Chronic spontaneous urticaria after BNT162b2 mRNA (Pfizer-BioNTech) vaccination against SARS-CoV-2

Clinical Features and Outcomes of Acute versus Chronic Urticaria Triggered by COVID-19 Infection

OBJECTIVE

Although acute urticaria (AU) and urticaria-like rash are commonly reported with COVID-19 infection, chronic spontaneous urticaria (CSU) triggered by COVID-19 is rare. The authors compared the features of COVID-19 infection-induced chronic CSU and AU to determine which patients' COVID-19 infection leads to CSU and possible indicators of chronicity.

METHODS

The authors retrieved the charts of patients diagnosed with AU or CSU following COVID-19 at the Urticaria Centers of Reference and Excellence and compared patients in terms of demographic characteristics, length of time between infection and onset of urticaria, duration of urticaria, COVID-19 disease severity, laboratory test results, vaccination, and treatment status.

RESULTS

A total of 92 patients were included in the study: 7 with CSU following COVID-19 and 85 with AU after COVID-19. The mean duration of urticaria for CSU and AU following COVID-19 was 13.0 ± 6.0 months and 7.1 ± 3.4 days, respectively. The average time between COVID-19 and the start of urticaria was longer in the CSU group (20.7 ± 3.9 days vs 4.5 ± 2.8 days, respectively; P = .000). No between-group differences were found for any other parameters.

CONCLUSIONS

The onset of urticaria more than 2 weeks after COVID-19 infection may serve as an indicator for urticaria chronicity beyond 6 weeks and may help physicians predict the possible course of urticaria associated with COVID-19 infection. The relevance of basopenia and eosinopenia needs to be determined.

mRNA-LNP COVID-19 Vaccine Lipids Induce Complement Activation and Production of Proinflammatory Cytokines: Mechanisms, Effects of Complement Inhibitors, and Relevance to Adverse Reactions

A small fraction of people vaccinated with mRNA-lipid nanoparticle (mRNA-LNP)-based COVID-19 vaccines display acute or subacute inflammatory symptoms whose mechanism has not been clarified to date. To better understand the molecular mechanism of these adverse events (AEs), here, we analyzed in vitro the vaccine-induced induction and interrelations of the following two major inflammatory processes: complement (C) activation and release of proinflammatory cytokines. Incubation of Pfizer-BioNTech's Comirnaty and Moderna's Spikevax with 75% human serum led to significant increases in C5a, sC5b-9, and Bb but not C4d, indicating C activation mainly via the alternative pathway. Control PEGylated liposomes (Doxebo) also induced C activation, but, on a weight basis, it was ~5 times less effective than that of Comirnaty. Viral or synthetic naked mRNAs had no C-activating effects. In peripheral blood mononuclear cell (PBMC) cultures supplemented with 20% autologous serum, besides C activation, Comirnaty induced the secretion of proinflammatory cytokines in the following order: IL-1α < IFN-γ < IL-1β < TNF-α < IL-6 < IL-8. Heat-inactivation of C in serum prevented a rise in IL-1α, IL-1β, and TNF-α, suggesting C-dependence of these cytokines' induction, although the C5 blocker Soliris and C1 inhibitor Berinert, which effectively inhibited C activation in both systems, did not suppress the release of any cytokines. These findings suggest that the inflammatory AEs of mRNA-LNP vaccines are due, at least in part, to stimulation of both arms of the innate immune system, whereupon C activation may be causally involved in the induction of some, but not all, inflammatory cytokines. Thus, the pharmacological attenuation of inflammatory AEs may not be achieved via monotherapy with the tested C inhibitors; efficacy may require combination therapy with different C inhibitors and/or other anti-inflammatory agents.

Chronic Spontaneous Urticaria Following mRNA COVID-19 Booster Vaccination at a Military Academy

Several adverse cutaneous reactions have been reported in the literature after SARS-CoV-2 vaccination with emerging reports on chronic spontaneous urticaria (CSU). However, there is little literature of chronic urticaria after COVID-19 boosters in a military population and the impact on operational readiness. We present a retrospective case series of CSU following Moderna COVID-19 booster vaccinations at the US Naval Academy (USNA). Demographics, clinical features, and impact on readiness were evaluated. Forty-nine students from the USNA were evaluated for urticaria after their third COVID-19 booster vaccination. Seventeen individuals were diagnosed with CSU. The median age was 20 years and predominantly male; the median time interval between vaccination and the onset of urticaria was 11 days. Out of 13 referred to Allergy, 7 patients had CU index performed and 2 were positive. Four patients received a second booster vaccination subsequently and did not have any exacerbation of symptoms. Symptoms were controlled with antihistamines, and none required immunomodulator or immunosuppressive therapies. All students were able to complete their commissioning, and none were referred for a medical board. In this series, USNA students who developed CSU after the mRNA COVID-19 Moderna booster vaccine did not have limitations from commissioning, duty status, or issues with subsequent COVID-19 vaccinations.

