Serum SARS-CoV-2-Specific Antibodies and Neutralization Capacity Following Vaccination

To editor:

In March 2020, the World Health Organization officially announced the coronavirus disease 2019 (COVID-19) pandemic, also known as the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outbreak, as a serious global health crisis due to the alarming levels of morbidity and mortality associated with the virus worldwide.¹ SARS-CoV-2 infected individuals may remain asymptomatic in the early stage, but as the disease progresses, it can lead to severe pneumonia, dyspnea, organ dysfunction, and even death. As of November 2023, COVID-19 has affected more than 772 million people and led to the deaths of more than six million individuals globally.² During pregnancy, women with symptomatic SARS-CoV-2 infection face an increased risk of developing severe complications, with elevated rates of hospital admission, intensive care unit admission, and death.³

Vaccination is essential in preventing the transmission of SARS-CoV-2 and reducing the incidence of its related morbidity and mortality. In 2021, it was estimated that vaccinations prevented more than 14 million deaths from COVID-19 worldwide.⁴ A study by Smith et al.⁵ in Hong Kong showed that the risk of death related to COVID-19 was more than thirty times higher among individuals who were not vaccinated compared to those who received at least two doses of vaccine.

We have previously reported a case series on evaluating the temporal changes in the levels and neutralization ability of anti-SARS CoV-2 antibodies in breast milk stimulated by natural infection or vaccination.⁶ Our findings revealed a progressive increase in both binding ability and immunoglobulin A (IgA) levels after both natural infection and COVID-19 vaccination. The neutralizing activity of antibodies became undetectable around 150 days after the first vaccine dose, but administering a booster dose restored the secretion of neutralizing IgA antibodies in breast milk. These findings underscore the importance of a vaccine booster dose in maintaining adequate levels of neutralizing antibodies. On the other hand, there is limited evidence regarding the antibody levels in maternal blood following vaccination, particularly after the vaccine booster doses. Therefore, our study aimed to evaluate the temporal changes of anti-SARS-CoV-2 antibodies in maternal serum among vaccinated women who had received booster doses.

This prospective case series included 35 randomly selected pregnant women who attended routine screening for Down syndrome at 11-13 weeks of gestation from November 2019 to October 2020. All participants gave birth at either Prince of Wales Hospital or Kwong Wah Hospital, Hong Kong SAR; the last delivery was in March 2021. Exclusion criteria were confirmed SARS-CoV-2 infection during the study period, spontaneous miscarriage or stillbirth, and termination of pregnancy. Eligible women were provided with written information describing the study, and those who consented to participate gave written informed consent. After enrollment, blood samples were collected in serum tubes from the participants before vaccination and at least 7 days after each of the three vaccine doses.

Serum anti-SARS-CoV-2 antibodies were analyzed by Elecsys Anti-SARS-CoV-2 assay (Roche Diagnostics, Rotkreuz, Switzerland) on a Cobas®e411 analyzer. This qualitative serological assay was able to detect immunoglobulin G (IgG) and immunoglobulin M (IgM) directed against the nucleocapsid protein of the SARS-CoV-2. A positive anti–SARS–CoV–2 antibody result was defined as a cut-off index ≥ 1.0.

Positive results from qualitative tests for anti-SARS–SARS-CoV-2 antibodies were subsequently confirmed by quantitative assays measuring IgG and IgM antibodies targeting the SARS-CoV-2 spike protein, conducted using enzyme-linked immunosorbent assays (ImmunoDiagnostics Limited, Hong Kong, China). All tests were performed in duplicate following the manufacturer’s protocols. An optical density < 0.2 was interpreted as negative, while a value of ≥ 0.2 was defined as positive. For samples with positive results, the concentration of anti–spike protein (ng/mL) was determined.

For the analysis of anti-SARS-CoV-2 antibody levels measured at multiple time points, a linear mixed-effects model was employed, incorporating both fixed and random effects. The fixed effects consisted of the number of vaccine doses and the time interval between vaccination and blood collection. The random effects included the intercept as well as the number of vaccine doses. The best-fitting model was determined using the Bayesian information criterion (BIC), comparing four models: fixed intercept with fixed slope, fixed intercept with random slope, random intercept with fixed slope, and random intercept with random slope. The final model was chosen based on the lowest BIC value. The estimated marginal means of antibody levels before and after vaccination are presented, with a p-value threshold of less than 0.05 indicating statistical significance. Data analyses were conducted using SPSS version 23.1 (IBM SPSS Statistics for Windows, Armonk, N.Y., USA).

Prior to vaccination, anti-SARS-CoV-2 antibodies were not detected in maternal serum. Using a mixed effect model, we found that the estimated marginal mean of log₁₀ anti-SARS-CoV-2 antibodies increased significantly following the first dose (0.11565; 95% confidence interval (CI): 0.16496–0.39627) compared to the pre-vaccination levels (–0.54223; 95% CI: 0.82944–0.25503) (P = 0.001). Additionally, the levels of anti-SARS-CoV-2 antibodies following the second and third-doses were approximately thirty-fold higher compared to the levels following the first dose, with estimated marginal means of 3.00031 (95% CI: 2.72652–3.27411) and 3.79196 (95% CI: 3.43801–4.14591), respectively (P < 0.001) (Fig. 1, Supplementary Tables 1 and 2, http://links.lww.com/MFM/A62).

Figure 1.

The changes of serum anti-SARS-CoV-2 antibodies before and after vaccination (n = 35). COVID-19: Coronavirus disease 2019; SARS-CoV-2: Severe acute respiratory syndrome coronavirus 2.

Our results have indicated that the levels of anti-SARS-CoV-2 antibodies significantly increase following vaccination, with even more pronounced increases observed after booster doses, which aligns with existing literature. Golan et al.⁷ reported that maternal blood IgM and IgG antibodies increased significantly following the first dose of SARS-CoV-2 vaccination, with a more substantial increase in IgG antibody levels at 4 to 10 weeks post-second dose. Similarly, a Singaporean study reported that the median neutralizing antibody levels at 0 to 10 days following the second dose (704.6 (163.1–2784.0 IU/ml) were significantly higher than at 0 to 3 days following the first dose (6.4 (3.4–8.9) IU/ml) (P < 0.001). The median levels of neutralizing antibody elevated from the first dose to the second dose, with a dramatic increase until 7-10 days post-vaccine booster (168.4 (100.7-288.5) vs. 2753.0 (1627.0–4712.0) IU/ml, P < 0.001).⁸ Although our study has demonstrated the efficacy of booster doses in enhancing serum-neutralizing antibody levels, the need for booster doses for the general community, especially pregnant women, remains an unanswered question.⁹ Hence, larger longitudinal studies are needed to specifically determine the necessity and optimal timing of COVID-19 vaccine boosters to maintain the protective levels of anti-SARS-CoV-2 antibodies in the maternal blood.

Acknowledgment

This work was supported by funding from Roche Diagnostics (Rotkreuz, Switzerland). The funder had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript.

Funding

None.

Conflicts of Interest

Liona C. Poon has been compensated with speaker fees and consulting payments from Ferring Pharmaceuticals and Roche Diagnostics. She has also received in-kind support from GE Healthcare, Revvity Inc. (formerly PerkinElmer Life and Analytical Sciences), Roche Diagnostics, Ningbo Aucheer Biological Technology Co., Ltd., and Thermo Fisher Scientific. Other authors report no conflict of interest.

Editor Note

Liona C. Poon and Chi Chiu Wang are editorial board members of Maternal-Fetal Medicine. The article was subject to the journal’s standard procedures, with peer-review handled independently of this editor and the associated group.

Data Availability

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.