COVID-19 post-vaccination lymphadenopathy: A review of the use of fine needle aspiration cytology

COVID-19 vaccine-associated clinical lymphadenopathy (C19-LAP) and subclinical lymphadenopathy (SLDI), which are mainly detected by 18F-FDG PET-CT, have been observed after the introduction of RNA-based vaccines during the pandemic. Lymph node (LN) fine needle aspiration cytology (FNAC) has been used to diagnose single cases or small series of SLDI and C19-LAP. In this review, clinical and LN-FNAC features


| INTRODUC TI ON
vaccine may cause side effects, 1,15 the most common being local pain and inflammation at the injection site, followed by headache, fatigue, chills, myalgia, arthralgia, fever, dizziness, nausea, and vomiting, often registered after the first administration; these effects depend on the vaccine type and individual responsivity. COVID-19 vaccine-associated lymphadenopathy (C19-LAP) may also occur, mostly in axillary, clavicular or cervical lymph nodes, in the arm of vaccine inoculation. In the Pfizer BioNTech COVID-19 vaccine trial, axillary and supraclavicular C19-LAP occurring on the same side as the injection was observed in 0.3% of the vaccine group vs less than 0.1% of the placebo group. [15][16][17] In Moderna's case, the incidence was 1.1%. 15,17 The site of the lymphadenopathy was axillary in 11% of the patients after the first dose and 16% after the second dose of the Moderna vaccine; similar findings have been reported for the Comirnaty-Pfizer/BioNTech vaccine. 18,19 It is likely that C19-LAP is merely the epiphenomenon of lymph node reactivity, since COVID-19 vaccine-associated subclinical lymphadenopathy (SLDI) has been reported in a higher percentage of cases than C19-LAP. 17 The detection of C19-LAP is mainly clinical and can be confirmed by ultrasound (US). Combined clinical data and US features allow a diagnosis of reactive SLDI or C19-LAP to be made; hence, a clinical follow-up is all that is required in most cases. Nonetheless, some C19-LAP and SLDI cases with equivocal presentation or in specific clinical contexts may raise differential diagnostic problems with lymph node metastases or lymphoma. [20][21][22][23][24][25][26] In these cases, a cytological or histological evaluation of the lymphadenopathy may be necessary. Fine needle aspiration cytology (FNAC) is routinely utilised in the diagnosis of lymphadenopathy. Numerous studies have reported that lymph node fine needle aspiration cytology (LN-FNAC) is an accurate, safe, effective, and well-tolerated diagnostic procedure, both for reactive processes and for lymphoma and metastases. 5,27-34 A review of studies on LN-FNAC in C19-LAP is reported herein.

| MATERIAL S AND ME THODS
A literature search was initially performed through PubMed and Google Scholar, on 11 January 2023, using the following keywords: "COVID-19", "vaccine", "lymphadenopathy", "cytology", and "fine-needle aspiration". No restrictions were placed on the year of publication. Only literature published in English was selected, including studies that reported histopathological and/or cytological findings in vaccine-related lymphadenopathy. 17,20,22,23, Studies on SLDI, which was generally detected by 18F-FDG PET-CT, were also selected and used for the general comprehension and description of the phenomenon and to retrieve cases evaluated by histology or LN-FNAC. [65][66][67][68] Recommendation articles, protocols, commentaries, and non-English articles were not considered. Data extracted from studies regarding LN-FNAC of C19-LAP and SLDI included the following: type of publication, number of patients and clinical data, type and dose of administered vaccine, delay from last vaccination to lymphadenopathy, LN site and size, cytological features and diagnosis, management and outcome. In studies reporting both LN-FNAC and histological evaluation by core needle biopsy (CNB) or surgical excision, only LN-FNAC cases were utilized for the present analysis. Finally, an unpublished case observed at our institution that met the same inclusion criteria was also added.

| COVID-19 vaccines and lymphadenopathy
Almost all COVID-19 vaccines may cause reactive lymphadenopathy; most cases are subclinical and detectable only instrumentally, whereas a minority of cases show overt clinical C19-LAP. Increased lymph node 18F-FDG uptake has been detected in up to 36% of vaccinated patients 69 up to 10 weeks after vaccination, with women and people aged over 65 years being most frequently affected. 69,70 Lymph node enlargement occurs less frequently; different studies have reported an incidence of ~1% among those vaccinated against COVID-19 15 [40][41][42]49,56,64 Rosai-Dorfman-Destombes disease (RDD; 1 case), 48 and Langerhans cell histiocytosis (LCH; 1 case). 57 In cases of reactive hyperplasia, the histological features were described as a preserved lymph node structure with cortical follicular hyperplasia, enlarged germinal centres and interfollicular expansion by small lymphocytes. Prominent germinal centres and tingible-body macrophages were frequently reported. In expanded interfollicular regions, capillaries with focally prominent endothelial cells have been described. 55 Diffuse or partial capsule thickness was frequently reported. The immunohistochemical phenotype (IHC) was reported in eight cases. 54,58,61,62,71 The results of flow cytometry were also reported in one case. 44 A case of atypical follicular hyperplasia with light chain-restricted germinal centres after a COVID-19 booster was described, 62 but the specific vaccine was not reported. A case of C19-LAP with eosinophilic abscesses has also been reported. 43 The histological features showed eosinophil-rich inflammation with micro-abscesses. 43 COVID-19 cases associated with KFD and RDD have been described as exhibiting the typical histological features of the corresponding, non-vaccine-related entities. [40][41][42]48,56,64

