Delayed Skin Reaction After mRNA-1273 Vaccine Against SARS-CoV-2

The coronavirus disease 2019 (COVID ‐ 19) is associated with a wide clinical spectrum of skin manifestations, including chilblain ‐ like, urticarial, vesicular and vasculitic lesions. Recently, delayed skin reactions following mRNA vaccination against SARS-CoV-2 have been reported. The exact pathomechanisms underlying these skin lesions remain unknown. Here, we describe eleven cases of delayed skin reactions after SARS-CoV-2 vaccination with the mRNA-1273 vaccine, discuss their transient and benign clinical courses and consider their potential pathomechanisms based on histopathological analyses. We conclude that further investigations to characterize the precise molecular and cellular mechanisms underlying this rare phenomenon are warranted.

Introduction SARS-CoV-2 continues to pose a major threat to health care systems worldwide. Ongoing vaccination programs are anticipated to control the COVID-19 pandemic, eventually. Novel mRNA-based vaccines assessed and made available at unprecedented pace constitute a most critical, enabling advance. At the same time, the novelty of mRNA-based vaccines calls for special attention to potential unexpected or extended spectra of side effects of this novel vaccination technology. 1 Baden et al. reported that immediate injection-site reactions were detected in 84.2% of the participants in the phase III trial after the rst dose of the mRNA-1273 vaccine. 2 The authors further reported the rare occurrence of late injectionsite reactions with an onset on or after day eight in 244 of the 30,420 participants of their trial after the rst dose (0.8%) and in 68 participants after the second injection (0.2%). 2 These reactions involved erythema, induration and soreness, which typically resolved within the next four to ve days. Based on histological ndings, the research group interpreted the side effect as a type IV hypersensitivity reaction. [3][4][5] Seemingly similar skin reactions have recently been reported with BNT162b2, the second currently approved mRNA-based vaccine. 6,7 The US Centers for Disease Control and Prevention (CDC) refers to these rare skin reactions associated with mRNA-based vaccines as "COVID arm". 8 "COVID arm" usually neither requires treatment nor should it discourage a 2nd dose vaccination if scheduled.
Here, we report on a series of eleven patients with these responses, all of which appeared near the injection site after complete resolution of the initial local and systemic symptoms associated with vaccination. In addition, we present possible therapeutic approaches and discuss potential pathomechanisms based on histological examinations.

Patients And Methods
We analyzed the reports on acute side effects in the Dermatology Departments at the University of Düsseldorf, Dermatology Department of Münster and in one private practice in Düsseldorf.

Case 1
56-year-old Caucasian male with no history of past medical problems. He developed three days after the injection of the mRNA-1273 vaccine a large local erythema with edema at the injection site. He also complained about local cutaneous hypersensitivity. Use of oral antihistamines gave a quick relief of the symptoms.

Case 2
60 year old Caucasian female with no history of past medical problems. She developed four days after the rst injection of the mRNA-1273 vaccine a large local erythema with edema at the injection site. She also developed a cervical lymphnode swelling starting 2 days after the rst injection. With use of oral antihistamines no skin symptoms occurred after the 2nd injection Because of the severe discomfort at the injection site, topical methylprednisolone aceponate for a few days as well as a systemic therapy with loratadine was initiated. After a few days, there was complete resolution of the skin lesion. The second vaccination was performed according to standard schedule. No further systemic or cutaneous reactions were observed.

Case 7
A healthy 44-year-old female developed local swelling, redness, and induration at the injection site and the surrounding area 3 days after the second vaccination with the mRNA-1273 vaccine. No other abnormalities were described except for cutaneous tenderness and mild itching. She complained of mild chills 24 hours after each vaccination. Otherwise, the vaccination was well-tolerated. No topical or systemic therapies were initiated. The skin lesions disappeared completely after a few days.

