NVX-CoV2373

Comparative IgG antibody responses on Day 14, summarized in terms of the ratio of IgG GMTs and difference in seroconversion rates (SCR; defined as ≥ 4-fold increase from baseline booster dose) between the vaccines. Non-inferiority will be demonstrated if: * The lower bound of the two-sided 95% CI on the ratio of the GMTS (GMTNVX-CoV2373/GMTBBIBP-CorV) is ≥ 0.6667, AND * The lower bound of the two-sided 95% CI on the difference between the SCRs (SCRNVX-CoV2373 - SCR BBIBP-CorV) is ≥ 10%.

All safety analyses will be summarized descriptively by vaccine group using the Safety Analysis Set. To compare the overall safety, the two-sided 95% CIs for the difference of incidence of solicited AEs for 7 days following each vaccination. Recording of solicited and unsolicited AEs may be conducted by electronic data capture (EDC)/reporting. All AEs will be followed until resolution or until clinically stable.

All safety analyses will be summarized descriptively by vaccine group using the Safety Analysis Set. 1085BUnsolicited AEs will be coded by preferred term and system organ class using MedDRA and summarized by vaccine group as well as by severity and relationship to booster vaccine. Unsolicited AEs through 28 days after the booster vaccination. Recording of solicited and unsolicited AEs may be conducted by electronic data capture (EDC)/reporting. All AEs will be followed until resolution or until clinically stable.

To compare the overall safety of a single booster injection of NVX-CoV2373 with Matrix-M adjuvant with a single booster injection of BBIBP-CorV in participants previously vaccinated with a primary two-dose series of the BBIBP-CorV vaccine. Recording of solicited and unsolicited AEs may be conducted by electronic data capture (EDC)/reporting. All AEs will be followed until resolution or until clinically stable.

Neutralizing Antibody responses to ancestral strain Novavax vaccine (NVX-CoV2373) for the participants in the ancestral strain NVX-2373 group and NVX-CoV2540 group at Day 29

Serum Immunoglobulin G (IgG) geometric mean ELISA unit concentrations (GMEU/mL) to the SARS-CoV-2 spike protein at Day 29 in each treatment arm.

Number of participants with first occurrence of positive (+) polymerase chain reaction (PCR)-confirmed SARS-CoV-2 illness with symptomatic mild, moderate, or severe COVID-19, with each symptom lasting for at least 2 consecutive days, with onset from Day 28 (7 days after second vaccination dose) through the length of the study.

Reactogenicity incidence and severity (mild, moderate or severe) recorded by all participants on their electronic patient-reported outcome diary application (eDiary) on days of vaccination and subsequent 6 days (total 7 days after each vaccine injection).

Number of participants with mild, moderate, or severe MAAEs through Day 49 i.e. 28 days after second injection of each set of vaccinations (initial and crossover).

Number of participants with mild, moderate, or severe AEs through Day 49 i.e. 28 days after second injection of each set of vaccinations (initial and crossover).

Number of participants with mild, moderate, or severe MAAEs attributed to study vaccine through specified time points approximately every 3 and 6 months.

Number of participants with mild, moderate, or severe SAEs attributed to study vaccine through specified time points approximately every 3 and 6 months.

Number of participants with mild, moderate, or severe AESIs attributed to study vaccine through specified time points approximately every 3 and 6 months.

Number of participants with mild, moderate, or severe SAEs attributed to study vaccine and AESIs through specified time points until Month 24 or the EoS.

Number of participants with mild, moderate, or severe MAAEs attributed to study vaccine and AESIs through specified time points until Month 24 or the EoS.

Number of participants with mild, moderate, or severe AESIs attributed to study vaccine and AESIs through specified time points until Month 24 or the EoS.

Number of participants with antibodies to SARS-CoV-2 NP at Day 35 to determine natural infection and to determine the incidence of asymptomatic infection acquired during study follow-up.

Number of participants who died during the study due to any cause.

Number of participants, testing serologically negative for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) at baseline, with first occurrence of positive (+) polymerase chain reaction (PCR)-confirmed SARS-CoV-2 illness with symptomatic mild, moderate, or severe COVID-19 with onset from Day 28 through the length of the study.

