Introduction

Mixed cryoglobulinemia syndrome (MCs) is a “benign” B-cell lymphoproliferative disorder characterized by a typical clinical triad—purpura, weakness, arthralgias—plus serum low complement C4 and mixed cryoglobulins [1,2,3,4,5,6]. The latter represent a variable proportion of circulating immune-complexes that reversibly precipitate at temperature below 37 °C; mixed cryoglobulins are composed by polyclonal IgGs, the autoantigens, and IgMs, the autoantibodies, with rheumatoid factor activity; the IgMs are mono-oligoclonal in type II MC and polyclonal in type III MC [1, 5, 7]. Type I cryoglobulins refer to the presence of only one, monoclonal, serum immunoglobulin, frequently associated to hematological neoplasias [1, 5, 7]. The presence of asymptomatic mixed cryoglobulins is commonly detectable in a large variety of well-known autoimmune, infectious, and neoplastic diseases [1, 5, 7], while only a minority of patients with mixed cryoglobulins develop systemic manifestations of MCs, also called cryoglobulinemic vasculitis (CV). This disorder is classified among small-medium vessel vasculitides, secondary to the intravascular deposition of immune-complexes, mainly mixed cryoglobulins, and complement [1, 5, 7].

In about 10% of cases, MCs has no identifiable cause, the so-called “essential” MCs [1, 5, 7]; while chronic hepatitis C virus (HCV) infection represents the main etiological factor in the large majority of MCs patients; in a small proportion of individuals MCs is associated with hepatitis B virus infection or with other autoimmune systemic disorders (ASD) as overlapping syndrome [8, 9]. MCs shows a chronic clinical course characterized by mild-moderate manifestations and not rarely by sudden disease flares or new organ involvement, more frequently skin ulcers, glomerulonephritis, peripheral sensory motor neuropathy, severe widespread vasculitis, and/or malignancies [8, 9]. The patients’ quality of life and the overall prognosis of MCs are generally poor if compared to the general population [10]; it is due to several factors, namely, the vasculitic multiple organ involvement, as well the frequent comorbidities and neoplastic complications [1, 5, 7, 9]. For these reasons, MCs patients may be counted among “frail” populations with high risk of disease worsening in the presence of different triggering factors, such as physical trauma or intercurrent infectious events.

In the early January 2020, a new coronavirus named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was identified as the infectious agent that caused a viral pneumonia epidemic in Wuhan, China [11, 12]. Symptomatic SARS-CoV-2 infection is termed coronavirus disease 19 (COVID-19); it can cause interstitial pneumonia, severe acute respiratory distress syndrome, kidney failure, diffuse vascular injury, and even death [11]. The World Health Organization declared COVID-19 a global pandemic on 11 March 2020 (https://www.who.int/dg/speeches/detail/who-director-general-s-opening-remarks-at-the-media-briefing-on-covid-19%2D%2D-11-march-2020).

Patients with ASD are considered frail subjects since the outbreak spread worldwide; during the last 2 years, several studies have been performed in order to assess either the overall impact of COVID-19 on ASD [13,14,15,16,17,18] and the immunogenicity and safety of the vaccination [19,20,21,22,23,24]. Previous analyses from different countries reported a variable, often increased prevalence of COVID-19 in ASD patients compared to the general population [13,14,15, 25,26,27].

Although MCs is included among ASD, very few studies focused on the impact of the ongoing pandemic as well as on the effects of COVID-19 vaccination in this disorder [19, 28, 29].

Here, we report the cumulative prevalence and outcomes of SARS-CoV-2 infection, as well as the safety and immunogenicity of COVID-19 vaccination in a very large series of Italian MCs patients after a long-term survey study.

Patients and Methods

Our multicenter, prospective survey study included 430 unselected patients with MCs (130 M, 300 F; mean age 70 ± 10.96 SD years, range 26–94; mean disease duration 8.9 ± 0.5SD years, range 1–38 years) consecutively recruited between March 2020 and October 2021 at 11 referral centers of the COVID-19 and ASD Italian Study Group; this long-term study included the first three waves and the beginning of the last wave of pandemic. The classification of MCs [2] and the patients’ clinical assessment of both cutaneous and internal organ involvement were carried out following the current criteria [30].