Case 8
A 63-year-old female noticed redness and swelling at the injection site on her left arm 2 days after her second mRNA-1273 vaccination. Apart from mild itching, no other symptoms were described. Concomitant diseases were denied. Allergies were denied. After the rst vaccination, the patient remained at home with a pronounced feeling of illness. Due to the mild symptoms, no therapeutic measures were taken in this case either. There was a signi cant improvement of the skin ndings already after two days and no further symptoms.

Case 9
A 50-year-old healthy female showed redness, swelling, and induration at the injection site 4 days after the second vaccination with mRNA-1273 vaccine, comparable to our other cases. Because of the extensive size of the lesion as well as the pronounced local burning and itching, a skin biopsy was taken.

Discussion
The here reported case series of delayed local skin reactions following vaccination with mRNA-1273 mirrors recently published case series of similar ndings with both currently approved mRNA-based vaccines mRNA-1273 3,9 and BNT162b2 6;7 . These delayed skin reactions are rare post-vaccination events of both approved mRNA-based vaccines, with incidences reported to be in the order of 0.8-1.0 % following the rst and 0.2-1.1% following the second dose 2,6 . Delayed skin reactions are typically observed several days after the vaccination, i.e. following a symptom-free interval and hence distinguishable from acute allergic and other immediate local reactions attributable to the vaccination intervention itself. The phenomenon is transient and typically resolves within 3-5 days, frequently without any treatment required. In cases in whom pharmacological intervention is needed, the experience of this case series as well as of earlier reports show that the condition may be expected to be well-responsive to topical glucocorticosteroids and oral histamines. In patients in whom a "COVID arm" occurs following the rst of two scheduled vaccinations, the recommendation is to seek the second vaccination as scheduled, potentially administered to the opposite arm if needed 8 .
An important question unanswered to date relates to the precise molecular and cellular mechanisms underlying the "COVID arm" phenomenon. In response to this question, we noted that the delayed skin reactions observed in our case serious are of two distinct time windows of onset and of two distinct (even though overlapping) clinical phenotypes, indicative of more than a single pathomechanism. Speci cally, we observed manifestations of comparatively early onset (days 2-3 post vaccination) characterized by diffuse, poorly demarcated erythema associated with variable degrees of local edema and symptoms of tenderness, local hypersensitivity and itch well-responsive to antihistamines (as exempli ed by case 1). These comparatively early skin manifestations appear to be distinct from manifestations of later onset (days 7-10 after the rst, days 2-4 after the second vaccination), which are characterized by more sharply demarcated erythema of irregular peri-and intra-lesional morphology. While the former type of skin manifestation clinically reminds of an allergic pathology, we were particularly interested in the pathology and pathomechanisms underlying the latter type of late-onset delayed skin reactions (mirroring events described in earlier reports), with the delayed time window suggesting the involvement of the patient's adaptive immune response to the vaccination. Even though patient numbers are small, the shorter time to onset following second vaccinations, as observed in our cohort, similarly suggests the involvement of patients' adaptive immune response, the more so as a similar shortening of time to onset following second vaccination has been observed before 3 . The exact mechanism of these reactions is not known, however a delayed hypersensitivity reaction has been hypothesized. 10 Focusing on these skin reactions of delayed onset, we considered for reference early skin manifestations associated with COVID-19 infection, per se. While COVID-19 associated skin lesions are of a wide clinical spectrum including chilblain-like, urticarial, vesicular and vasculitic lesions, the occurrence of distinct types of skin lesions within distinct windows of onset during the COVID-19 clinical course has been noted 11 . Histopathological skin biopsy analyses of these COVID-19 associated cases show diverse ranges of morphologies. A consistent histological feature, however, appears to be the presence of prominently dilated blood vessels with swollen endothelial layers and vessels engulfed with red blood cells and perivascular in ltrates 12 .
