Effectiveness of vaccination against SARS-CoV-2 Omicron variant infection, symptomatic disease, and hospitalisation: a systematic review and meta-analysis
Authors: Meggiolaro Angela , Schepisi M. Sane , Farina Sara , Castagna Carolina , Mammone Alessia , Siddu Andrea , Stefanelli Paola , Boccia Stefania , Rezza Giovanni 
Published: June 2022

AbstractBackgroundThe rapid rise of Sars-Cov2 B.1.1.529 variant (named Omicron) in the late November 2021 prompted the health authorities to estimate the potential impact on the existing countermeasures, including vaccines. This meta-analysis aims to assess the effectiveness of the current Sars-Cov2 vaccine regimens against laboratory-confirmed Omicron infection. A secondary endpoint aims to investigate the waning effectiveness of primary vaccination against symptomatic Omicron infection and related hospitalization.MethodsThe systematic review started on December 1, 2021 and was concluded on March 1, 2022. Random-effects (RE) frequentist meta-analyses are performed to estimate the primary vaccination course and the booster dose effectiveness against Omicron. Multiple meta-regressions are performed under mixed-effects model. This study is registered with PROSPERO, CRD42021240143.FindingsIn total, 15 out of 502 records are included in the quantitative synthesis. The meta-analysis on B.1.1.529 infection risk produces an OR=0·69 (95%CI: 0·57 to 0·83; τ2=0·225; I2=99·49%) after primary vaccination and an OR=0·30 (95%CI: 0·23 to 0·39; τ2=0·469; I2=99·33%) after one additional booster dose. According to the multiple meta-regression models, one booster dose significantly decreases by 69% the risk of symptomatic Omicron infection (OR=0·31; 95%CI: 0·23 to 0·40) and by 88% the risk of hospitalization (OR=0·12; 95%CI: 0·08 to 0·19) with respect to unvaccinated. Six months after primary vaccination, the average risk reduction declines to 22% (OR=0·78; 95%CI: 0·69 to 0·88) against symptomatic infection and to 55% against hospitalization (OR=0·45; 95%CI: 0·30 to 0·68).InterpretationDespite the high heterogeneity, this study confirms that primary vaccination does not provide sufficient protection against symptomatic Omicron infection. Although the effectiveness of the primary vaccination against hospitalization due to Omicron remains significantly above 50% after 3 months, it dramatically fades after 6 months. Therefore, the administration of one additional booster dose is recommended within 6 months and provides a 76% decrease in the odds of symptomatic Omicron after five months.FundingThere was no funding source for this study.ARTICLE HIGHLIGHTSthe primary vaccination decreases the risk of Omicron infection by 31%, while one additional booster dose decreases the risk by 70%the primary vaccination course reduces the risk of symptomatic Omicron infection by 24% and the risk of hospitalization by 50%one additional booster dose decreases by 69% the risk of symptomatic Omicron infection and the risk of hospitalization by 88%the effectiveness of the primary vaccination against hospitalization dramatically wanes after 3 months from vaccination, reaching a minimum of 45% in risk reduction after more than 6 monthsPANEL: research in contextEvidence before this studyOmicron variant’s higher transmissibility combined with an increased risk of infection among individuals vaccinated with primary vaccination have prompted health authorities to introduce a booster vaccination. The systematic review including “vaccine effectiveness”, “Covid-19”, “SARS-CoV-2”, and “Omicron” search terms, is performed over three web engines and one early stage research platform (i.e., WHO COVID-19 DATABASE, PubMed, medRxiv + bioRxiv) Additionally, all relevant web sources reporting living data on vaccine effectiveness (i.e., https://view-hub.org/covid-19/ and https://covid-nma.com/), electronic databases and grey literature are considered. The last search update was on March 1, 2022. No country, language, study design restrictions are applied.Added value of this studyPrimary vaccination provides relatively low protection against the Omicron VOC, while one additional booster dose decreased substantially the risk of symptomatic Omicron infection and of hospitalization.Implications of all the available evidenceThe booster dose should be recommended after three months and no later than six months after the primary course vaccination, in order to avoid severe consequences, in particular among the elderly population.