## 4 Discussion
Confusion emerged around patient identification as \"case 3\" in both papers (with the same pictures in both papers) described the same person, although a different therapeutic approach was reported: in the first paper, Medrol administration solved the case; in the second paper, Medrol was not mentioned and only linisopril administration was reported as a medical treatment. [PERSON] was the only one who clearly reported two cases of patients experiencing swelling following vaccine administration: in one case, a watch-and-wait approach was resolutive, with no medication, and in the other one, a hyaluronidase injection solved the case [24]. Three more cases were described during the Moderna clinical trial [20], although only data regarding the first two patients were clear: patients received an injection of fillers into the cheeks and developed facial swelling following a COVID-19 vaccine. The last case described in the Moderna clinical trial was characterized by lip angioedema following a vaccine injection, although it was not known how long prior to vaccination the lip implant was injected or what kind of filler was injected (i.e., was it permanent?). The medical history of the patient also revealed the same lip angioedema after recieving an influenza vaccine in the past [20]. If we take account for the high number of patients who received dermal filler injections in recent years and the high number of patients vaccinated since vaccine development, 19 cases of AE (15 from published research, 3 cases during the Modenal clinical trial and the case reported in this work) would seem to suggest a really low rate of incidence. Notably, facial AE were only described following Moderna and Pfizer vaccinations. Following COVID-19 vaccine administration, allergic and anaphylactic reactions have been reported and several hypothesis have emerged [32; 33]. [PERSON] and [PERSON] suspected that polyethylene glycol (PEG), an inactive ingredient in the vaccine preparation, was involved with these AE [34]. PEG is widely used and is present in laxatives, makeup, chemotherapy drugs, etc. [35]. Moreover, HA fillers cross-linked with PEG, instead of 1,4-butanediol diglycidyl ether (BDDE), are commercially available [36; 37]. To date, no AE were recorded in this class of fillers, showing a high biocompatibility and safety profile from an immunological perspective [38; 39]. Based on the available published data, facial AE in areas previously injected with filler were recorded 5 times following the first dose of Moderna vaccine, 8 times following the second dose of the same vaccine, 3 times after the administration of the first Pfizer vaccine dose and 3 times following the second dose of the same vaccine. Data regarding AE treatments and the outcomes were available for only nine patients. In 6 cases, medical therapy was administrated, in 2 cases a watch-and-wait approach was employed, and in 1 case, a hyaluronidase injection was utilized. Every AE described for the patients were resolved. Concerning our report, the patient had a long history of different facial filler injections, from 2012 to 2020. During this long time-lapse, two adverse events were recorded: a lip angioedema in 2013 using a Belotero product and a tardive bilateral swelling about 10 months after receiving Volbella (Allergan, Irvine CA) injections in the tear trough area. Lip angioedema was resolved with medical therapy, whereas tear trough swelling was unresponsive to medical approaches. To solve the latter AE, three sessions of hyaluronidase injections were administered. Although the patient was re-injected with another brand of filler, following the second dose of a COVID-19 vaccine, she developed swelling in the right lower eyelid, which self-resolved within a few days. Due to the great resistance to hyaluronidase injection evidenced in this case, the persistence of Volbella (Allergan, Irvine CA) cannot be excluded. Data regarding the type of filler previously injected were available in only 9 out of 19 patients.