According to the classification criteria, all MCs patients showed a clinical history characterized by the typical clinical triad, i.e., purpura, weakness, arthralgias, plus serum mixed cryoglobulins, and low complement C4 [31]. In addition, the MCs series recruited for the study was further subgrouped taking into account the association with a triggering agent and/or overlapping with other well-defined diseases. Therefore, MCs was classified in four subsets as follows: [1] associated to hepatitis C virus (HCV) infection (over two third of cases: 334/430, 77.6%; at the time of the study HCV was eradicated in 327 patients); [2] associated to hepatitis B virus infection (10/430, 2.3%); [3] overlapping with other well-known autoimmune systemic diseases (39/430, 9.1%), namely, rheumatoid arthritis (RA) in 4, systemic lupus erythematosus (SLE) in 10, and Sjögren’s syndrome (SS) in 11, with a miscellany of other conditions in 14; and [4] “essential” MCs (47/430, 10.1%).

Informed consent was obtained from all patients before participation; the study protocol was approved by local ethic committee (RETRO-CoV2 study code #17886_bio).

Prevalence of COVID-19

The clinical status of MCs patients and the possible development of COVID-19 were evaluated by trained physicians during the telephone survey or ambulatory visits (accounting for one-third of the evaluations) using a standardized symptom-assessment questionnaire at regular 6-month intervals, or in the event of significant clinical variations [30]. Together with the updating of MCs symptoms and ongoing treatments, the patients’ interview focused on the possible COVID-19 manifestations. At the first interview, the patients were also advised to contact the physician in case of proven SARS-Cov-2 infection and/or novel symptoms suggesting a COVID-19. In all cases, data of positive SARS-CoV-2 detection by polymerase chain reaction (PCR) testing at oral/nasopharyngeal swab and COVID-19 symptom onset were collected. Symptoms assessed in the survey included fever, sore throat, new cough, nasal congestion/rhinorrhea, dyspnea, chest pain, rash, myalgias, fatigue/malaise, headache, nausea/vomiting, diarrhea, anosmia, dysgeusia, and joint pain.

COVID-19 was classified as (i) definite COVID-19 (the presence of both COVID-19 manifestations and positive oral/ nasopharyngeal swabs at PCR testing) or (ii) highly suspected COVID-19 (the presence of clinical manifestations highly suggestive of symptomatic SARS-CoV-2 infection, in the absence of PCR testing, mostly during the first months of pandemic because of the scarce availability of virological tests). Acute COVID-19 course, including treatment, hospitalization, supplemental oxygen, and mechanical ventilation, was obtained by survey or medical records if available during the clinical visit. Time to COVID-19 symptom resolution and vaccination status (before infection, after infection, declined, and planning to receive) at time of survey were also assessed.

Severity of COVID-19 infection was defined according to the World Health Organization (WHO) criteria: asymptomatic infection, mild, moderate, severe, and critical illness [32].

Moreover, MCs flares and/or worsening that appeared within the first 4 weeks of COVID-19 were classified as possible COVID-19-related manifestations.

COVID-19 Vaccination

In our MCs patients, the peri-vaccination management of immune-modifier medications followed the recommendations of the Italian Society of Rheumatology (https://www.reumatologia.it/vaccinazioni), taking in account both individual patient’s clinical characteristics (e.g. major comorbidities), disease activity, and ongoing treatments at the time of vaccination. In all cases, the timing and doses of some immunomodifiers, especially corticosteroids and rituximab, were carefully recorded.

Patients who received COVID-19 vaccine were carefully monitored by recording the possible vaccine-related (within the first 4 weeks of single dose) adverse events (AEs) and/or MCs flares or worsening.

