Robust humoral and cellular recall responses to AZD1222 attenuate breakthrough SARS-CoV-2 infection compared to unvaccinated

Abstract

Background: Breakthrough severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in coronavirus disease 2019 (COVID-19) vaccinees typically produces milder disease than infection in unvaccinated individuals.

Methods: To explore disease attenuation, we examined COVID-19 symptom burden and immuno-virologic responses to symptomatic SARS-CoV-2 infection in participants (AZD1222: n=177/17,617; placebo: n=203/8,528) from a 2:1 randomized, placebo-controlled, phase 3 study of two-dose primary series AZD1222 (ChAdOx1 nCoV-19) vaccination (NCT04516746).

Results: We observed that AZD1222 vaccinees had an overall lower incidence and shorter duration of COVID-19 symptoms compared with placebo recipients, as well as lower SARS-CoV-2 viral loads and a shorter median duration of viral shedding in saliva. Vaccinees demonstrated a robust antibody recall response versus placebo recipients with low-to-moderate inverse correlations with virologic endpoints. Vaccinees also demonstrated an enriched polyfunctional spike-specific Th-1-biased CD4+ and CD8+ T-cell response that was associated with strong inverse correlations with virologic endpoints.

Conclusion: Robust immune responses following AZD1222 vaccination attenuate COVID-19 disease severity and restrict SARS-CoV-2 transmission potential by reducing viral loads and the duration of viral shedding in saliva. Collectively, these analyses underscore the essential role of vaccination in mitigating the COVID-19 pandemic.

Keywords: AZD1222 (ChAdOx1 nCoV-19); COVID-19 vaccine; SARS-CoV-2; breakthrough infection; cell-mediated immunity; serology.

Figure 1

AZD1222 vaccinees had an overall lower incidence and shorter duration of COVID-19 symptoms compared with placebo recipients upon symptomatic SARS-CoV-2 infection. (A) Incidence and (B) mean duration of self-reported COVID-19 symptoms recorded in participant e-Diaries during the 28-day illness period. (A) Symptoms with differences of ≤1.5% between arms were excluded from this plot. (B) Symptoms with differences in mean durations of ≤1 day between arms are excluded from this plot. Error bars depict standard deviation.

Figure 2

Virologic outcomes to breakthrough infection are attenuated in AZD1222 vaccinees compared to unvaccinated. (A) SARS-CoV-2 genome copies from participant nasopharyngeal swabs collected at illness visits determined by quantitative (q)RT-PCR. Line plot with geometric means and 95% CI. Viral genome copies were imputed to 1 when the SARS-CoV-2 nasopharyngeal swab qualitative result was not detected. (B) SARS-CoV-2 quantitation (Log10 viral copies/mL) in participant saliva over time. Line plot with mean ± SD. Not detected values of viral quantitation are treated as 0. (C) Cumulative incidence plot of SARS-CoV-2 clearance (saliva samples). The median time to clearance of viral shedding for each group is marked by a circle. (D, E) Viral load by SARS-CoV-2 variant (viral genome copies) in nasopharyngeal swabs (D) and saliva samples (E) collected at first illness visit determined by qRT-PCR. The bottom and top edges of the box indicate the first and third quartiles, the difference is the IQR, the line inside the box is the median, and the marker inside the box is the geometric mean. The whiskers that extend from the box indicate the minimum and maximum after removing outliers (i.e., datapoints >1.5 x IQR from the box). Viral genome copies are imputed to 1 when the SARS-CoV-2 nasopharyngeal swab qualitative result is not detected.

