1948. Evaluation of a heterologous booster vaccine regimen: Pfizer-BioNTech BNT162b2 mRNA booster vaccine following priming with Novavax NVX-CoV2373


 
 
 In the United States, booster vaccines for persons 18 years and older were approved under Emergency Use Authorization (EUA) in September 2021. Waning immunity following SARS-CoV-2 primary vaccination series led to recommendations for booster vaccination. Emerging data suggest that providing boosters different from the primary series (heterologous vaccination) may provide a broader immune response than boosting with the same vaccine (homologous vaccination). CDC recommended the Pfizer-BioNTech BNT162b2 30-μg mRNA booster vaccine to clinical trial participants >6 months post study vaccines if not planned for boosting within the study.
 
 
 
 We conducted an observational study of persons who received 2 doses of Novavax protein-based NVX-CoV2373 vaccine 21 days apart, in a Phase 3 clinical trial, and subsequently received a Pfizer BNT162b2 booster vaccine under EUA. Serologic assays, including the Roche anti-nucleocapsid (N) IgG and anti-Spike (S) IgG, were performed on blood collected pre-booster (D0) and on days 18 (D18) and 34 (D34) post-booster vaccine. The anti-S IgG geometric means (GMTs) were calculated over study time points. Wilcoxon signed rank test was performed to compare anti-S IgG response between D0 and D18 and D0 and D34.
 
 
 
 Of 26 participants enrolled, 16 (57%) were women; the median age was 47 years (range 29-67). Roche anti-N antibodies were negative at all visits. Time from second NVX-CoV2373 vaccine to Pfizer BNT162b2 booster was a median of 10.4 months in 54% of participants and 7 months in 46% of participants. Anti-S IgG GMTs were 222 BAU/ml D0, 24,723 BAU/ml D18, and 24,584 BAU/ml D34 (p< 0.0001 for comparisons of D0 with D18 & D34). Overall, participants tolerated the booster vaccine without significant adverse events. Cell mediated immunity and D614G pseudovirus neutralizing antibody assays are in progress. Figure 1.Anti-S IgG titers pre and post-booster vaccine
 16 participants included with all 3-time study time points for comparison.
 
 
 
 Two doses of NVX-CoV2373 vaccine followed by the Pfizer BNT162b2 booster vaccine resulted in ∼100-fold increase in anti-S IgG against SARS-CoV-2. No participant had evidence of prior SARS-CoV-2 infection by anti-N IgG. Two doses of NVX-CoV2373 vaccine followed by one dose of Pfizer BNT162b2 vaccine is an effective and well-tolerated regimen for boosting anti-S IgG against SARS-CoV-2.
 
 
 
 Christine Johnston, MD, MPH, AbbVie: Advisor/Consultant|Gilead: Grant/Research Support|GSK: Advisor/Consultant Kerry J. Laing, PhD, Curevo Vaccine: Advisor/Consultant|MaxHealth Biotechnology: Advisor/Consultant|Sanofi Pasteur: Grant/Research Support David M. Koelle, MD, Curevo Vaccines: Advisor/Consultant|MaxHealth LLC: Advisor/Consultant|Oxford Immunotec: gift of reagents|Sanofi: Grant/Research Support|Sensei: Grant/Research Support Alexander L. Greninger, MD, PhD, Abbott: Contract Testing|Cepheid: Contract Testing|Gilead: Grant/Research Support|Gilead: Contract Testing|Hologic: Contract Testing|Merck: Grant/Research Support|Novavax: Contract Testing|Pfizer: Contract Testing Anna Wald, MD, MPH, Aicuris: Advisor/Consultant|Auritec: Advisor/Consultant|Crozet: Advisor/Consultant|DXNow: Advisor/Consultant|GSK: Grant/Research Support|Merck: Advisor/Consultant|sanofi: Grant/Research Support|VIR: Advisor/Consultant|X-Vax: Advisor/Consultant.
