Titers and capacity of neutralizing antibodies against SARS-CoV-2 variants after heterologous booster vaccination in health care workers primed with two doses of ChAdOx1 nCov-19: a single-blinded, randomized clinical trial

Background Booster vaccination is important because of waning immunity and variant immune evasion. We conducted a single-blinded, randomized trial to evaluate the safety, reactogenicity, and immunogenicity of heterologous booster vaccination in health care workers (HCW) who had received two doses of ChAdOx1 nCov-19. Methods and findings HCW at least 90 days after the second dose were enrolled to receive one of the four vaccines: BNT162b2, half-dose mRNA-1273, mRNA-1273, and MVC-COV1901. The primary outcomes were humoral and cellular immunogenicity and the secondary outcomes safety and reactogenicity 28 days post-booster. 340 HCW were enrolled: 83 received BNT162b2 (2 excluded), 85 half-dose mRNA-1273, 85 mRNA-1273, and 85 MVC-COV1901. mRNA vaccines had more reactogenicity than protein vaccine. Anti-spike IgG increased by a fold of 8.4 for MCV-COV1901, 32.2 for BNT162b2, 47.6 for half-dose mRNA-1273 and 63.2 for mRNA1273. The live virus microneutralization assay (LVMNA) against the wild type, alpha and delta variants were consistent with anti-spike IgG for all booster vaccines. The LVMNA in the four groups against omicron variant were 6.4 to 13.5 times lower than those against the wild type. Serum neutralizing antibody against omicron variant was undetectable in 60% of the participants who received MCV-COV1901 as a booster by LVMNA. By using pseudovirus neutralizing assay, we found that neutralization activity in the four groups against omicron variant were 4.6 to 5.2 times lower than that against the D614G. All booster vaccines induced comparable T cell response. Conclusions Third dose booster not only increases neutralizing antibody titer but also enhances antibody capacity against SARS-CoV-2 variants. mRNA vaccines are preferred booster vaccines for those after primary series of ChAdOx1 nCov-19. Trial registration [ClinicalTrials.gov][1] [NCT05132855][2] ### Competing Interest Statement The authors have declared no competing interest. ### Clinical Trial [ClinicalTrials.gov][1] [NCT05132855][2] ### Funding Statement The trial was sponsored and primarily funded by the Medigen Vaccine Biologics Corporation and Chang Gung Memorial Hospital (both to CHC), under award numbers XMRPG3L0101, CORPG3K0311, CORPG3L0371, CORPG3L0481, and CORPG3K0242 and with support from the Research Center for Epidemic Prevention Science, Chang Gung University and Chang Gung Memorial Hospital (to CHC and Shih SR) (award number: MOST 109-2327-B-182-002). 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 The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The study protocol was approved by Chang Gung Medical Foundation Institutional Review Board (202101767A3). 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 study are available upon reasonable request to the authors. All data produced in the present work are contained in the manuscript * HCW : health care workers PNA : pseudovirus neutralizing assay LVMNA : live virus microneutralization assay COVID-19 : Coronavirus Disease 2019 CGMH : Chang Gung Memorial Hospital FDA : Food and Drug Administration VOCs : variants of concern [1]: http://ClinicalTrials.gov [2]: /lookup/external-ref?link_type=CLINTRIALGOV&access_num=NCT05132855&atom=%2Fmedrxiv%2Fearly%2F2022%2F06%2F16%2F2022.06.14.22276236.atom