Waning of SARS-CoV-2 vaccine-induced immunity: A systematic review and secondary data analysis

Background The emergence of Omicron (B.1.1.529) variant of SARS-CoV-2 in late 2021 was followed by a marked increase of breakthrough infections. Estimates of vaccine effectiveness (VE) in the long term are key to assess potential resurgence of COVID-19 cases in the future. Methods We conducted a systematic review of manuscripts published until June 21, 2022 to identify studies reporting the level of protection provided by COVID-19 vaccines against SARS-CoV-2 infection and symptomatic disease at different time points since vaccine administration. An exponential model was used to perform a secondary data analysis of the retrieved data to estimate the progressive waning of VE associated with different vaccine products, numbers of received doses, and SARS-CoV-2 variants. Findings Our results show that VE of BNT162b2, mRNA-1273, ChAdOx1 nCoV-19 vaccines against any laboratory confirmed infection with Delta might have been lower than 70% at 9 months from second dose administration. We found a marked immune escape associated with Omicron infection and symptomatic disease, both after the administration of two and three doses. The half-life of protection against symptomatic infection provided by two doses was estimated in the range of 178-456 days for Delta, and between 66 and 73 days for Omicron. Booster doses were found to restore the VE to levels comparable to those acquired soon after administration of the second dose; however, a fast decline of booster VE against Omicron was observed, with less than 20% VE against infection and less than 25% VE against symptomatic disease at 9 months from the booster administration. Conclusions This study provides a cohesive picture of the waning of vaccine protection; obtained estimates can inform the identification of appropriate targets and timing for future COVID-19 vaccination programs. ### Competing Interest Statement M.A. has received research funding from Seqirus. The funding is not related to COVID-19. All other authors declare no competing interest. ### Funding Statement P.P. and V.M. acknowledge funding from EU grant 694160 VERDI. S.M., G.G., M.M. acknowledge funding from EU grant 874850 MOOD. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes 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. Yes I 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. Yes All data produced in the present work are contained in the manuscript