Neutralization of SARS-CoV-2 omicron after BBIBP-CorV and ZF2001 booster vaccination

The COVID-19 epidemic has been raging for three years, and vaccines have become one of the most promising irreplaceable means of preventing and controlling COVID-19. The fifth variants of concern (VOCs) identified by the World Health Organization (WHO), omicron, is sweeping the world. The omicron variant carries a large number of mutations in the spike protein, with at least 15 mutations in the receptor-binding domain (RBD), the primary target of neutralizing antibodies [1]. The neutralization capacity of vaccines against omicron variant and the identification of booster strategies are of global concern [2,3].

We recruited 353 participants who had completed basal immunization with two doses of inactivated vaccine to perform BBIBP-CorV or ZF2001 as the third dose respectively. The participants were divided into 5 groups based on age and vaccination details(Fig. 1 A). We performed live virus neutralization assays using wild-type SARS-CoV-2(EPI_ISL_13499639) and omicron BA.2.68 sub-variant(EPI_ISL_14675318) to evaluate neutralizing activity against SARS-CoV-2 before booster and 14 and 28 days after booster. The IgG antibodies were detected using an indirect ELISA kit(Vazyme, China) based on SARS-CoV-2 spike protein. Significance was assessed with the use of Mann-Whitney test and unpaired t-test. The characteristics of the participants and detailed experimental methods are provided in the Supplementary Appendix. All participants have signed written informed consent, and this study was approved by the Ethical Approval Committee of Shandong Center for Disease Control and Prevention (Ethical approval number: SDJK2022-003-01).

Fig. 1.

Neutralizing antibodies against wild-type and BA.2.68, and serum IgG titers induced by BBIBP-CorV and ZF2001 booster vaccines. (A) The details of this study design, including age characteristics of participants, booster vaccine type, booster interval and serum sample collection time in the 5 booster groups. (B–D) Box violin plots show neutralizing antibodies against wild-type SARS-CoV-2 and Omicron BA.2.68, and IgG titers before booster and 14 and 28 days after booster from participant's serum samples in 5 groups. The geomatic mean titers(GMTs) are shown above each column. The number of serum samples is shown below each column.

The participants' serum samples before booster had low neutralization ability against wild-type and BA.2.68(mean GMT = 4.04), and there was no statistical difference between the 5 groups. On the 14th day after booster, the neutralization ability of wild-type increased to 2.91–32.34 times, and the neutralization ability of BA.2.68 increased to 1.66–9.61 times. On the 28th day after booster, the neutralization ability of the samples from the participants against wild-type and BA.2.68 increased to 10.44–53.01 and 3.37–15.23 times (Fig. 1 B-D and Table 1 ). The results clearly showed that although neutralizing antibodies were gradually reduced after two doses of inactivated vaccine, the antibody response could be awakened by third dose rapidly.

Table 1.

GMTs and statistical significance in neutralizing antibodies.

Group	wild-0d	wild-14d	wild-28d	BA.2.68-0d	BA.2.68–14d	BA.2.68–28d
group A	boosted by BBIBP-CorV, 6 months interval, 18–59 years old
GMT	3.97	53.29	80.50	3.62	7.37	15.45
Fold change(to 0d)		13.41	20.26		2.04	4.26
P value		****	****		****	****
Fold change(to wild)				−1.1	−7.23	−5.21
P value				ns	****	****
group B	boosted by BBIBP-CorV, 6 months interval, over 60 years old
GMT	4.24	12.34	44.23	3.86	6.39	13.04
Fold change(to 0d)		2.91	10.44		1.65	3.37
P value		****	****		****	****
Fold change(to wild)				−1.1	−1.93	−3.39
P value				ns	****	**
Fold change(to group A)	−0.94	−4.32	−1.82	−0.94	−1.15	−1.18
P value	ns	ns	ns	ns	ns	ns
group C	boosted by ZF2001, 6 months interval, 18–59 years old
GMT	4.06	131.32	204.25	3.89	37.40	54.93
Fold change(to 0d)		32.33	50.29		9.62	14.13
P value		****	****		****	****
Fold change(to wild)				−1.04	−3.51	−3.72
P value				ns	****	**
Fold change(to group A)	1.02	2.46	2.54	1.07	5.07	3.56
P value	ns	***	*	ns	****	**
group D	boosted by ZF2001, 6 months interval, over 60 years old
GMT	4.36	54.57	121.24	4.00	15.62	37.10
Fold change(to 0d)		12.52	27.82		3.91	9.27
P value		****	****		****	****
Fold change(to wild)				−1.09	−3.49	−3.27
P value				ns	****	*
Fold change(to group B)	1.03	4.42	2.74	1.04	2.45	2.85
P value	ns	ns	ns	ns	***	*
Fold change(to group C)	−0.93	−2.41	−1.68	−0.97	−2.39	−1.48
P value	ns	***	ns	ns	****	ns
group E	boosted by ZF2001, 12 months interval, 18–59 years old
GMT	4.39	102.18	232.85	4.04	34.60	61.50
Fold change(to 0d)		23.26	53.01		8.57	15.23
P value		****	****		****	****
Fold change(to wild)				−1.09	−2.95	−3.79
P value				ns	***	*
Fold change(to group C)	1.08	0.78	1.14	1.04	0.93	1.12
P value	ns	ns	ns	ns	ns	ns