Neutralizing antibody To determine if the combination of antigen and adjuvant levels of NVX-CoV2601 GMTR against the Omicron subvariant XBB.1.5 superior(LB of the 95% CI for GMTR \> 1.0) to that elicited by NVX-CoV2373

Neutralizing antibody SRR against the Omicron XBB.1.5 subvariant elicited by NVXCoV2601 is non-inferior (NI)to the SRR elicited by NVX-CoV2373in participants ≥ 50 years of age previously vaccinated with ≥ 3 doses of a COVID-19 prototype or bivalent licensed mRNA vaccine.

Reactogenicity incidence, duration, and severity (mild, moderate, severe, or potentially life-threatening) recorded by parent(s)/caregiver(s) on an electronic patient-reported outcome diary application (eDiary) on days of vaccination and subsequent 6 days (total 7 days after each vaccine injection).

Incidence and severity of MAAEs through 28 days after second injection of each set of vaccinations (initial and crossover), and after a booster dose.

Incidence and severity of unsolicited AEs through 28 days after second injection of each set of vaccinations (initial and crossover), and after a booster dose.

Incidence and severity of MAAEs attributed to study vaccine after initial vaccination at Day 0 through Month 24 or the EoS.

Incidence and severity of SAEs after initial vaccination at Day 0 through Month 24 or the EoS.

Incidence and severity of AESIs (including multisystem inflammatory syndrome in children \[MIS-C\], and myocarditis and/or pericarditis) after initial vaccination at Day 0 through Month 24 or the EoS.

Death due to any cause occurring from Day 0 to EoS.

Number of PLWH with unsolicited AEs stratified by level of control of HIV infection.

Number of HIV-Negative participants with unsolicited AEs.

Number of PLWH with unsolicited AEs stratified by level of control of HIV infection.

Number of PLWH with solicited systemic AEs for 7 days following each vaccination stratified by baseline severity of disease as determined by the level of control of HIV infection into well-controlled and less-well-controlled treatment groups.

Number of HIV-Negative participants with solicited systemic AEs for 7 days following each vaccination.

Number of PLWH with solicited local AEs for 7 days following each vaccination stratified by baseline severity of disease as determined by the level of control of HIV infection into well-controlled and less-well-controlled treatment groups.

Number of HIV-Negative participants with solicited local AEs for 7 days following each vaccination.

Serum IgG antibody levels assayed with the SARS-CoV-2 rS protein antigen expressed as GMEUs in PLWH stratified by level of control of HIV infection.

Serum IgG antibody levels assayed with the SARS-CoV-2 rS protein antigen expressed as GMFRs in PLWH stratified by level of control of HIV infection.

Serum IgG antibody levels assayed with the SARS-CoV-2 rS protein antigen expressed as SCRs in PLWH stratified by level of control of HIV infection.

Epitope-specific immune responses assayed with the SARS-CoV-2 rS protein receptor-binding domain measured by serum titers in an hACE2 receptor binding inhibition assay expressed as GMT in PLWH stratified by level of control of HIV infection.

Epitope-specific immune responses assayed with the SARS-CoV-2 rS protein receptor-binding domain measured by serum titers in an hACE2 receptor binding inhibition assay expressed as GMFR in PLWH stratified by level of control of HIV infection.

Epitope-specific immune responses assayed with the SARS-CoV-2 rS protein receptor-binding domain measured by serum titers in an hACE2 receptor binding inhibition assay expressed as SCR in PLWH stratified by level of control of HIV infection.

Titers of neutralizing antibody to the prototype virus expressed as GMT in PLWH stratified by level of control of HIV infection.

Titers of neutralizing antibody to the prototype virus expressed as SCR in PLWH stratified by level of control of HIV infection.

Titers of neutralizing antibodies to the prototype virus expressed as GMFR in PLWH stratified by level of control of HIV infection.