The speci c mechanisms underlying these endothelial, vascular, and perivascular in ammatory changes associated with COVID-19 infection have not been fully elucidated. A possible lead hypothesis involves direct viral infection of endothelial cells, with preliminary evidence for this derived from existing data of both electron microscopy and polymerase chain reaction (PCR) analyses within skin lesions 13,14 15 .
Not dissimilar to the cutaneous histomorphology observed in COVID-19 infection, the histology images of the current mRNA-1273 case series and similarly of a previously reported histology analysis of skin lesions following mRNA-based BNT162b2 vaccination 6 share prominent features of a super cial and deep perivascular dermatitis with scattered eosinophils and intraluminal neutrophil accumulation (Fig. 9,   10). This histology pattern is commonly referred to as dermal hypersensitivity reaction 16 . It is not diagnostic for any speci c condition or etiology. Additional analyses will be required, therefore, to further dissect the phenomenon itself as well as its molecular triggers.
At least two categorically different molecular pathomechanisms will need to be considered in these future investigations: Firstly, polyethylene glycol (PEG), an excipient contained in both mRNA-based vaccines yet not in DNA vector vaccines 1 may trigger allergic post-vaccination reactions as a net consequence of prior sensitization to PEG 17 or by direct T cell stimulatory action 18 . With direct proof lacking, the hypothesis of excipient-mediated pathology may appear to be supported indirectly, at least at rst glance, by the fact that delayed local skin reactions have not been reported the same with DNA vector vaccines, i.e. with vaccines also encoding the S protein yet neither containing PEG nor other excipients contained in mRNAbased lipid nanoparticle vaccines 1,17,19 . An alternative hypothesis still to be considered, however, is the potential direct transfection of endothelial cells with mRNA locally released from lipid nanoparticles, with aberrantly endothelially expressed S antigen responded to by building adaptive cellular immunity. The seeming absence of similar delayed skin reactions with DNA vector vaccines could nd its explanation, if this alternative hypothesis would be proven correct, in the differential distribution, dosage, in vivo stability and cargo release characteristics of viral vector DNA vaccines as compared to lipid nanoparticles delivering mRNA. Therefore, while "COVID arm" is a transient, localized, and clinically benign rare side effect upon vaccination with mRNA-based vaccines encoding the S-protein, the deciphering of the precise molecular and cellular mechanisms underlying this local phenomenon (directly accessible by biopsy) could still become the springboard of a more precise understanding of very rarely occurring endothelitic pathology observed with S-protein encoding vaccines, in general. In-depth analyses of the pathomechanisms underlying delayed skin reactions following mRNA-based vaccinations are called for, in consequence. It could evolve to be of signi cant clinical relevance to discern whether either (1) excipients exclusive to mRNA-based vaccines (with PEG as the most prominent candidate molecule) or (2) the S-protein encoded by both mRNA-based and DNA vector vaccines and potentially expressed on endothelial cells, or (3) both pathomechanisms in conjunction will constitute the triggering event(s) of very rare and rare adverse events affecting different vascular compartments and endothelial cells, with the spectra of clinical sequelae dependent on the tissue compartments a icted.

Conclusion
Delayed local skin reactions, also referred to as "COVID arm", are a rare side effect that can present as a localized, transient erythematous and edematous plaque several days after the rst or second dose of the mRNA-based COVID-19 vaccines. Even though most cases resolve spontaneously, topical glucocorticosteroids and oral anti histamines are effective in resolving the skin lesions and controlling symptoms. Patients should be noti ed that a "COVID" arm is a non-threatening benign potential side effect that should not discourage from obtaining a second dose of mRNA-based vaccine. Further investigations regarding the precise molecular and cellular mechanisms underlying this cutaneous pathology may inform the understanding of very rare and rare adverse events associated with S protein encoding vaccines, in general, and are called upon, therefore.   A 50-year-old healthy female showed redness, swelling, and induration at the injection site 4 days after the second vaccination with mRNA-1273 vaccine, comparable to our other cases. Because of the extensive size of the lesion as well as the pronounced local burning and itching, a skin biopsy was taken. (Figures 9 and 10).

Figure 11
Redness, swelling and induration 8 days after second vaccination with mRNA-1273   Redness, swelling and induration 12 days after second vaccination with mRNA-1273