Post-Vaccination Syndrome: A Descriptive Analysis of Reported Symptoms and Patient Experiences After Covid-19 Immunization

Introduction

A chronic post-vaccination syndrome (PVS) after covid-19 vaccination has been reported but has yet to be well characterized.

Methods

We included 241 individuals aged 18 and older who self-reported PVS after covid-19 vaccination and who joined the online Yale Listen to Immune, Symptom and Treatment Experiences Now (LISTEN) Study from May 2022 to July 2023. We summarized their demographics, health status, symptoms, treatments tried, and overall experience.

Results

The median age of participants was 46 years (interquartile range [IQR]: 38 to 56), with 192 (80%) identifying as female, 209 (87%) as non-Hispanic White, and 211 (88%) from the United States. Among these participants with PVS, 127 (55%) had received the BNT162b2 [Pfizer-BioNTech] vaccine, and 86 (37%) received the mRNA-1273 [Moderna] vaccine. The median time from the day of index vaccination to symptom onset was three days (IQR: 1 day to 8 days). The time from vaccination to symptom survey completion was 595 days (IQR: 417 to 661 days). The median Euro-QoL visual analogue scale score was 50 (IQR: 39 to 70). The five most common symptoms were exercise intolerance (71%), excessive fatigue (69%), numbness (63%), brain fog (63%), and neuropathy (63%). In the week before survey completion, participants reported feeling unease (93%), fearfulness (82%), and overwhelmed by worries (81%), as well as feelings of helplessness (80%), anxiety (76%), depression (76%), hopelessness (72%), and worthlessness (49%) at least once. Participants reported a median of 20 (IQR: 13 to 30) interventions to treat their condition.

Conclusions

In this study, individuals who reported PVS after covid-19 vaccination had low health status, high symptom burden, and high psychosocial stress despite trying many treatments. There is a need for continued investigation to understand and treat this condition.

COVID-19 and Its Impact on Common Diseases in the Allergy Clinics

Coronavirus disease 2019 (COVID-19) is a highly contagious viral disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). It has various effects on asthma, allergic rhinitis, atopic dermatitis, and urticaria and may change the course of the disease depending on the severity of the infection and control status of the disease. Conversely, these diseases may also impact the course of COVID-19. Patients with chronic urticaria and atopic dermatitis may have COVID-19-induced disease exacerbations and biological treatments reduce the risk of exacerbations. Poor asthma control is linked to severe COVID-19 while allergic asthma is associated with lower risk of death and a lower rate of hospitalization due to COVID-19 compared with nonallergic asthma. The use of intranasal corticosteroids is associated with lower rates of hospitalization due to COVID-19 in patients with allergic rhinitis, whereas the effect of inhaled corticosteroids is confounded by asthma severity. These observations reinforce the importance of keeping allergic diseases under control during pandemics. The use of biologicals during COVID-19 is generally regarded as safe, but more evidence is needed. The pandemic substantially changed the management of allergic disorders such as home implementation of various biologicals, allergen immunotherapy, food introduction, and increased use of telemedicine and even home management of anaphylaxis to reduce emergency department burden and reduce risk of infection. Physicians need to be aware of the potential impact of COVID-19 on allergic diseases and educate their patients on the importance of continuing prescribed medications and adhering to their treatment plans to maintain optimal control of their disease.

New-onset chronic spontaneous urticaria post-COVID-19 vaccination-South African case series

Background

Chronic spontaneous urticaria (CSU) is defined as the spontaneous occurrence of hives, angioedema, or both for more than 6 weeks; several inciting triggers including vaccines have been implicated. Coronavirus disease 2019 (COVID-19) vaccinations have been well tolerated by patients with CSU. However, reports have emerged of CSU triggered by COVID-19 vaccination and this study describes a South African case series.

Objective

To provide details of the first case series of new-onset CSU post-COVID-19 vaccination in Africa and summarize the global literature of reported cases to date.

