This is a Phase 2/3, randomized, double-blind study to evaluate the safety and immunogenicity of different booster dose levels of the monovalent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) recombinant (r) spike (S) protein nanoparticle (SARS-CoV-2 rS) vaccines with Matrix-M™ adjuvant (NVX-CoV2373 [prototype Wuhan vaccine with Matrix-M adjuvant] or NVX-CoV2601 [Omicron XBB.1.5 subvariant vaccine with Matrix-M adjuvant]).

Clinical Study

The present study aims to investigate the safety and immunogenicity of different booster dose levels of monovalent and bivalent vaccines in adults ≥ 50 years of age who have already been immunized with ≥ 3 doses of a COVID-19 prototype or bivalent licensed mRNA vaccine. The Boosters of investigational products will be administered ≥ 90 days after the participants receive their third dose of a COVID-19 prototype or bivalent licensed mRNA vaccine.

Approximately 1,980 participants ≥ 50 years of age who have received a regimen of ≥ 3 doses of a coronavirus disease 2019(COVID-19) vaccine (the last vaccine could have been a bivalent licensed mRNA vaccine) will be included in this study. The last COVID-19 vaccine dose should have been administered ≥ 90 days before Day 0.

Approximately 1,800 participants will be randomly assigned in a 1:2:2:2:2:1 ratio to receive NVX-CoV2373 or NVC-CoV2601 in a double-blinded fashion into 1 of 6 monovalent vaccine groups (vaccine groups A to G). Following completion of enrollment into the six monovalent vaccine groups, 180 participants will be enrolled in vaccine group G to receive a bivalent licensed mRNA vaccine in an open-label fashion.

In clinical trials, the most common adverse reactions associated with Novavax's prototype COVID-19 vaccine (NVX-CoV2373) included headache, nausea or vomiting, muscle pain, joint pain, injection site tenderness, injection site pain, fatigue, and malaise.

Non-clinical data demonstrated that Novavax's vaccine-induced neutralizing antibody responses to subvariants BA.2.86, EG.5.1, FL.1.5.1, and XBB.1.16.6 as well as CD4+ polyfunctional cellular (T-cell) responses against EG.5.1 and XBB.1.16.6. These data indicate Novavax's vaccine can stimulate both arms of the immune system and may induce a broad response against currently circulating variants.