Moreover, the immunogenicity of COVID-19 vaccines was evaluated in the serum samples of a subgroup of 45 unselected MCs patients and in 100 age-matched healthy individuals (mean age 70.17 years, ± SD 2.020) at three different time points: 3 weeks and 24 weeks after completion of the vaccination cycle and 2–4 weeks after the booster dose. The titer of IgG neutralizing antibody (NAb) against SARS-CoV-2 trimeric spike S1/S2 glycoproteins was measured by the SARS-CoV-2 IgG II Quant antibody test kit (Abbott Laboratories, Chicago, IL). As recommended by the World Health Organization (WHO), antibody titers were expressed as binding antibody units (BAU)/mL, with a cut-off for positive testing of 7 BAU/mL.

On the basis of NAb titers, MCs patients and controls were categorized as full responders (≥ 70 BAU), suboptimal responders (< 70 and ≥ 7 BAU), and non-responders (< 7 BAU).

Statistical Analysis

Data are expressed as mean ± standard deviation (SD), median (interquartile IQ range: 25–75th percentile), or numbers (percentage) as appropriate. The one-way analysis of variance (ANOVA) and the Kruskal–Wallis H test were used for comparing three or more groups, as appropriate. Non-parametric tests as Mann-Whitney or Wilcoxon were used for comparing continuous variables between two groups as appropriate. Fisher’s exact test was used to compare categorical variables. Logistic regression model was built to assess the predictivity of categorical variables for a dichotomic dependent variable, expressed as odds ratio (OR) and 95% confidence interval. All tests were two tailed. Analyses were performed using GraphPad Prism v 9 Software.

Results

Table 1 summarizes the main demographic features of the 430 MCs patients showing some significant differences among the four MCs subsets. Namely, patients with MCs overlapping with other autoimmune systemic disorders were characterized by significantly lower patients’ mean age when compared with the largest subgroup of HCV-related MCs (67.3 ± 10.3 vs 71.2 ± 10.96 SD; p = 0.03), as well as by higher female/male subject ratio when compared to other three subsets (6.8% in overlapping syndromes vs 2.3% in HCV-related MCs, p = 0.02; vs 0.66% in HBV-related, p = 0.004; and vs 2.8% in essential MCs, p = 0.01). Finally, patients with essential MCs showed a lower disease duration than that observed in the HCV-related MCs (6.6 ± 5SD vs 9.5 ± 6.2 SD; p = 0.0002).

Table 1 Demographic characteristics and clinical subsets of 430 patients with mixed cryoglobulinemia syndrome
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Prevalence and Outcome of COVID-19

Considering the whole series of 430 MCs, SARS-CoV-2 infection was recorded in 51/430 patients (11.9%, Table 2), during telephone surveys and/or during check-ups in the outpatient clinics (one third of cases). Stratifying patients based on the age (< 60 years and ≥ 60 years), according to the COVID-19 pandemic national data [33], we found a higher prevalence of SARS-CoV-2 infection in the younger MC subgroup (22% vs 9.8%, p = 0.0084, Table 2).

Table 2 Demographic characteristics of mixed cryoglobulinemia syndrome patients with/without COVID-19
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The cumulative prevalence of SARS-CoV-2 infection in MC patients, with (44 pts) or without (7 pts) COVID-19 manifestations, was significantly higher than that observed in the Italian general population (National update of Task force, Istituto Superiore di Sanità, October 20, 2021) [33] over the same time interval (11.9% vs 8.0%, p < 0.005) (Table 2) [33].

In addition, we performed a comparison between the infection rate in the MC patients and in the Italian general population [33] by stratifying the sample based on age groups. The prevalence of COVID-19 recorded in MC patients ≥ 60 years revealed again significantly higher than that reported in the Italian general population [35 cases out of 357 patients, 9.8% vs 1,224,596/18,977,236, 6.45%; OR: 1.57 (1.11–2.23), p = 0.0135].

No significant difference between COVID-19-positive and COVID-19-negative MCs patients emerged regarding both sex and disease duration (Table 2).

