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.

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.

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.