Methods

All patients referred to our Urticaria Center of Excellence in Cape Town from the initiation of the COVID-19 vaccine rollout in South Africa (from February 2021 to August 2022) were reviewed to identify patients who developed new-onset CSU within 12 weeks of receiving a COVID-19 vaccine. Medical history, physical examinations, and laboratory investigations were reviewed.

Results

More than 20 million adults received COVID-19 vaccinations in South Africa during the study period. Eight patients had new-onset chronic urticaria post-COVID-19 vaccination; 6 of the 8 patients were female, the median age was 41 years (interquartile range [IQR], 38-44), and all had a history of atopy. Only 1 reported COVID-19 infection post vaccination. Chronic urticaria occurred following Pfizer-BioNTech, AstraZeneca, and Janssen Ad26.COV2.S vaccination in 6, 1, and 1 patient, respectively, with a median of 12 days (IQR, 3-38) from vaccination to symptoms onset. The baseline median score for Urticarial Activity Score 7 was 34 (IQR, 29-40), and 5 of the 8 patients (63%) had a total IgE level of more than 43 IU/L. All patients received high-dose antihistamines, with only 3 patients controlled.

Conclusions

New-onset CSU can rarely be triggered by COVID-19 vaccinations, most commonly mRNA vaccines. COVID-19 vaccine-triggered CSU appears to have a phenotype similar to that triggered by other inciting agents and across populations.

Urticaria exacerbations and adverse reactions in patients with chronic urticaria receiving COVID-19 vaccination: Results of the UCARE COVAC-CU study

BACKGROUND

Concern about disease exacerbations and fear of reactions after coronavirus disease 2019 (COVID-19) vaccinations are common in chronic urticaria (CU) patients and may lead to vaccine hesitancy.

OBJECTIVE

We assessed the frequency and risk factors of CU exacerbation and adverse reactions in CU patients after COVID-19 vaccination.

METHODS

COVAC-CU is an international multicenter study of Urticaria Centers of Reference and Excellence (UCAREs) that retrospectively evaluated the effects of COVID-19 vaccination in CU patients aged ≥18 years and vaccinated with ≥1 dose of any COVID-19 vaccine. We evaluated CU exacerbations and severe allergic reactions as well as other adverse events associated with COVID-19 vaccinations and their association with various CU parameters.

RESULTS

Across 2769 COVID-19-vaccinated CU patients, most (90%) received at least 2 COVID-19 vaccine doses, and most patients received CU treatment and had well-controlled disease. The rate of COVID-19 vaccination-induced CU exacerbation was 9%. Of 223 patients with CU exacerbation after the first dose, 53.4% experienced recurrence of CU exacerbation after the second dose. CU exacerbation most often started <48 hours after vaccination (59.2%), lasted for a few weeks or less (70%), and was treated mainly with antihistamines (70.3%). Factors that increased the risk for COVID-19 vaccination-induced CU exacerbation included female sex, disease duration shorter than 24 months, having chronic spontaneous versus inducible urticaria, receipt of adenovirus viral vector vaccine, having nonsteroidal anti-inflammatory drug/aspirin intolerance, and having concerns about getting vaccinated; receiving omalizumab treatment and Latino/Hispanic ethnicity lowered the risk. First-dose vaccine-related adverse effects, most commonly local reactions, fever, fatigue, and muscle pain, were reported by 43.5% of CU patients. Seven patients reported severe allergic reactions.

CONCLUSIONS

COVID-19 vaccination leads to disease exacerbation in only a small number of CU patients and is generally well tolerated.

Covid-19 Vaccination and Chronic Urticaria: A Paradigm Study

Background

Multiple vaccines were introduced during 2020-2021 to combat Covid-19 pandemic, being one of the successful vaccine programmes in the present era. Very few studies are available on status of chronic urticaria (CU) post Covid-19 vaccination.

Aim

The aim of this study was to study effect of Covid-19 vaccination on our urticaria cohort.

Materials and Methods

In this retrospective study, case records of CU patients registered in urticaria clinic, who had received any type of Covid-19 vaccine during the interval of March 2021-2022 were retrieved. Patients were classified as 'vaccine induced urticaria' (VIU) when CU developed for first time post-vaccination and 'vaccine exacerbated urticaria' (VEU) when administration of vaccine exacerbated disease activity in previously diagnosed CU.