COVID-19-related death rate was increased in MCs patients (5.9%) compared to Italian general population (2.76%), even if without statistical significance (Table 2). Considering the whole MCs series, during the time interval of survey, the death rate observed in the 51 patients complicated by COVID-19 (3/51, 5.9%) was significantly higher than that recorded in the COVID-19-negative group (2/379, 0.5%) due to concurrent malignancies (p < 0.01) (Table 2).

In addition, the percentage of MCs patients with a disease duration ≥ 10 years was significantly higher in the group of COVID-19-positive compared to COVID-19-negative ones (55% vs 37%, p = 0.0217, OR = 0.4922; CI 0.28–0.8856).

Logistic regression analysis showed that the age < 60 years is an independent factor associated to COVID-19 (p = 0.013, OR: 2.55; Supplementary Table 1).

Among the 51 COVID-19-positive MCs subjects, 7 (13.7%) remained asymptomatic, while 44 (96.3%) individuals developed COVID-19 manifestations. The group of 44 MCs patients with symptomatic COVID-19 included 33 individuals with mild-moderate manifestations and 11 with severe respiratory symptoms requiring hospitalization (Table 2). Noteworthy, severe COVID-19 manifestations (n = 11) were invariably observed in older MCs patients (aged ≥ 60 years; Fig. 1D) and more frequently in male sex (male subjects 7/11, 64%; female subjects 4/11, 36%; Fig. 2D), while the MCs duration did not correlate with the severity of COVID-19 manifestations (Table 2).

Fig. 1
figure 1

A COVID-19 prevalence; B asymptomatic infection; C mild-moderate disease; D severe disease. **p < 0.001

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Fig. 2
figure 2

A COVID-19 prevalence; B asymptomatic infection; C mild-moderate disease; D severe disease. *p < 0.05

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The possible relationship between the COVID-19 prevalence and ongoing immune-modifier treatments was also investigated. Among MCs patients who experienced COVID-19, 21/51 (41%) underwent immune-modulating therapies vs 92/379 (24%) of COVID-19-negative MCs subjects (p 0.0166; OR = 2.184; CI 1.188–4.005). Moreover, the percentage of patients undergoing glucocorticoids was higher in the COVID-19-positive compared to the COVID-19-negative individuals (p = 0.0333; OR = 2.007; CI 1.092–3.764).

COVID-19 Vaccination

The large majority of MCs patients (374/430, 87%) underwent to complete COVID-19 vaccination, namely, two doses of BNT162b2 or mRNA-1273 vaccines in 368/374 (98%) or a single dose of Ad26.COV2.S in 6/374 (2%) (Table 3), followed by a booster dose of vaccine in half cases (216/430, 50%). A significant lower percentage of patients (27/39, 69%) with overlapping syndromes received the first two doses of vaccine if compared to both HCV-related MCs (69% vs 90%, p = 0.0012) and essential MCs subsets (69% vs 94%, p = 0.004); moreover, patients with overlapping syndromes underwent less frequently to the booster dose of COVID-19 vaccine if compared to other subsets (Table 3).

Table 3 Immunogenicity and safety of COVID-19 vaccines in 430 mixed cryoglobulinemia syndrome patients
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The cumulative percentage of MCs patients developing one or more side effects within the first 4 weeks after vaccine administration was 35% (130/374), without significant differences among various MCs subsets (Table 3). In addition, MCs flares following COVID-19 vaccination were observed in 5% of cases (20/374); disease flares were significantly more frequent in essential MCs compared to HCV-related MCs (6/44, 14%, vs 11/299, 4%; p = 0.013). Interestingly, the prevalence of MCs flares possibly triggered by vaccination was significantly lower than that caused by COVID-19 (20/374, 5% vs 10/51, 14%; p = 0.0285).

In a subgroup of 45 unselected MCs patients, we measured the serum titer of NAb at different time intervals, namely, at 3 weeks and at 24 weeks after the first two doses of vaccine in order to evaluate the early and late response, and at 3 weeks after the booster vaccination (Table 3, Fig. 3).

