In order to investigate whether a germinal center response occurs following vaccine boost and to characterize it based on the type of vaccine received, we aim to analyze Spike and/or RBD-specific germinal center B cells within the draining lymph nodes.

To achieve this, we will perform lymph node fine needle aspiration of the draining lymph node in 6 subjects in each study arm, 3 to 6 weeks after the booster vaccine injection.

Lymph nodes are secondary lymphoid organs essential for the establishment of the germinal center reaction, allowing the generation of long-lived B cell memory and high-affinity antibodies. In order to investigate whether a germinal center response occurs following vaccine boost and to characterize it based on the type of vaccine received, we aim to analyze Spike and/or RBD-specific germinal center B cells within the draining lymph nodes. Accessing germinal center through fine needle aspirations under ultrasound guidance has recently emerged as an innovative strategy in the United States to investigate vaccine response. This procedure under local anesthesia is well-tolerated, and no major side effects have been reported in a hundred patients who underwent fine needle aspiration. The technical details for performing this examination are now well-described.

Our previous work has highlighted the importance of repertoire diversity in responding to new variants of SARS-CoV-2. We thus aim to investigate whether a booster dose with mRNA or protein-based vaccine mobilizes memory B cells or naive cells, thereby contributing to a greater diversity of the Memory B cell comportment. To preserve the diversity of the memory B cell pool is absolutely essential for our immune system to adapt against variants.

To achieve this, we will perform lymph node fine needle aspiration of the draining lymph node in 6 subjects in each study arm, 3 to 6 weeks after the booster vaccine injection. Indeed, it has been shown that the response of germinal centers in response to a booster vaccination was detectable at 2- and 8-weeks post-injection vaccination.

This outpatient procedure will be performed by a radiologist under ultrasound guidance, aiming to collect 4 fine needle aspirations from 4 axillary draining lymph nodes under local anesthesia. Concurrently, a 35 mL blood sample will be collected from the subject to correlate the response in secondary lymphoid organs with the circulating response.

Inclusion Criteria:

• Male or female aged 18 years and over included in COVIBOOST 2

Exclusion Criteria:

• Hypersensitivity to lidocaine, to anaesthetics of the amide type or to any of the excipients
• Patient with recurrent porphyrias

The IPD will not be share. The 2 vaccines used in this study are marketed vaccine and used in routine care