Results

Overall, 353 CU patients registered with us during this period, 265 had received atleast one dose of a Covid-19 vaccine, of which 12 reported VEU (ten of whom had received adenovirus vector vaccine), and three patients were diagnosed with VIU (all had received inactivated virus vaccine). Mean vitamin D3 levels were significantly higher in patients who had VEU as compared to those CU patients without exacerbation (p = 0.003). Significant correlation was observed between level of concern regarding adverse effects of vaccination, pre-vaccination, and post-vaccination urticaria activity score (UAS-7), (Pearson correlation coefficient = 0.66, P = 0.007) in both VEU and VIU. Urticaria symptoms were controlled in 75% and 66.6% patients, respectively, of VEU and VIU, after one month of initiating standard antihistamine treatment.

Conclusion

Hence, we conclude that though Covid-19 vaccines can trigger CU, standard treatment protocols control disease activity in most patients.

Updated guidance regarding the risk of allergic reactions to COVID-19 vaccines and recommended evaluation and management: A GRADE assessment and international consensus approach

This guidance updates 2021 GRADE (Grading of Recommendations Assessment, Development and Evaluation) recommendations regarding immediate allergic reactions following coronavirus disease 2019 (COVID-19) vaccines and addresses revaccinating individuals with first-dose allergic reactions and allergy testing to determine revaccination outcomes. Recent meta-analyses assessed the incidence of severe allergic reactions to initial COVID-19 vaccination, risk of mRNA-COVID-19 revaccination after an initial reaction, and diagnostic accuracy of COVID-19 vaccine and vaccine excipient testing in predicting reactions. GRADE methods informed rating the certainty of evidence and strength of recommendations. A modified Delphi panel consisting of experts in allergy, anaphylaxis, vaccinology, infectious diseases, emergency medicine, and primary care from Australia, Canada, Europe, Japan, South Africa, the United Kingdom, and the United States formed the recommendations. We recommend vaccination for persons without COVID-19 vaccine excipient allergy and revaccination after a prior immediate allergic reaction. We suggest against >15-minute postvaccination observation. We recommend against mRNA vaccine or excipient skin testing to predict outcomes. We suggest revaccination of persons with an immediate allergic reaction to the mRNA vaccine or excipients be performed by a person with vaccine allergy expertise in a properly equipped setting. We suggest against premedication, split-dosing, or special precautions because of a comorbid allergic history.

How Infection and Vaccination Are Linked to Acute and Chronic Urticaria: A Special Focus on COVID-19

Since more than a century ago, there has been awareness of the connection between viral infections and the onset and exacerbation of urticaria. Our knowledge about the role of viral infection and vaccination in acute and chronic urticaria improved as a result of the COVID-19 pandemic but it has also highlighted knowledge gaps. Viral infections, especially respiratory tract infections like COVID-19, can trigger the onset of acute urticaria (AU) and the exacerbation of chronic urticaria (CU). Less frequently, vaccination against viruses including SARS-CoV-2 can also lead to new onset urticaria as well as worsening of CU in minority. Here, with a particular focus on COVID-19, we review what is known about the role of viral infections and vaccinations as triggers and causes of acute and chronic urticaria. We also discuss possible mechanistic pathways and outline the unmet needs in our knowledge. Although the underlying mechanisms are not clearly understood, it is believed that viral signals, medications, and stress can activate skin mast cells (MCs). Further studies are needed to fully understand the relevance of viral infections and vaccinations in acute and chronic urticaria and to better clarify causal pathways.

The Impact of COVID-19 Vaccination on Inflammatory Skin Disorders and Other Cutaneous Diseases: A Review of the Published Literature

Background: Four vaccines have been authorized by the European Medicines Agency (EMA): viral vector-based vaccines (AstraZeneca; AZD1222 and Johnson & Johnson; Ad26.COV2. and 2 mRNA-based vaccines (Pfizer/BioNTech; BNT162b2 and Moderna; mRNA-1273). Adverse events (AEs) related to vaccination have been described in the literature. The main aim of the dermatological practice was to avoid the diffusion of COVID-19, allowing the continuity of care for patients. Objective: The aim of this review article is to investigate current literature regarding cutaneous reactions following COVID-19 vaccination, mainly inflammatory dermatological diseases. Materials and methods: Investigated manuscripts included metanalyses, reviews, letters to the editor, real-life studies, case series, and reports. Results: We selected a total of 234 articles involving more than 550 patients. We have divided the results section into various sub-sections to ensure greater understanding for readers. Conclusions: Clinicians should keep in mind the possibility of new onsets or the worsening of several dermatoses following vaccination in order to promptly recognize and treat these AEs. Certainly, vaccination should not be discouraged.