Number of human immunodeficiency virus negative (HIV-) participants with first occurrence of positive (+) polymerase chain reaction (PCR), (+) PCR-confirmed, SARS-CoV-2 illness with symptomatic mild, moderate, or severe COVID-19 assessed from Day 28 (7 days after second vaccination dose) through the length of the study.

Number of HIV+ participants with first occurrence of (+) PCR-confirmed SARS-CoV-2 illness with symptomatic mild,moderate or severe COVID-19 assessed from Day 28 (7 days after second vaccination) through the length of the study.

Number of HIV- participants with solicited AEs, local and systemic, for 7 days following each vaccination (Days 0 and 21) by severity score, duration, and peak intensity.

Number of HIV- participants with unsolicited AEs (eg, treatment-emergent, serious, suspected unexpected serious, those of special interest, MAAEs) through Day 35 by Medical Dictionary for Regulatory Activities (MedDRA) classification, severity score, and relatedness.

Number of HIV+ participants with solicited AEs, local and systemic, for 7 days following each vaccination (Days 0 and 21) by severity score, duration, and peak intensity.

Number of HIV+ participants with unsolicited AEs (eg, treatment-emergent, serious, suspected unexpected serious, those of special interest, MAAEs) through Day 35 by MedDRA classification, severity score, and relatedness.

Serum IgG antibody levels specific for the SARS-CoV-2 rS protein antigen(s) as detected by enzyme-linked immunosorbent assay (ELISA) expressed as GMTs at Day 35.

Serum IgG antibody levels specific for the SARS-CoV-2 rS protein antigen(s) as detected by ELISA expressed as GMFRs at Day 35.

Serum IgG antibody levels specific for the SARS-CoV-2 rS protein antigen(s) as detected by ELISA expressed as SCR at Day 35. SCR is defined as the percentage of participants with a post-vaccination titer ≥ 4 fold over naïve background and ≥ 2 fold over pre existing titer.

Number of healthy adult HCW, with AESIs for 14 days post second vaccination (Day 35) by severity score, duration, and peak intensity.

Number of healthy adult HCW, with AESIs for 14 days post second vaccination (Day 70) by severity score, duration, and peak intensity.

Percentage of participants with solicited AEs (local, systemic) for 7 days following each primary vaccination (Days 0, 21) by severity score, duration, and peak intensity.

Safety laboratory values (serum chemistry, hematology) by FDA toxicity scoring (absolute and change from baseline where identified) at 7 days after each vaccination.

Serum IgG antibody levels specific for the SARS-CoV-2 rS protein antigen(s) as detected by enzyme-linked immunosorbent assay (ELISA) expressed as GMTs through Day 21.

Serum IgG antibody levels specific for the SARS-CoV-2 rS protein antigen(s) as detected by ELISA expressed as GMFRs through Day 35.

Serum IgG antibody levels specific for the SARS-CoV-2 rS protein antigen(s) as detected by ELISA expressed as SCRs through Day 35. SCR is the proportion of participants with ≥4-fold rises in ELISA units.

Serum IgG antibody levels specific for the SARS-CoV-2 rS protein antigen(s) as detected by ELISA expressed as GMTs for the two-dose regimens by dose at Day 35 regardless of baseline immune status and stratified by baseline immune status.

Serum IgG antibody levels specific for the SARS-CoV-2 rS protein antigen(s) as detected by ELISA expressed as GMFRs for the two-dose regimens by dose at Day 35 regardless of baseline immune status and stratified by baseline immune status.

Serum IgG antibody levels specific for the SARS-CoV-2 rS protein antigen(s) as detected by ELISA expressed as SCRs (≥4-fold rises) for the two-dose regimens by dose at Day 35 regardless of baseline immune status and stratified by baseline immune status.

Percentage of participants with solicited AEs (local, systemic) for 7 days following each primary vaccination (Days 0 and 21) by severity score, duration, and peak intensity.

Percentage of participants with unsolicited AEs (eg, treatment-emergent, serious, suspected unexpected serious, those of special interest, all medically attended adverse events \[MAAEs\]) through the 35 days by Medical Dictionary of Regulatory Activities (MedDRA) classification, severity score, and relatedness.