Monovalent XBB.1.5 booster vaccination induces a broad spectrum of SARS-CoV-2 neutralizing antibodies

KEYWORDS:

• SARS-CoV-2
• Omicron variant
• XBB.1.5
• booster vaccine
• neutralizing antibody

The COVID-19 pandemic is ongoing, with a “variant soup” after Omicron emerging. Omicron subvariants BQ and XBB exhibit a higher immune evasion due to more mutations in the key region of the spike protein (Figure 1(a)), resulting in a waning immune response among the vaccinees even boosted with a BA.5 bivalent vaccine [1]. The XBB subvariants EG.5 and FL.1.5.1 are the dominant strain now, which makes up 20.6% and 13.3% of new infections in the US. on Aug.19, respectively [2]. In the circumstances of a swarm of SARS-CoV-2 variants in prevalent, a booster COVID-19 vaccine that can induce a broad-spectrum immune response is in need.

Figure 1. The PNAb response induced by different booster vaccination strategies. (a) Mutation prevalence in the spike proteins of BA.5, BQ.1.1, CH.1.1, XBB.1.5, XBB.1.9, XBB.1.16, EG.5.1, and FL.1.5.1 [2]. NTD, N-terminal domain of S1; RBD, receptor-binding domain; S2, C-terminal furin cleavage fragment of spike. (b-m) The PNAb titers of Ad5-WT (b), Ad5-BA.4/5 (c), Ad5-BQ.1.1 (d), Ad5-XBB.1.5 (e), Ad5-WT+BA.4/5 (f), Ad5-WT+BQ.1.1 (g), Ad5-WT+XBB.1.5 (h), Ad5-BA.4/5+XBB.1.5 (i), Ad5-BQ.1.1+XBB.1.5 (j), Ad5-WT+BA.4/5+XBB.1.5 (k), Ad5-WT+BQ.1.1+XBB.1.5 (l), Ad5-BA.4/5+BQ.1.1+XBB.1.5 (m) at 28 days after booster vaccination against pseudovirus of WT and Omicron sublineages (BA.4/5, BQ.1.1, CH.1.1, XBB.1.5, XBB.1.9, XBB.1.16, and EG.5.1). The GMTs of the PNAb are shown, the lower limit of detection (LoD) is 30, neutralizing antibody levels that fell below the LoD were counted as 15. figure 1(n–p) The GMTs to the pseudovirus were normalized by Min-max normalization. For every PNAb value, the minimum value of PNAb gets transformed into a 0, the maximum PNAb value gets transformed into a 1, and every other value gets transformed into a decimal between 0 and 1. The transfer function is x* = (x-min)/(max-min). Among them, max/min is the maximum/minimum PNAb value of the same vaccine combination in different pseudovirus. The broad-spectrum neutralization abilities of monovalent (n), bivalent (o), Ad5-XBB.1.5 contained (p) groups are shown.

We evaluated the neutralization response of a series of monovalent or multivalent spike antigens of SARS-CoV-2 variants based on adenovirus type 5 virus, including the BA.4/5, BQ.1.1, and XBB.1.5 as a booster vaccination. BALB/c mice (n = 10 per group) were injected with 2 × 10⁶ infectious units (ifu) of wild-type (WT) vaccine as a prime vaccination, and intranasal booster with the same dose of the candidates in the form of monovalent, bivalent, trivalent or tetravalent at day 30 (Figure S1(a)). Anti-WT RBD IgG-binding antibodies were detected before the booster, and no significant differences were observed among the groups (Figure S1(b)). Anti-WT, BA.4/5, BQ.1.1, CH.1.1, XBB.1.5, XBB.1.9, XBB.1.16, and EG.5.1 pseudovirus neutralizing antibody (PNAb) responses were detected at 28 days after the booster to evaluate the immunoreactivity of the different vaccination strategies.

For the monovalent (Ad5-WT, Ad5-BA.4/5, Ad5-BQ.1.1, or Ad5-XBB.1.5) booster strategy, all candidates induced a high level of PNAb against the prototype strain, with geometric mean titers (GMTs) ranging from 6550 to 9733 (Figure 1(b–e)), demonstrated that the prime Ad5-WT injection stimulated sufficient memory B cells target to the prototype spike and can be recalled by the WT or variant strains. The Ad5-WT booster induced a low- or under-detection level of BA.4/5, BQ.1.1, CH.1.1, XBBs, and EG.5.1 PNAb response, with GMTs ranging from 16 to 260 (Figure 1(b)), indicating that those variants exhibit a strong immune evasion against wild-type SARS-CoV-2. Booster with Ad5-BA.4/5 or Ad5-BQ.1.1 results in a high level of PNAb against BA.4/5 and BQ.1.1 (GMTs ranging from 4009 to 7582) (Figure 1(c,d)), but a low level of PNAb to the XBBs (GMTs ranging from 62 to 530). However, the Ad5-XBB.1.5 booster induced a high level of PNAb against prototype (GMT = 6550), XBBs (GMTs ranging from 3935 to 4142) and the latest EG.5.1 (GMT = 3889), and a moderate level of PNAb against BA.4/5 (GMT = 1629), BQ.1.1 (GMT = 1494), and CH.1.1 (GMT = 796) (Figure 1(e)). These results indicate that a monovalent Ad5-XBB.1.5 booster vaccination can induce broad-spectrum neutralizing reactions against the prototype and variants, including XBBs, BA.4/5, BQ.1.1, CH.1.1, and the latest EG.5.1 (Figure 1(n)).