Figure 3

The kinetics and magnitude of the breakthrough anti-SARS-CoV-2 antibody response are impacted by age and vaccination status. Levels of anti-SARS-CoV-2 (A) spike-binding, (B) neutralizing, and (C) nucleocapsid antibodies in AU/mL. The bottom and top edges of the box indicate the first and third quartiles, the difference is the IQR, the line inside the box is the median, and the marker inside the box is the geometric mean. The whiskers that extend from the box indicate the minimum and maximum after removing outliers (i.e., datapoints >1.5 x IQR from the box). (A, B) Yellow shaded region denotes peak antibody titers observed during primary analysis [13].Statistical evidence between groups was determined by post-hoc two-tailed Mann-Whitney tests. Not significant (NS), p>0.05; *p ≤ 0.05; **p ≤ 0.01; ***p ≤ 0.001.

Figure 4

Initial breakthrough SARS-CoV-2 spike-binding antibody titers are influenced by the interval since second dose primary series vaccination. Levels of anti-SARS-CoV-2 spike-binding antibodies by time since second dose of primary series vaccination. The bottom and top edges of the box indicate the first and third quartiles, the difference is the IQR, the line inside the box is the median, and the marker inside the box is the mean. The whiskers that extend from the box indicate the minimum and maximum after removing outliers (i.e., datapoints >1.5 x IQR from the box).

Figure 5

Illness visit day 1 SARS-CoV-2 nAb titers display low-moderate negative correlations with SARS-CoV-2 virologic outcomes. Scatterplot analysis depicting the relationship between (A) SARS-CoV-2 viral load in nasopharyngeal swabs (B) SARS-CoV-2 viral load in saliva samples, and (C) duration of viral shedding in saliva (y-axes) and illness visit day 1 nAb titers (x-axes) in vaccinees and placebo recipients. Blue and red shading denotes 95% CI. Dotted line denotes 95% prediction limits. Clustering of participants along the y-axis occurs due to levels of serum anti-SARS-CoV-2 neutralizing antibody falling below the assay lower limit of quantification (LLOQ). LLOQ= 40 ID50. 50% of LLOQ =20 ID50.

Figure 6

AZD1222 vaccinees possess an enriched SARS-CoV-2 spike-specific T-cell response upon symptomatic SARS-CoV-2 infection compared with placebo recipients. T-cell responses from vaccinees and placebo recipients were assessed following stimulation of PBMCs with SARS-CoV-2 spike peptide pools. Frequencies of SARS-CoV-2 spike protein-specific CD4+ T cells (A) expressing CD154, interferon gamma (IFNγ), IL-2, and tumor necrosis factor alpha (TNFα), or any combination of all four (Any Response), and CD8+ T cells (B) expressing IFNγ, IL-2, and TNFα, or any combination of all three (Any Response) are shown. Illness visit day 1 cytokine profiles and frequencies of (C) CD4+ and (D) CD8+ T cell populations upon breakthrough infection in vaccinees and placebo recipients. Spike-specific CD4+ (E) and CD8+ (F) T-cell frequencies (Any Response) from participants at ILL-D1 and ILL-D14 are shown. Bars indicate median values within each group. Frequencies of spike-specific CD4+ (G) and CD8+ (H) T-cells (Any Response) against ancestral SARS-COV-2 and Omicron BA.1 variant spike proteins from vaccinee and placebo recipient “responders” at illness visit day 1. In the box and whisker plots the horizontal line represents median, boxes represent IQR, whiskers extend to minimum and maximum, and each symbol represents a participant. Dotted line indicates “responder” threshold. Statistical evidence between groups were determined by two-tailed Mann-Whitney tests. Not significant (NS), P>0.05; *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001. An. = Ancestral; O = Omicron.

Figure 7

Illness visit day 1 CD8+ titers display strong negative correlations with SARS-CoV-2 virologic outcomes. Scatterplot analysis depicting the relationship between (A) SARS-CoV-2 viral load in nasopharyngeal swabs (B) SARS-CoV-2 viral load in saliva samples, and (C) duration of saliva viral shedding (y-axes) and illness visit day 1 spike-specific CD8+ T cell frequencies (x-axes) in vaccinees and placebo recipients. Blue and red shading denotes 95% confidence limits. Dotted line denotes 95% prediction limits.