Determinants of early antibody responses to COVID-19 mRNA vaccines in a cohort of exposed and naïve healthcare workers

Abstract

Background: Two doses of mRNA vaccination have shown >94% efficacy at preventing COVID-19 mostly in naïve adults, but it is not clear if the second dose is needed to maximize effectiveness in those previously exposed to SARS-CoV-2 and what other factors affect responsiveness.

Methods: We measured IgA, IgG and IgM levels against SARS-CoV-2 spike (S) and nucleocapsid (N) antigens from the wild-type and S from the Alpha, Beta and Gamma variants of concern, after BNT162b2 (Pfizer/BioNTech) or mRNA-1273 (Moderna) vaccination in a cohort of health care workers (N=578). Neutralizing capacity and antibody avidity were evaluated. Data were analyzed in relation to COVID-19 history, comorbidities, vaccine doses, brand and adverse events.

Findings: Vaccination induced robust IgA and IgG levels against all S antigens. Neutralization capacity and S IgA and IgG levels were higher in mRNA-1273 vaccinees, previously SARS-CoV-2 exposed, particularly if symptomatic, and in those experiencing systemic adverse effects (p<0·05). A second dose in pre-exposed did not increase antibody levels. Smoking and comorbidities were associated with 43% (95% CI, 19-59) and 45% (95% CI, 63-18) lower neutralization, respectively, and 35% (95% CI, 3-57%) and 55% (95% CI, 33-70%) lower antibody levels, respectively. Among fully vaccinated, 6·3% breakthroughs were detected up to 189 days post-vaccination. Among pre-exposed non-vaccinated, 90% were IgG seropositive more than 300 days post-infection.

Interpretation: Our data support administering a single-dose in pre-exposed healthy individuals as primary vaccination. However, heterogeneity of responses suggests that personalized recommendations may be necessary depending on COVID-19 history and life-style. Higher mRNA-1273 immunogenicity would be beneficial for those expected to respond worse to vaccination and in face of variants that escape immunity such as Omicron. Persistence of antibody levels in pre-exposed unvaccinated indicates maintenance of immunity up to one year.

Funding: This work was supported by Institut de Salut Global de Barcelona (ISGlobal) internal funds, in-kind contributions from Hospital Clínic de Barcelona, the Fundació Privada Daniel Bravo Andreu, and European Institute of Innovation and Technology (EIT) Health (grant number 20877), supported by the European Institute of Innovation and Technology, a body of the European Union receiving support from the H2020 Research and Innovation Programme. We acknowledge support from the Spanish Ministry of Science and Innovation and State Research Agency through the "Centro de Excelencia Severo Ochoa 2019-2023" Program (CEX2018-000806-S), and support from the Generalitat de Catalunya through the CERCA Program. L. I. work was supported by PID2019-110810RB-I00 grant from the Spanish Ministry of Science & Innovation. Development of SARS-CoV-2 reagents was partially supported by the National Institute of Allergy and Infectious Diseases Centers of Excellence for Influenza Research and Surveillance (contract number HHSN272201400008C). The funders had no role in study design, data collection and analysis, the decision to publish, or the preparation of the manuscript.

Keywords: Antibody; Avidity; COVID-19; Health care workers; Neutralization; SARS-CoV-2; mRNA vaccines.

Figure 1

Study flowchart. Participants selection, recruitment, sample sizes in all study visits, and subjects used for avidity and neutralization assays. HCW, health care workers.

Figure 2

Pre- and post-vaccination antibody levels after 1 dose and 2 doses of the COVID-19 mRNA vaccines. Plots show IgA, IgG and IgM levels (median fluorescence intensity, MFI) against the receptor-binding domain (RBD) of the SARS-CoV-2 Spike glycoprotein (S), the S protein and its subunit S2 at pre- and post-vaccination after 1 dose (N=44) (a) and 2 doses (N=253) (b). All plasma samples were analyzed at 1:500 dilution. Pre-vaccination samples were collected at study month 6 for those who were already vaccinated at month 9, and at month 9 for those vaccinated at month 12. Post-vaccination samples analyzed are those collected >10 days after the 1^st dose (a) and 2 weeks after the 2^nd dose (b). Paired samples are joined by grey lines. The center line of boxes depicts the median of MFIs; the lower and upper hinges correspond to the first and third quartiles; the distance between the first and third quartiles corresponds to the interquartile range (IQR); whiskers extend from the hinge to the highest or lowest value within 1·5 × IQR of the respective hinge. Wilcoxon signed-rank test was used to assess statistically significant differences in antibody levels between pre- and post-vaccination.

