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.