Randomized controlled trial reveals no benefit to a 3-month delay in COVID-19 mRNA booster vaccine

Abstract

BACKGROUNDThere is uncertainty about the timing of booster vaccination against COVID-19 in highly vaccinated populations during the present endemic phase of COVID-19. Studies focused on primary vaccination have previously suggested improved immunity with a longer interval between the first and second vaccine doses.METHODSWe conducted a randomized, controlled trial (November 2022-August 2023) and assigned 52 fully vaccinated adults to an immediate or a 3-month delayed bivalent Spikevax mRNA booster vaccine. Follow-up visits were completed for 48 participants (n = 24 per arm), with collection of saliva and plasma samples following each visit.RESULTSThe rise in neutralizing antibody responses to ancestral and Omicron strains were almost identical between the immediate and delayed vaccination arms. Analyses of plasma and salivary antibody responses (IgG, IgA), plasma antibody-dependent phagocytic activity, and the decay kinetics of antibody responses were similar between the 2 arms. Symptomatic and asymptomatic SARS-CoV-2 infections occurred in 49% (21 of 49) participants over the median 11.5 months of follow-up and were also similar between the 2 arms.CONCLUSIONSOur data suggest that there was no benefit in delaying COVID-19 mRNA booster vaccination in preimmune populations during the present endemic phase of COVID-19.TRIAL REGISTRATIONAustralian New Zealand Clinical Trials Registry number 12622000411741 (https://anzctr.org.au/Trial/Registration/TrialReview.aspx?ACTRN=12622000411741).FUNDINGNational Health and Medical Research Council, Australia (program grant App1149990) and Medical Research Future Fund (App2005544).

Keywords: Adaptive immunity; Antigen; COVID-19; Immunoglobulins; Vaccines.

Figure 1. CONSORT flow diagram.

Figure 2. Neutralizing antibodies following bivalent mRNA booster vaccination.

Plasma neutralizing activity was measured using a live virus neutralization assay against SARS-CoV-2 Omicron BA.1 (A–E) and XBB.1.5 (F–J) variants. Pre-booster (A and F) and post-booster day-14 (D14) (D and I) and day-84 (D84) (E and J) neutralizing antibody responses were compared between the delayed (blue/purple diamonds, n = 24) and immediate arms (pink triangles, n = 24) at the respective sampling time points. Line graphs describe the kinetics of plasma neutralization activity of the delayed (B and G) and immediate (C and H) arms after receiving the bivalent booster. Numbers above each time point describe the respective median neutralization IC₅₀ against each viral variant. Dotted lines depict the detection threshold for the assay (neutralization IC₅₀ = 20). Dark purple diamonds and lines show the antibody responses of the 3 individuals who received the BA.5 bivalent booster in the delayed arm of the study. Saliva neutralizing activity against ancestral SARS-CoV-2 was measured using the sVNT (Genscript). Pre-booster (K) and post-booster day-14 (N) neutralizing antibody responses were compared between the delayed (purple diamonds, n = 18) and immediate arms (pink triangles, n = 19), respectively. Line graphs describe the change in saliva neutralization activity following the bivalent booster (L and M). Numbers describe the percentage of surrogate neutralization observed at each time point. Dotted lines depict the sVNT cutoff for neutralizing activity (30%). Statistical significance was calculated between cohorts and time points using the 2-tailed Mann-Whitney U test or the Kruskal-Wallis test followed by Dunn’s multiple-comparison test. Paired saliva analysis (day 0 vs. day 14) was performed using the Wilcoxon matched-pairs, signed-rank test. Experiments were performed in duplicate. Graphs are displayed as the median, and where significant, P values are reported (**P ≤ 0.01, ***P ≤ 0.001, and ****P ≤ 0.0001).

Figure 3. Antibody kinetics following bivalent mRNA booster vaccination.

Kinetics of plasma (A–O) and saliva (P–U) antibody responses against the SARS-CoV-2 variant Omicron XBB.1.5. Line graphs depict the plasma neutralization responses in the delayed (purple diamonds, n = 24) (A) and immediate (pink triangles, n = 24) (B) arms as previously described in Figure 2, G and H. Line graphs also illustrate the rise and decay of plasma total IgG levels (D and E), total IgA responses (G and H), Fc-γR2a binding (J and K), and antibody-dependent phagocytic activity (M and N), as well as salivary total IgG levels (P and Q) and total IgA (S and T) responses in the delayed (purple diamonds, n = 24) (D, G, J, M, P, and S) and immediate (pink triangles, n = 24) (E, H, K, N, Q, and T) arms, respectively. Dark purple diamonds and lines show the antibody responses of the 3 individuals who received the BA.5 bivalent booster in the delayed arm of the study. Modeled decay slopes (C, F, I, L, O, R, and U) describe the half-life and time taken for the respective antibody responses to return to pre-booster baseline levels. Statistical significance was calculated between cohorts using the likelihood ratio test, and where significant, P values are reported (*P ≤ 0.05). Experiments were performed in duplicate.

Figure 4. Breakthrough COVID-19.

Kaplan-Meier probability of remaining negative for symptomatic COVID-19 during the study in the delayed (purple) and immediate (pink) arms (A). Analysis includes all first on-study COVID-19 symptomatic infections (pre- and post-study vaccination, self-reported). The probability for the delayed arm reaches zero because the final 3 delayed arm participants were positive/censored just after 12 months, whereas there were 5 final immediate arm participants who remained at risk. The numbers below the graph show the remaining number of participants at risk (number censored) during the study at baseline (0 mo), month 3 (3 mo), month 6 (6 mo), month 9 (9 mo), and month 12 (12 mo). Statistical significance between survival curves was calculated by log-rank Mantel-Cox test. Line graphs show the plasma (B–D) and salivary (E–G) antibody responses against Omicron XBB.1.5 from 4 representative individuals (green) with COVID-19 breakthrough infections (rapid antigen test–positive [RAT-positive]). Total IgG (B and E), total IgA responses (C and F), and Fc-γR2a binding (D and G) against Omicron XBB.1.5 are shown following symptom onset. Line graphs also depict the kinetics of N-specific IgG for both the delayed (purple diamonds) (H) and immediate (pink triangles) (I) arms across sampling time points, highlighting individuals with known symptomatic (RAT-positive; green) and asymptomatic (>4-fold rise in N-specific IgG from the previous time point; yellow) breakthrough infections. Experiments were performed in duplicate. (J) Kaplan-Meier plot showing the probability of remaining COVID-19 negative during the study in the delayed (purple) and immediate (pink) arms. Analysis includes all first on-study COVID-19 infections (pre- and post-study vaccination, self-reported, and asymptomatic laboratory diagnosis). The probability for the delayed arm reaches zero because the final 2 delayed arm participants were positive/censored just after 12 months, whereas there were 3 final immediate arm participants who remained at risk (log-rank P = 0.838, by Mantel-Cox test).