Fig. 3
figure 3

A Median levels of IgG neutralizing antibody (NAb) against SARS-CoV-2 trimeric spike S1/S2 glycoproteins in mixed cryoglobulinemia patients and healthy controls, at 3 and 24 weeks after a complete vaccination and 2–4 weeks after the booster shot; results are expressed in binding antibody unit/mL (BAU/mL); interquartile range: 25–75th percentile. B Rate of no response at 3 and 24 weeks after a complete vaccination and 2 weeks after the booster shot in mixed cryoglobulinemia patients and healthy controls. *p < 0.05, **p < 0.001, ***p < 0.0001

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The NAb serum levels at the three different measurement times (Fig. 3A) showed a significant reduction of serum NAb levels after 6 months from complete vaccination (median NAb from 211.0 (range IQ 6.250–1218) BAU/mL to 46.00 (range IQ 3.000–273.0) BAU/mL, p < 0.05), which was followed by a marked booster effect caused by the third dose of vaccine (median NAb 1722 (range IQ 135.5–2500) BAU/mL, p < 0.001).

Moreover, the comparison between the NAb levels assessed in MCs patients and in the control group at the three different time points showed that the NAb levels were significantly lower in MCs vs controls at each measurement (3 weeks after complete vaccination: p < 0.05; 24 weeks after complete vaccination: p < 0.0001; and 3 weeks after booster dose: p < 0.0001) (Fig. 3A).

After the first two doses of vaccine, a full or suboptimal response was observed in 65% (29/45) and 11% (5/45) of MCs patients, respectively, while no response was recorded in 24% of cases (11/45).

Moreover, MCs patients showed a significant higher rate of no response at 3 and 24 weeks after the first two doses of vaccine compared to controls (patients vs controls: 24% vs 6%, p = 0.0039; and 29% vs 3%, p = 0.0001, respectively), while after the booster dose of vaccine, the absence of seroconversion was recorded in a smaller proportion of patients (14%) and in any of the healthy controls (p = 0.05) (Fig. 3B).

Due to the low number of patients with a complete humoral response profile, it was not possible to assess the correlation between immunogenicity and other parameters such as MC severity and/or presence of lymphoma

Considering the levels of NAb induced by a complete vaccination, patients on immune-modifier treatments showed a higher prevalence of a no response or a suboptimal response (10/16, 63%) compared to those without immune-modifiers (8/29 (28%), p = 0.0298 OR = 0.228; CI 0.07–0.79) (Supplementary Table 2).

In particular, rituximab (RTX), administered within 6 months preceding the first shot of vaccine, was associated to an increased rate of no response (full response vs no response, p = 0.015; Supplementary Table 2).

Discussion

To the best of our knowledge, this is the first study concerning the COVID-19 prevalence and outcome and the COVID-19 vaccine immunogenicity in a large MCs patients’ population; it provides the results of a long-term survey period covering the first three waves of the pandemic and partly the fourth. The multicenter approach allowed to enroll a large series of patients with this rare disorder; all patients showed a typical MCs encompassing four distinct settings, namely, the HCV- and HBV-related MCs, the MCs in overlap with other ASDs, and the “essential” MCs; the whole of these subgroups may represent quite well the entire spectrum of MCs one deals with in the clinical practice. As expected, the majority of MCs patients had a viral etiology, mostly HCV- and very rarely the HBV-related disease, reflecting the current epidemiology of these hepatotropic viruses in Italy. In addition, the higher female/male subject ratio, observed in the overlap syndrome subgroup compared to other subgroups, is in keeping with the coexistence of MCs with other ASDs, which are characterized by a marked prevalence of female sex, as well as by a younger mean age compared to HCV-related MCs [34], while the finding of different mean disease duration among MCs subsets, particularly between HCV-related and essential MCs, could be not completely reliable since this parameter is difficult to assess in real-life studies, as previously suggested [35].

