 and BA.2, respectively, were the dominating virus variants in Scania county, Sweden. We used continuous density case-control sampling matched for sex and age, and with further adjustment for differences in comorbidities and prior infection. VE remained relatively stable after the transition from BA.1 to BA.2 among people with at least three doses but decreased markedly among those with only two doses. Protection from prior infection was also lower after the transition to BA.2. These findings suggest that booster vaccination is needed to maintain sufficient protection against severe COVID-19.Competing Interest StatementThe authors have declared no competing interest.Funding StatementThis study was supported by Swedish Research Council (VR; grant numbers 2019-00198 and 2021-04665), Sweden's Innovation Agency (Vinnova; grant number 2021-02648) and by internal grants for thematic collaboration initiatives at Lund University held by JB and MI. FK is supported by grants from the Swedish Research Council and Governmental Funds for Clinical Research (ALF), and CB is supported by Swedish Research Council for Health, Working life and Welfare (Forte; grant number 2020-00962). The funders played no role in the design of the study, data collection or analysis, decision to publish, or preparation of the manuscript.Author DeclarationsI confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.YesThe details of the IRB/oversight body that provided approval or exemption for the research described are given below:Ethical approval was obtained from the Swedish Ethical Review Authority (2021-00059).I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.YesI understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable.YesAggregated surveillance data from the present study are publicly available.https://sodrasjukvardsregionen.se/kliniskastudier/covid-vacciner-skyddseffekt