*, p < 0.05; **, p < 0.01; ***, p < 0.001; ****, p < 0.0001, and “ns” indicates no statistical significance.

The neutralizing activity against BA.2.68 variant in the serum samples of all participants after booster was lower than that of the wild-type, which decreased by 1.93–6.36 times at 14 days, and decreased by 3.27–5.21 times at 28 days(Table 1). While it is true that omicron variants carrying a large number of spike protein mutations could lead to immune escape, a third dose still provides protection against omicron variants.

In the comparison of the effects in booster dose between younger and older groups, there was no significant difference in neutralizing antibody levels between the group B and group A after BBIBP-CorV booster, and between the group D and group C at 28 days after ZF2001 booster(Table 1). Both our result and a previous study [4] suggest that we may be able to view the effectiveness of booster vaccination in the elderly population in a positive way. Importantly, similar to a recent pseudovirus neutralization assay study on booster doses [5], the ZF2001 heterologous boost induced significantly higher antibody titers compared to the homologous boost(Fig. 1 B-D and Table 1). Recent studies on boost interval of ZF2001 mentioned that extending the booster interval from 60 days to 140 days and 1 month to 4 months appears to result in higher neutralizing antibody titers [6,7]. We found that no significant difference was observed after extending the booster interval from 6 months(group C) to 12 months (group E) (Table 1).

The rapid emergence of new variants poses a severe challenge for the protective efficacy of vaccines developed with wild-type SARS-CoV-2 vaccine strains. The third dose of booster vaccination is being actively implemented in many countries. Our study further demonstrate that the importance of the third dose of vaccine is credible, and provide an important basis for optimizing booster immune strategies. It should be noted that cellular immune responses also play an important role in vaccine efficacy evaluation, our study only conducted humoral immunity studies, and our results did not analyze the relationship between clinical severity and vaccination. Also limited by the single type of omicron sub-variant and the short follow-up time, the broad-spectrum effectiveness and durability of the third dose of COVID-19 vaccine needs to be further studied.

Author contributions

Xiaolin Jiang: Conceptualization, Methodology, Writing-Review & Editing, Funding acquisition. Lin Zhang: Study design and Statistical analysis. Jinzhong Zhang and Xiangkun Jiang: Resources. Yuwei Zhang, Xingyu Guo, Shanshan Han, Mingxiao Yao, Shu Zhang, Bo Pang, Jianxing Wang, Shuang Wang, Ming Fang, Xiaolin Liu and Zengqiang Kou: Investigation. Yuwei Zhang, Xingyu Guo, Shanshan Han and Lianxiang Zhao: Formal analysis, Visualization, Writing-Original Draft. All of the authors approved the manuscript.

Funding

This work was supported by the Major Scientific and Technological Innovation Project in Shandong Province (grant numbers: 2020SFXGFY02-1), Key Research and Development plan of Shandong Province(grant numbers: 2021RZA01021) and Natural Science Foundation of Shandong Province(grant numbers: ZR202112040005).

Declaration of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Appendix A. Supplementary data

The following is the Supplementary data to this article.