Overall, SARS-CoV-2 infection was recorded in a significantly higher percentage of MCs patients than the Italian general population, even when the comparison was performed considering subjects ≥ 60 years. Also, even though our survey did not explore the patient exposure history, COVID-19 prevalence was higher in younger individuals (< 60 years), consistently with the national data; it is possibly due to their social lifestyle that may increase the infectious risk. While the severity of COVID-19 manifestations revealed significantly worse in older (> 60 years) MC patients, in particular, COVID-19-positive patients complicated by severe respiratory symptoms requiring hospitalization (11/51, 21.6%; 3 deceased) were invariably observed in the subgroup of older subjects (≥ 60 years). Of note, the patients’ older age was the only adverse prognostic parameter evidenced by multiple regression analysis; the high frailty of this patients’ subgroup is possibly due to the progressive gathering of different negative factors, mainly MC-related visceral organ damages, comorbidities, and/or long-term steroid treatment [6].

This relationship is in keeping with previous observations regarding the older age as one of major risk factors for COVID-19 worse outcomes [36, 37]. Similarly, we observed a higher rate of men among the 11 hospitalized MCs patients, even if the overall prevalence of SARS-CoV-2 infection was equal in both genders, confirming that the male sex represents an additional risk factor for more severe COVID-19 manifestations [38].

The significantly higher prevalence and severity of COVID-19 in MCs compared to the Italian general population may be related to seriously compromised health conditions present in this patients’ population; in fact, if a household lifestyle and a more limited exposure justify the lower prevalence in older people, the poor health conditions could explain the higher prevalence of worse outcome of COVID-19 in MC patients ≥ 60 years compared to the same age group of Italian general population. An increased frailty against the pandemic has been observed in different ASDs series, particularly in patients with other systemic vasculitides or connective tissue diseases [25, 26, 36]. Nevertheless, a previous study conducted by the COVID-19 and ASD Italian Study Group on a large sample of ASDs [37] revealed a very low COVID-19 prevalence in rheumatoid arthritis (0.5%, namely, 5 COVID-19 cases out of 1114 patients), a disease that shares numerous similarities with MCs. In this context, the MCs patients represent one of the most “frail” population due its diffuse vascular multiorgan involvement, advanced mean age, frequent chronic steroid administration, and possibly long disease duration [6]. Although the actual disease duration in MCs is hard to assess in real life [35], we found an increased prevalence of COVID-19 in patients with a long-lasting cryoglobulinemic syndrome. It is conceivable that a longer disease duration may contribute to generally worse health condition, thus increasing the patient’s vulnerability and morbidity of SARS-CoV-2 infection.

The COVID-19-related mortality rate was increased in our MCs patients compared to the Italian general population (5.9 % vs 2.76%), even if not statistically different [33], possibly due to the relatively low number of observed events. Of note, the mortality rate among COVID-19-positive individuals, during the time interval of our survey, was significantly higher than that observed in COVID-19-negative patients; this finding underlines the increased frailty of MCs patients against the pandemic and suggests the need of prevention measures against SARS-CoV-2 infection.

Data about the COVID-19-related mortality are available in the world literature for other ASDs, even if they are often contradictory and largely variable [13,14,15, 25, 39]. The majority of previous studies lacks a comparison with national data, and mostly, they report a rather high COVID-19 mortality reflecting the initial period of COVID-19 pandemic when no treatments and vaccine were available [13,14,15, 39, 40].

The strong restriction measures of the first year of pandemic followed by the intense vaccination campaign, with the administration of two doses of vaccine in the large majority of MCs patients plus a booster dose in half cases, may have contributed to limit the worst consequences for our MCs patients during the COVID-19 pandemic exacerbation of the last 2021 [29]. Of note, the subgroup of MCs in overlap with another ASD showed the lowest vaccination rate; this is in line with previous reports [41], highlighting an alarming vaccine hesitancy in patients with severe ASDs that may be due to a baseless fear of serious adverse event risk and/or disease worsening/reactivation.

