Delta breakthrough infections elicit potent, broad and durable neutralizing antibody responses

Abstract

The SARS-CoV-2 Delta variant is currently responsible for most infections worldwide, including among fully vaccinated individuals. Although these latter infections are associated with milder COVID-19 disease relative to unvaccinated subjects, the specificity and durability of antibody responses elicited by Delta breakthrough cases remain unknown. Here, we demonstrate that breakthrough infections induce serum binding and neutralizing antibody responses that are markedly more potent, durable and resilient to spike mutations observed in variants of concern than those observed in subjects who were infected only or received only two doses of COVID-19 vaccine. However, wee show that Delta breakthrough cases, subjects who were vaccinated after SARS-CoV-2 infection and individuals vaccinated three times (without infection) have serum neutralizing activity of comparable magnitude and breadth indicate that multiple types of exposure or increased number of exposures to SARS-CoV-2 antigen(s) enhance spike-specific antibody responses. Neutralization of the genetically divergent SARS-CoV, however, was moderate with all four cohorts examined, except after four exposures to the SARS-CoV-2 spike, underscoring the importance of developing vaccines eliciting broad sarbecovirus immunity for pandemic preparedness.

Figure 1:. Repeated exposures to SARS-CoV-2 antigens through vaccination or infection enhance S-specific serum IgG and IgA binding titers.

(A) Serum IgG binding titers at 30 or 60 days post infection or 10, 112, or 180 days post second or third vaccine dose were evaluated for longitudinal samples by ELISA using prefusion-stabilized SARS-CoV-2 S Hexapro as antigen. Serum samples were obtained from individuals who had a Delta breakthrough infection (n=15, magenta triangle), were previously infected then vaccinated (n=15, teal diamond), have only been vaccinated (n=15, orange circle), were infected in 2020 in Washington State (n=15, gray square), or were SARS-CoV-2 naive (samples taken prior to vaccination, n=15, open hexagon). (B) Serum IgA binding titers at 30 days post infection or 10 days post second vaccine dose were evaluated by ELISA using prefusion-stabilized SARS-CoV-2 S Hexapro as antigen. (C) Serum IgM binding titers at 30 days post infection or 10 days post second vaccine dose were evaluated by ELISA using prefusion-stabilized SARS-CoV-2 S Hexapro as antigen. (D) Serum IgG binding titers were evaluated by ELISA at 30 days post infection, 10 days post second or third vaccine dose or prior to SARS-CoV-2 exposure (SARS-CoV-2 naive) using prefusion-stabilized SARS-CoV 2P S as antigen. (E-F) Serum IgG binding titers were evaluated at 30 days post infection, 10 days post second or third vaccine dose, or prior to SARS-CoV-2 exposure (SARS-CoV-2 naive) by ELISA using OC43 S (E) or HKU1 2P S (F) as antigen. # of doses: number of vaccine doses received. Statistical significance was determined by Kruskal Wallis and Dunn’s multiple comparisons test and shown only when significant. *P < 0.05; **P < 0.01; ***P < 0.001; and ****P < 0.0001. LOD is shown as a gray horizontal dotted line when above the x axis. Raw fits are shown in Figure S1 and S2.

Figure 2:. Delta breakthrough, infected/vaccinated and triple vaccinated individuals have exceptionally high serum neutralizing activity.

Serum samples were obtained from individuals who had a Delta breakthrough infection (n=14, magenta triangle), who were previously infected then vaccinated (n=15, teal diamond, infected/vaccinated), who have been vaccinated only (n=15, orange circle), or who were infected only in 2020 in Washington State (n=15, gray square, HCS). All neutralization assays were performed using VeroE6-TMPRSS2 as target cells at least in duplicate. (A) SARS-CoV-2 G614 S VSV pseudotype neutralization. (B) SARS-CoV-2 Delta S VSV pseudotype neutralization. (C) SARS-CoV-2 Beta S VSV pseudotype neutralization. (D) SARS-CoV S VSV pseudotype neutralization. # of doses: number of vaccine doses received. Statistical significance was determined by Kruskal Wallis and Dunn’s multiple comparisons test and shown only within group or at matched timepoint for ease of viewing when significant. *P < 0.05; **P < 0.01; ***P < 0.001; and ****P < 0.0001. Normalized curves and fits are shown in Fig. S3–S13.