Early kinetics of cellular immunity in recipients of bivalent BNT162b2 vaccine: a proof-of-concept study

To the Editor,

There is now incontrovertible evidence that the accurate characterization of immunity against various pathogens such as the severe acute respiratory syndrome coronavirus disease 2 (SARS-CoV-2) should encompass the assessment of both humoral and cellular immunity, whereby T cell response plays a vital, irreplaceable role in preventing the risk of developing severe forms of coronavirus disease 2019 (COVID-19) [1]. This is mostly attributable to the many essential actions that T cells (both CD4+ and CD8+) exert against acute viral infections, namely cytotoxic activity, cytokines secretion, recruitment and/or priming of other immune cells [2].

Reliable evidence is accumulating that a blunted response or a faster decline of cellular immunity may significantly predict COVID-19 vaccine breakthrough infections, as well as the risk of developing more severe SARS-CoV-2 related illness in recipients of COVID-19 vaccines. For example, a recent study published by Garofalo et al. showed that the response of SARS-CoV-2 spike-specific interferon (IFN)-γ releasing cells was significantly lower in patients with severe COVID-19 vaccine breakthrough infection compared to those who developed mild or moderate illness (i.e., 8 vs. 135 s.f.c. × 10⁶; p<0.001), whilst the levels of anti-SARS-CoV-2 IgG antibodies did not differ significantly between these groups [3]. This implicitly confirms that cellular immunity, probably more than humoral immunity, plays a crucial role in arresting or slowing the progression of COVID-19 toward severe and/or critical forms.

In a previous study published in this journal [4], we evaluated both the humoral and cellular response of a cohort of ostensibly healthcare workers assayed 14 days after administration of the new bivalent BNT162b2 vaccine. In this subsequent proof-of-concept study, we extend our investigation to address the very early T cell response in BNT162b2 bivalent vaccine recipients, by monitoring the progressive variation of IFN-γ release, together with total anti-SARS-CoV-2 antibodies, from day 1 through day 14 after vaccination, thus providing information that may be used to increase our understanding of this essential arm of the immune response against SARS-CoV-2.

The specific protocol used in this study has been described previously [4]. In this further investigation, we studied three ostensibly healthy healthcare workers who received a single booster of the novel Pfizer/Biontech mRNA BNT162b2 bivalent vaccine (Comirnaty, Pfizer Inc, NY, USA) in February 2023. Blood was drawn immediately before vaccine injection (i.e., D0) and at fixed times afterward (after 1 [D1], 4 [D4], 5 [D5], 7 [D7] and 14 [D14] days), within a serum blood tube (Greiner Bio-One, Kremsmünster, Austria) and the three Roche Cobas IGRA SARS-CoV-2 blood Tubes (PC, NC and AG; Roche Diagnostics, Basel, Switzerland). The level of total anti-SARS-CoV-2 serum antibodies was assayed with Roche Elecsys Anti-SARS-CoV-2 electrochemiluminescence immunoassay (ECLIA; Roche Diagnostics, Basel, Switzerland), whilst cellular immunity was quantified using the Elecsys IGRA SARS-CoV-2 test (Roche Diagnostics, Basel, Switzerland), as earlier described [4]. Individual test results were reported as variation from the baseline (i.e., [Time point value]/[Baseline value]) and were finally plotted as mean and standard deviation (SD) of individual values. All subjects provided a written informed consent for participating to this study, whose protocol was conducted in accordance with the Declaration of Helsinki and cleared by the Ethics Committee of Verona and Rovigo Provinces (59COVIDCESC; November 8, 2021).

The results of this study are shown in Figure 1. The value of IGRA SARS-CoV-2 started to increase already at day 1 after COVID-19 bivalent vaccination, peaked at day 4 (5.1 ± 3.4 folds increase), but then sharply declined already at day 5, then stabilizing at values around 2.6–2.9 folds higher than the baseline in the following period. The value of total anti-SARS-CoV-2 antibodies started to increase at day 5 after vaccination, and then continued to gradually increase up to the end of the observational period (i.e., plateauing at 3.6 ± 1.6 folds increase at day 14).

Figure 1:

Early variation of humoral (i.e., total anti-SARS-CoV-2 antibodies) and cellular (i.e., IGRA SARS-CoV-2) response after administration of the new bivalent BNT162b2 vaccine in three ostensibly healthy healthcare workers. Results are shown as mean and standard deviation of individual ratios of increase from baseline. SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; IGRA, interferon gamma release assay.

Monitoring both the early and later immune response to COVID-19 vaccination is essential for at least two primary reasons. First, early quantification of both cellular and humoral immunity would enable to identify individuals with lower or even blunted immune response (i.e., “non-responders”), who may benefit from receiving additional boosters, or may be informed about their higher vulnerability to developing infection (due to blunted humoral response) or severe illness (due to suboptimal cellular response) [5]. Then, longitudinal monitoring of humoral and cellular immunity over time would permit to timely detect faster or shaper decline in “normal responders”, who may also be targeted with specific vaccination strategies encompassing higher doses and/or shorter boosters [6]. As specifically concerns the former aspect, despite the very low number of subjects included in this pilot study, we showed that the SARS-CoV-2 IFN-γ response is characterized by a very short lag time of around 1–2 days, and gradually increases up to 4–5 days after administration, after which period begins to decline and stabilizes at around 2–3 folds higher values compared to baseline. These results are basically aligned with those preliminarily presented by Painter et al. [7], who showed that spike-specific CD4+ and CD8+ T cells could be robustly activated by SARS-CoV-2 infection early during the first week, with activation seen between 3 and 5 days from symptom onset in most individuals, then peaking at around day 7. Moreover, our findings also provide preliminary biologic evidence to support the clinical observation that COVID-19 vaccines administered within a few days after SARS-CoV-2 exposure may be effective in halving the risk of developing severe COVID-19 illness [8], whereby we showed that vaccine efficacy in boosting T cell response begins to manifest very early (i.e., within few days) after administration.

In conclusion, although this case series comprised only three subjects and our findings will need to be validated in larger studies, we found that the T cell response after COVID-19 bivalent vaccination undergoes a rapid “acute” boost (i.e., within 4–5 days from vaccine administration), then exhibiting a progressive decline to stabilize at values 2–3 folds higher than the baseline in the following 10 days.