Figure 3

Neutralizing capacity of plasma samples before and after COVID-19 mRNA vaccination. Dots depict antibody neutralizing capacity, as a percentage of RBD-ACE2 binding inhibition by plasma samples from 33 participants. Paired samples are joined by grey lines. The center line of boxes depicts the median of the neutralization percentage; the lower and upper hinges correspond to the first and third quartiles; the distance between the first and third quartiles corresponds to the interquartile range (IQR); whiskers extend from the hinge to the highest or lowest value within 1·5 × IQR of the respective hinge. Wilcoxon signed-rank test was used to assess statistically significant differences between pre- and post-vaccination neutralization. Pre-vaccination levels correspond to visit M6. Pre-vaccination samples were analyzed at a 1:50 dilution while post vaccination samples were analyzed at 1:400. We standardized the post-vaccination results to make them comparable by dividing them by 8.

Figure 4

Antibody levels against S antigens after one and two doses of mRNA vaccines in previously SARS-CoV-2 infected and uninfected individuals. Plots show IgA, IgG and IgM levels (log₁₀ MFI) against the receptor-binding domain (RBD) of the SARS-CoV-2 Spike glycoprotein (S), the S protein and its subunit S2 after 1 dose (N=64, 20 naive and 44 pre-exposed) and 2 doses (N=263, 211 naive and 52 pre-exposed). Post-vaccination samples analyzed were those collected >7 days after the 1^st dose and 2 weeks after the 2^nd dose. The center line of boxes depicts the median of MFIs; the lower and upper hinges correspond to the first and third quartiles; the distance between the first and third quartiles corresponds to the interquartile range (IQR); whiskers extend from the hinge to the highest or lowest value within 1·5 × IQR of the respective hinge. Wilcoxon rank test was used to assess statistically significant differences in antibody levels between naive and pre-exposed participants for a same dosage, and between 1^st and 2^nd dose into each group. We selected all dilutions at 1:500 to make levels comparable.

Figure 5

Antibody neutralization capacity and avidity after two doses of COVID-19 mRNA vaccines in naive and pre-exposed participants. a) Antibody neutralizing capacity, as a percentage of RBD-ACE2 binding inhibition by plasma samples assayed at 1:400 dilution (N=92, 47 naive and 45 pre-exposed). b) Antibody avidity, as % of IgA and IgG levels against RBD, S and S2 antigens measured incubating samples with a chaotropic agent over the IgA and IgG levels measured in the same samples without chaotropic agent, all at 1:5000 dilution (N=58, 48 naive and 10 pre-exposed). The center line of boxes depicts the median of MFIs; the lower and upper hinges correspond to the first and third quartiles; the distance between the first and third quartiles corresponds to the interquartile range (IQR); whiskers extend from the hinge to the highest or lowest value within 1·5 × IQR of the respective hinge. Wilcoxon rank test was used to assess statistically significant differences in antibody neutralization and avidity between naive and pre-exposed participants.

Figure 6

Comparison of antibody levels, neutralization and avidity between the two COVID-19 mRNA vaccines after two doses. a) Antibody levels elicited by BNT162b2 and mRNA-1273 among naive and pre-exposed participants (N = 263, 207 BNT162b2 / 56 mRNA-1273, 211 naïve, 52 exposed). Plasma samples were analyzed at 1:5000 dilution for IgG and 1:500 for IgA/IgM. b) Plasma neutralization capacity elicited by BNT162b2 and mRNA-1273 among naive and pre-exposed participants (N=92, 45 BNT162b2r/47 mRNA-1273, 47 naive, 45 exposed). Plasma dilution used was 1:400. c) Antibody avidity elicited by BNT162b2 vs mRNA-1273 among naive and pre-exposed participants (N=58, 36 BNT162b2 and 22 mRNA-1273, 48 naive, 10 pre-exposed). Plasma dilution used was 1:5000. Red and green dots correspond to naive and pre-exposed participants, respectively.

Figure 7

Kinetics of SARS-CoV-2 antibody levels since onset of symptoms in non-vaccinated participants. Levels (median fluorescence intensity, MFI) of IgA, IgG and IgM against each antigen (Nucleocapsid full length protein (N), and its C-terminal domain, the Receptor Binding Domain (RBD), full S protein and its subunit S2) measured in 338 samples from 102 symptomatic participants collected in up to 6 time points per participant (paired samples joined by lines). The black solid line represents the fitted curve calculated using the LOESS (locally estimated scatterplot smoothing) method. Shaded areas represent 95% confidence intervals. Dashed line represents the positivity threshold. Samples were analyzed at the 1:500 dilution.