Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causing coronavirus disease 2019 (COVID-19) was first described in December 2019 in Wuhan, China [1]. SARS-CoV-2 has, since then, been responsible for almost 650 million cases and more than 6.5 million deaths worldwide [2]. The severity of COVID-19 depends on both the variant of the virus and the host’s comorbidities [3]. Patients with advanced age, diabetes mellitus, hypertension, cardiovascular disease, endocrine, and respiratory diseases are considered to be at high risk for severe COVID-19 [3]. The liver is commonly affected in patients with COVID-19, with severe transaminasemia being present in a substantial number of patients, reaching up to 50% of all SARS-CoV-2 infected individuals. High serum transaminases levels seem to be predictive of poor outcome, however acute liver failure is uncommon in patients with no prior liver disease ^[4][5][4,5]. Patients with chronic liver diseases (CLD) have been recognized as a high-risk group for severe COVID-19 since the beginning of the pandemic ^[6][7][8][9][6,7,8,9]. Among them, patients with liver cirrhosis (PWLC), irrespective of the cause, seem to carry the heaviest burden, as shown by an increased morbidity and mortality ratio ^[6][7][8][9][6,7,8,9].

Vaccination is the most effective tool against severe COVID-19 and, given the high morbidity and mortality of the pandemic, many different anti-SARS-CoV-2 vaccines, broadly divided into mRNA, inactivated or weakened virus, viral vector, and protein subunit ones, have been licensed with fast-track authorization ^[10][11][12][10,11,12]. During the months following the initiation of global vaccination, these vaccines proved to be safe and effective, even in special populations, such as patients with chronic kidney disease and immunocompromised, pediatric, pregnant, and older individuals ^[13][14][15][13,14,15]; however adverse events of variable significance, mainly due to autoimmune phenomena, have been reported ^{[16][17][18][19][20][21][22][23][24]}[16,17,18,19,20,21,22,23,24]. Among these side effects, pain at the injection site, fatigue, headache, nausea and low-grade fever were the most common, while others, such as anaphylaxis, thrombosis with thrombocytopenia syndrome, Guillain-Barré syndrome, myocarditis and pericarditis, autoimmune-like hepatitis, and thrombosis of various veins, though severe, proved to be extremely rare ^{[16][17][18][19][20][21][22][23][24]}[16,17,18,19,20,21,22,23,24].