A retrospective analysis of clinically confirmed long post-COVID vaccination syndrome

BACKGROUND AND AIM

Long post-COVID vaccination syndrome (LPCVS) is an increasingly recognized disease that occurs after SARS-CoV-2 vaccinations and lasts >4 weeks. However, little is known about the clinical presentation, underlying pathophysiology, treatment, and outcome of LPCVS. This study aims to present a series of patients with LPCVS, their treatment, and outcomes.

METHODS

This was a retrospective analysis of three patients with LPCVS.

RESULTS

In an observation period of 2 months (January and February 2022), three patients were collected in whom side effects after vaccination against COVID-19 lasted >4 weeks and in whom instrumental examinations were largely unremarkable. All three patients received only symptomatic therapy and only partially recovered within 6-8 months after vaccination. LPCVS significantly impaired the quality of life of the included patients.

CONCLUSIONS

SARS-CoV-2 vaccinations may cause not only short-term but also long-term side effects that include not only known diseases but also non-specific symptoms with normal or slightly abnormal clinical and instrumental findings. Although LPCVS leads to long-term disability, it is not widely recognized and not always accepted by manufacturers, health authorities, and even scientists. LPCVS should not be dismissed as a functional disorder and patients with LPCVS should be taken seriously.

RELEVANCE FOR PATIENTS

The possible causal relation between some long side effects and SARS-CoV-2 vaccines cannot be ignored. The pathophysiology of LPCVS should be further studied to lay a foundation for further improvement of the vaccines.

A Case Report: Long Post-COVID Vaccination Syndrome During the Eleven Months After the Third Moderna Dose

It is undisputed that anti-SARS-CoV-2 vaccines can have side effects. Long post-COVID vaccination syndrome (LPCVS) is one of them and is often neglected. It persists 11 months after the third mRNA-1273 (Moderna) vaccine dose has not been reported. Our patient is a 39-year-old male with a largely uneventful previous history who developed severe adverse reactions immediately after the third dose of the mRNA-1273 (Moderna) vaccine. In addition to brief fever, headache, flickering eyes, skin rashes, tiredness, disorientation, dizziness (brain fog), tiredness, impaired thinking and concentration, and emotional disorders occurred as a result. Cerebral MRI showed non-specific white matter lesions in a frontotemporal distribution. Some of the immune parameters were deflected. Non-steroidal anti-inflammatory drugs, antihistamines, sartans, and statins have occasionally provided temporary relief. In conclusion, LPCVS is a definite complication of anti-SARS-CoV-2 vaccinations and can severely impact the quality of life and lead to disability. Despite extensive work-up, a clear cause for the long-term neuro-cognitive deficits cannot be identified. Symptomatic treatment can provide some relief. Patients with LPCVS should be taken seriously and treated appropriately.

Cutaneous Reactions Following COVID-19 Vaccination: A Review of the Current Literature

The outbreak of coronavirus disease 2019 (COVID-19) represented a new worldwide challenge, strongly impacting on the global economy, overall health and lifestyle. Since then, several strategies have been adopted to contain the widespread of infection. Among these, vaccination is currently the most important measure to fight against the pandemic. However, several concerns such as slower-than-hoped-for rollout, the hurried approval with limited data, the mechanism of action (in particular mRNA-based), and the uncertain duration of protection they afforded were initially raised. Moreover, even if cutaneous reactions have been rarely reported in clinical trials, global mass vaccination showed several dermatologic reactions not initially recognized, leaving dermatologists to decide how to diagnose and treat them. In this scenario, dermatologists should be ready to promptly recognize these clinical manifestations. Thus, the aim of this manuscript is to review current literature on cutaneous reactions following COVID-19 vaccination, particularly inflammatory dermatological diseases, in order to help clinicians to better understand these dermatological conditions and to provide an extensive overview of all the vaccine-related skin manifestations.

Clinical Effects of BNT162b2 Vaccine on the Short-Term Course of Chronic Spontaneous Urticaria Patients

Background

The clinical effects of Pfizer-BioNTech coronavirus disease 2019 (COVID-19; BNT162b2) vaccine on the clinical course of chronic spontaneous urticaria (CSU) is unclear.

Aims and Objectives

To evaluate the clinical effects of BNT162b2 vaccine on the clinical course of CSU.