Bivalent booster vaccines can induce potent, durable, and broad antibody responses against multiple SARS-CoV-2 variants [3], and Omicron-containing bivalent vaccines against SARS-CoV-2 are being used in multiple geographies [4]. The bivalent booster strategies (Ad5-WT+BA.4/5, Ad5-WT+BQ.1.1, Ad5-WT+XBB.1.5, Ad5-BA.4/5+BQ.1.1, Ad5-BA.4/5+XBB.1.5, and Ad5-BQ.1.1+XBB.1.5) were conducted here, which intranasal inoculation with 2 × 10⁶ ifu at day 30 after Ad5-WT prime. The bivalent candidates without Ad5-XBB.1.5 (Ad5-WT+BA.4/5, Ad5-WT+BQ.1.1, and Ad5-BA.4/5+BQ.1.1) did not induce high levels of PNAb against XBBs, EG.5.1, and CH.1.1 (GMTs ranging from 62 to 1100) (Figure 1(f,g), Figure S2(a)). In contrast, the bivalent strategies containing Ad5-XBB.1.5 (Ad5-WT+XBB.1.5, Ad5-BA.4/5+XBB.1.5, and Ad5-BQ.1.1+XBB.1.5) all induced broad-spectrum PNAb response (Figure 1(h–j)), with the Ad5-BA.4/5+XBB.1.5, and Ad5-BQ.1.1+XBB.1.5 groups induced a superior breadth of neutralization (GMTs ranging from 726 to 8652). It demonstrated that a bivalent booster vaccine containing Ad5-XBB.1.5 can enhance the breadth of the immune response (Figure 1(o)). The trivalent and tetravalent booster strategies were also evaluated. Similar to the bivalent booster strategies, the trivalent booster containing Ad5-XBB.1.5 showed enhanced immunity and induced broad PNAb responses (GMTs ranging from 487 to 11,145) (Figure 1(k–m)). However, the booster of a tetravalent vaccine containing Ad5-WT, Ad5-BA.4/5, Ad5-BQ.1.1, and Ad5-XBB.1.5 induced low levels of XBBs, EG.5.1, and CH.1.1 PNAb response (Figure S2(c)).

When comparing the PNAb response of the booster strategies which contains Ad5-XBB.1.5, a similar PNAb response was observed in the monovalent, bivalent, and trivalent vaccine groups (Figure 1(p), Figure S3(a–h)). The Ad5-XBB.1.5 monovalent vaccine induced no significant difference in PNAb titers against WT, BQ.1.1, CH.1.1 XBB.1.5, XBB.1.16, and EG.5.1 compared to the Ad5-XBB.1.5 contained bivalent or trivalent groups and a significant higher XBB.1.9 PNAb titers compared to Ad5-WT+XBB.1.5 and Ad5-WT+BA.4/5+XBB.1.5 groups, but a significantly lower BA.4/5 PNAb titers compared to the groups which contain the Ad5-BA.4/5 (Figure S3(a–h)).

Last year, the bivalent COVID-19 mRNA vaccines containing BA.5/WT spike were licenced for use because the bivalent COVID-19 mRNA vaccine showed better immunogenicity than that of the monovalent BA.5 booster [5]. WHO recommended the use of a monovalent XBB.1 descendent lineage when updating the antigen composition of the COVID-19 vaccine in May 2023 [6]. Here, we demonstrated that a booster of the monovalent XBB.1.5 spike vaccine induces a broad-spectrum PNAb response against the circulating variants on mice and therefore, the development of next-generation COVID-19 vaccine containing monovalent XBB.1 spike is encouraged at this time.

There are some constraints and limitations in this study. We used the prototype vaccine to initiate primary immune response in a mouse model, while most people have multiple exposures to SARS-CoV-2 variant spike antigens, some conclusions here may not be generalizable to the human species. Another limitation is the absence of data on cellular immune response. Both neutralizing antibody and T-cell response are immune correlates of protection against COVID-19 [7–9], but unlike neutralizing antibodies, SARS-CoV-2 T-cell responses generated upon vaccination or previous infection are highly cross-reactive with Omicron [10]. As a result, a booster vaccination, which can induce a broadspectrum of SARS-CoV-2-neutralizing antibodies in the context of complex immune backgrounds, is critical to the prevention and control of the current COVID-19, and how immune imprinting affects immunity to XBB antigen remains complex to explore.

Ethics declarations

This study was performed in strict accordance with ethical guidelines for research in animal proved by the Ethics Committee of the Beijing Institute of Biotechnology Animal Care and Use Committee.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

All data are available in this article, supplementary materials, or available from the corresponding.