| Fine needle aspiration cytology in cases of SLDI and C19-LAP
The present study is a pooled review based on 14 report s [20][21][22][35][36][37][38][44][45][46][47]52,72 and one unpublished case observed by the authors. In total, 26 cases are reported, including 17 (65%) female patients, 8 (32%) male patients, and one patient whose gender was not reported, 21  Lymphadenopathies were developed on the same side as the vaccine shot in 13 cases, and on the opposite side in 3 cases; the side on which the lymphadenopathy occurred was not reported in 10 cases. The specific lymph nodes reported were supraclavicular (10 cases), cervical (6 cases), axillary (7 cases), mediastinal (1 case) and submandibular (2 cases) ( Table 2). All lymph nodes were evaluated using US, and were reported as enlarged and oval, with major diameters ranging from 5 to 50 mm (mean 23.7 mm) (Figure 1); spherical shape was reported in one case (Hagen et al. 38 ). Another reported US feature was diffuse or focal cortical thickening and preserved, visible hilum in almost all cases; effaced hilum was reported in one case (Hagen et al. 38 ). The 18F-FDG-PET/CT data were not reported for any of these cases.

| C19-LAP Cytological Features
Our research found 14 detailed reports describing the FNAC features of SLDI and C19-LAP. 20-22,35-38,4 4-47,52,59,72 From these 14 reports, FNAC findings for 25 cases of C19-LAP were retrieved; one unpublished case observed at our institution was also added, for a total of 26 cases. Aalberg et al. 21 reported a case in the axillary lymph node of a patient suffering from stage IV renal cell carcinoma; FNAC smears were negative for neoplasia and showed reactive lymphoid cells. Trikannad et al. 59 reported a case in the mediastinal lymph node of a patient suffering from melanoma.
Cytological findings showed a reactive granulomatous process, TA B L E 1 Main COVID-19 vaccines and corresponding lymphadenopathy as a side effect. one showed polymorphous lymphoid cells suggestive of a reactive process; the second showed a reactive lymphoid population with scattered large, atypical cells of uncertain significance; in the third, they observed atypical lymphocytes suggestive of a lymphoproliferative disorder. In the last two cases a biopsy was performed, and in one case flow cytometry assessed a polyclonal reactive process.
García-Molina et al. 36    Histological controls and follow-up confirmed the FNAC diagnoses.
The clinical and cytological features of the reported cases are summarised in Table 2.

| Post-vaccine lymphadenopathy
Post-vaccine lymphadenopathy due to reactive changes is a well- LAP has been reported as caseating granulomatous inflammation not dissimilar from tubercular lymphadenitis. 77 Rubella LAP has been reported as lymphadenopathy with a prevalence of sinus histiocytosis. 11 A case of tetanus LAP was suspected to be a T-cell lymphoproliferative disorder and was then diagnosed as vaccinerelated "pseudolymphomatous" CD4 -cells, florid proliferation. 14 All other LAPs (associated with measles, rubella, smallpox, cholera, typhus, tetanus, diphtheria, pertussis, salk (polio), influenza, and HPV) have been reported as diffuse, follicular, or combined diffuse and follicular lymph node hyperplasia with increased numbers of lymphoblasts, vascular and sinusoidal changes, and a variable number of eosinophils, plasma cells, and mast cells. 11,12 In addition to granulomatous or follicular hyperplasia, non-Covidvaccine-related cases of lymphadenopathy caused by KFD, 10 RDD and Langerhans cell histiocytosis, 61 as well as post-transplantation like lymphadenopathy, have been reported. 78

| Pathology of COVID-19 post-vaccine lymphadenopathy
Lymph node excision and core-needle biopsy (CNB) were the most frequently performed procedures for the pathological evaluation of COVID-19 post-vaccine lymphadenopathy. We re-  Zeppa.

ACK N OWLED G EM ENT
Open Access Funding provided by Universita degli Studi di Salerno within the CRUI-CARE Agreement.

FU N D I N G I N FO R M ATI O N
All co-authors have seen and agree with the contents of the manuscript and there is no financial interest to report.

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors have no conflicts of interest to declare.

DATA AVA I L A B I L I T Y S TAT E M E N T
All data supporting this article are available upon reasonable request to the corresponding author.