Methods

In this study, 90 CSU patients vaccinated with one or two repeated doses of BNT162b2 vaccine were included. Urticaria Activity Score over 28 days (UAS28), Urticaria Control Test (UCT), Chronic Urticaria Quality of Life Questionnaire (CU-Q2oL), and Medication Scores (MSs) were obtained before the vaccination, 28 days after the first and, if available, after the second dose of BNT162b2 vaccine. The demographic, clinical, and laboratory features were compared between the subjects with exacerbated (group A) and non-exacerbated (group B) disease activity.

Results

Among the 90 study participants, 14 (15.5%) experienced exacerbations in their urticarial activity after the first or repeated doses of BNT162b2 vaccinations. The demographic, clinical, and laboratory features were similar between the exacerbated and non-exacerbated CSU patients. However, the rate of adverse reactions within 48 hours, such as hives, injection site reactions and wheals lasting <1 hour, were significantly higher in group A than in group B (P = 0.004, P < 0.001, P = 0.001, P = 0.018).

Conclusions

BNT162b2 vaccination caused an exacerbation in 15.5% of CSU patients during the short-term follow-up. The long-term evaluation can be informative about the lasting effects of BNT162b2 vaccine on the clinical course of CSU patients.

Curbing COVID-19 Vaccine Hesitancy from a Dermatological Standpoint: Analysis of Cutaneous Reactions in the Vaccine Adverse Event Reporting System (VAERS) Database

OBJECTIVES

Adverse reactions to the COVID-19 vaccines have been of interest since their emergency authorization. Cutaneous manifestations of the vaccines are not well studied. We aimed to characterize cutaneous reactions to the Moderna (mRNA-1273) and the Pfizer-BioNTech (BNT162b2) COVID-19 vaccines on a large, national scale.

METHODS

The Vaccine Adverse Event Reporting System was filtered for cutaneous and hair and nail reactions to the COVID-19 vaccines. Patient demographics and past medical histories, vaccine manufacturer and dosing, symptom timing, reaction location, and patient outcomes were extracted from each report.

RESULTS

As of December 24, 2021, there were 67,273 cutaneous reactions to all COVID-19 vaccines, with most patients receiving the Moderna (mRNA-1273) or Pfizer-BioNTech (BNT162b2) vaccines. The most common reactions overall were injection-site reaction, urticaria, and papular rash, with injection-site reaction more common after the Moderna (mRNA-1273) vaccine, and all other cutaneous reactions more common after the Pfizer-BioNTech (BNT162b2) vaccine. Patients with past histories of psoriasis, urticaria, and local site reactions to a vaccine were more likely to report these same symptoms after the COVID-19 vaccine.

CONCLUSION

Patients should be counseled about these potential dermatologic reactions to the COVID-19 vaccines. Most occur within the first few days after vaccination, and are mild and self-limiting. Patients should therefore be encouraged that it is safe to receive the COVID-19 vaccine from a dermatological perspective.

Urticarial vasculitis: Clinical and laboratory findings with a particular emphasis on differential diagnosis

Urticarial vasculitis (UV) is a rare cutaneous vasculitis of small vessels characterized by recurrent episodes of wheal-like lesions that tend to last more than 24 hours, healing with a residual ecchymotic postinflammatory hyperpigmentation. The histopathologic pattern of UV is that of leukocytoclastic vasculitis, consisting of fibrinoid necrosis of dermal vessels' walls and neutrophil-rich perivascular inflammatory infiltrates. Although its etiopahogenesis remains still undefined, UV is now regarded as an immune complex-driven disease with activation of the complement cascade, leading to exaggerated production of anaphylatoxins that are responsible for neutrophil recruitment and activation. This condition can be categorized into 2 main entities according to serum complement levels: normocomplementemic UV and hypocomplementemic UV, the latter being associated with circulating anti-C1q autoantibodies and possible extracutaneous manifestations. Systemic multiorgan involvement may be seen particularly in syndromic hypocomplementemic UV, also known as McDuffie syndrome. This review summarizes the clinicopathological and laboratory features as well as the underlying pathophysiological mechanisms of UV. A focus on its main differential diagnoses is provided, that is, chronic spontaneous urticaria, bullous pemphigoid, IgA (Henoch-Schönlein purpura) and IgM/IgG immune complex vasculitis, lupus erythematous tumidus, Wells syndrome, erythema multiforme, cutaneous mastocytosis, cryopyrin-associated periodic syndromes, and coronavirus disease 2019-associated and anti-severe acute respiratory syndrome coronavirus 2-vaccine-associated urticarial eruptions.