In the MCs series, we observed that the adverse events of COVID-19 vaccination, mostly transient local or systemic reactions, were generally self-limiting. Our observation is consistent with findings reported by other studies on similar disease settings that showed an adverse event rate comparable with controls [42]. Overall, the risk-benefit evaluation to vaccinate patients with ASDs is a particularly debated issue in the early phase of pandemic, mostly given the possible exacerbation of the autoimmune disease triggered by immunization [43, 44]. Moreover, anecdotal reports described de novo cases of vasculitis developing in healthy individuals shortly after COVID-19 vaccination [45, 46]. Of interest, the only study regarding the impact of COVID-19 on MCs patients by Visentini and colleagues [28] reported an overall rate of post-immunization flares compared to that recorded in the present survey. Considering that these flares were clinically mild-moderate and subsided spontaneously, the authors suggested that COVID-19 vaccine should be encouraged in this particular patient setting [28].

Even more, the results of the present study showing the significant lower rate of MCs flares induced by COVID-19 compared to those triggered by COVID-19 vaccine strongly support the actual advantages of the immunization.

The immunogenicity of COVID-19 vaccination was assessed in a subgroup of MCs patients and in healthy controls at three different time points, namely, 3 and 24 weeks after the complete vaccination schedule and 3 weeks after the booster dose administration. The median NAb levels were significantly lower in MCs compared to controls at the three longitudinal dosages, showing a vaccine response impairment comparable to that observed for other ASDs [19, 47, 48]. In addition, the behavior of the NAb levels showed a relevant decay 24 weeks after the complete vaccination, similarly to that described in the world literature [49]; this observation is particularly worrying in patients with basic immune system dysregulation and often under immune-suppressor treatments. However, the marked reduction of serum NAb can be overcome by a booster shot in the majority of MCs patients as observed in healthy controls.

Patients with MCs showed a significantly higher rate of no response after the first doses of vaccine if compared to other ASDs and healthy controls [19]; our preliminary observation was confirmed by the significantly higher percentage of MCs patients who experienced a no response compared to healthy controls, more pronounced soon after the vaccination and at the 6-month evaluation. Rather, we observed that a relevant number of MCs (14%) did not show a valuable seroconversion even after the booster dose of vaccine, while all controls experienced a full response; this not negligible percentage of patients with undetectable NAb level must be considered clinically at high risk for COVID-19. In view of the cost-benefit ratio, the identification of these particularly frail subjects is crucial in the clinical practice; the inclusion of NAb evaluation among other virological testing should be mandatory in the routine monitoring of MCs patients, as well as of other ASD subgroups. The evaluation of the cellular response could also be considered since a compensatory role was described in people with a low humoral response to SARS-CoV-2 infection and/or to COVID-19 vaccine [50].

Concerning the effects of immune-modifier drugs on the immunogenicity induced by COVID-19 vaccine, the results observed in the MCs series are in keeping with the available literature [19]; similarly to that reported for other ASDs [19, 21, 23], recent RTX infusions and glucocorticoids hampered an effective humoral response in MCs.

Conclusions

In summary, we can affirm that the results of the present survey study including a very large MCs patients’ population of such rare disorder provided some useful information:

  • Patients with MCs can be included among numerous patients’ populations at high risk to be infected, mostly due to the use of immunomodulating therapies, and to develop severe COVID-19 manifestations.

  • The absence of detectable seroconversion, which is hampered by recent RTX infusions and glucocorticoids in a proportion of vaccinated individuals even after the booster dose, represents the major challenge in the clinical practice.

  • A good safety of vaccines was recorded; of note, the rate of vaccine-related side effects/disease flares was significantly lower if compared to that caused by COVID-19.

  • Our data are not conclusive; as regards the long-term efficacy of vaccination against SARS-CoV-2 infection, only a long follow-up period of observation may clarify this crucial issue.

Finally, the current course of the pandemic suggests a progressive evolution to an endemic condition that does not necessarily mean mild disease, with periodic exacerbations and unpredictable appearance of more harmful SARS-CoV-2 variants. Therefore, specific prevention/treatment strategies [51,52,53,54], possibly tailored on individual patients, should be elaborated for MCs, as well as for other frail patients’ populations.