2. mRNA Vaccines

Although the idea of using messenger RNA (mRNA) for the production of specific proteins has been present since the 1980s, it was only because of the COVID-19 pandemic that this technology was developed and widely used, in a relatively short time ^[25][33]. The mRNA-based vaccines introduce an mRNA molecule that is capable of coding a part of the SARS-CoV-2 spike protein ^[26][34]. To date, two mRNA vaccines are approved against SARS-CoV-2, namely the BNT162b2 from Pfizer, BioNTech and the mRNA-1273 from Moderna ^[27][28][35,36]. These novel vaccines have proved to be safe and effective in a large variety of patients, including individuals with obesity, chronic lung disease, diabetes mellitus and cardiac disease, among others ^[27][28][35,36]. Only a small number of patients with chronic liver diseases (CLD) and/or cirrhosis, were included in phase I-III clinical trials of mRNA vaccines; therefore a lot of concern was raised regarding their safety and efficacy in these patients [17]. This concern was further increased after the association of mRNA vaccination with liver-related adverse events, with the most important being autoimmune-like hepatitis; this side effect was not attributed exclusively to mRNA vaccination, but in the largest so far case series by Efe C et al., 67 out of 87 patients were mRNA-vaccinated ^{[19][29][30][31]}[19,37,38,39]. The efficacy of mRNA vaccines in PWLC was proven quite early in the pandemic, through a study from John B et al., showing that even one dose of mRNA vaccination was associated with a 64.8% reduction in COVID-19 infections and 100% protection against hospitalization or death due to COVID-19; these percentages rose up to 78.6% and 100% respectively after 2nd dose [9]. Interestingly enough, the effect of anti-SARS-CoV-2 vaccination was lower in patients with decompensated cirrhosis, even though the number of patients and events among decompensated PWLC in this study were low ^[32][40]. Likewise, in a more recent study by Ge J et al., anti-SARS-CoV-2 vaccination was associated with a 66% all cause mortality in PWLC suffering from COVID-19 [9], while in another study by John BV et al., comparing vaccine-induced versus infection-induced immunity, in a large cohort of PWLC, vaccination led to a reduced risk of developing symptomatic SARS-CoV-2 infection; notably, when symptomatic, the infection had a lower risk of being moderate or severe ^[33][41]. Most importantly, in another study by John BV et al., a third dose of a mRNA vaccine was shown to significantly ameliorate immunity against SARS-CoV-2, leading to an even greater reduction in overall and symptomatic SARS-CoV-2 infection, as well as COVID-19-related mortality ^[34][42]. This effect was more evident among patients with compensated liver cirrhosis and those receiving the BNT162b2 vaccine. As far as antibody responses are concerned a variety of immunological responses have been investigated. Regarding post-vaccination antibody responses, Thuluvath P., et al., showed that the vast majority of PWLC had adequate antibody responses, with 91% of the patients in this study having received mRNA-based vaccines; interestingly, vaccination with the viral vector-based Ad.26.COV2.S vaccine was associated with poor immune response in multivariable analysis ^[35][43]. In the first ever study measuring neutralizing antibodies in a small cohort of PWLC, vaccinated exclusively with mRNA vaccines, Bakasis AD et al., showed that the presence of liver cirrhosis was not associated with statistically significant differences in total or neutralizing antibodies when compared with patients with CLD and healthy individuals; three months post-vaccination all patients exhibited lower levels of total and neutralizing antibodies, again with no differences between patients with and without liver cirrhosis ^[36][44]. In contrast with the preliminary studies, the subsequent studies of mRNA vaccinated PWLC showed contradictory results. In agreement with the aforementioned studies, Ruether D et al., showed that all 53 PWLC developed adequate anti-SARS-CoV-2 antibodies, with no differences in antibody-titers when compared with healthy controls ^[37][45]. On the contrary, Willuweit K et al., showed that even though 96% of their PWLC developed anti-SARS-CoV-2 antibodies after two doses of the BNT162b2 vaccine, their levels were lower than those of healthy controls, showing furthermore a rapid and significant decline ^[38][46]. Likewise, Al-Dury S et al., Giambra V et al., and Iavarone M et al., showed suboptimal antibody responses in PWLC when compared with controls, even though none of these studies showed higher rates of SARS-CoV-2 infection post-vaccination or COVID-19 severity in PWLC ^[39][40][41][47,48,49]. Interestingly, two of these studies showed defective T-cell reactivity, while, in the third one, T-cell responses were similar to controls, further complicating the scenery of post mRNA-vaccination T-cell responses in PWLC ^[39][40][41][47,48,49]. In the study by Giambria V et al., a booster dose of the BNT162b2 vaccine was administered; after that third dose both humoral and cellular responses improved ^[39][47]. In another, very interesting study by Chauhan M et al., PWLC and those with liver transplantation with low anti-SARS-CoV-2 after the initial vaccination received a booster dose ^[42][50]. A total of 18 PWLC were included in the study, with 12 of them having received mRNA vaccines as initial vaccination and 6 the Ad.26.COV2.S vaccine. Seventeen of these patients received mRNA as a booster vaccine and 1 the Ad.26.COV2.S vaccine, with 12 of them having good antibody responses ^[42][50]. Overall, mRNA vaccines seem to lead to adequate antibody responses, though in lower titers than healthy individuals; moreover PWLC exhibit faster antibody waning, reflecting their cirrhosis-related immune dysfunction. The different results found in the abovementioned studies most probably reflect differences in methods of antibody measurement and cut-off points rather than the real outcome of mRNA vaccination in these patients. In any case, booster doses seem to overcome any immune deficiency leading to higher antibody production. Despite their differences regarding T- and B- cell responses adverse events of mRNA-based vaccines were rare and non severe in all of the above-mentioned studies. Among them, local pain, fatigue and low-grade fever were the most common, with severe adverse events being extremely rare. Moreover, no differences among patients with cirrhosis and healthy controls were noted, proving the safety of mRNA vaccination among PWLC ^{[35][36][37][38][39][40][41]}[43,44,45,46,47,48,49].

3. Viral Vector-Based Vaccines

Viral vector vaccines use an unrelated harmless virus, more often an adenovirus, to deliver genetic material which can be transcribed by the recipient’s host cell as mRNA coding for a desired protein to elicit an immune response ^[43][44][51,52]. These vaccines are further divided in two types: non-replicating and replicating; non-replicating use replication-deficient viral vectors to deliver genetic material of a particular antigen to the host cell to induce immunity against the desired antigen, while replicating ones produce new viral particles in the cells they enter. Currently available anti-SARS-CoV-2 adenovirus vector-based vaccines include Ad.26.COV2.S by Johnson and Johnson (Janssen) along with Beth Israel Deaconess Medical Center, ChAdOX1-nCOV by Oxford-AstraZeneca, Gam-COVID-Vac and Sputnik Light by Gamaleya Research Institute of Epidemiology and Microbiology, and Ad5-nCoV-S vaccine by CanSino Biologics, making viral vector-based vaccines the most commonly used anti-SARS-CoV-2 vaccines worldwide ^[45][46][53,54] Adenovirus vector-based vaccines have proven to be safe and effective in the general population ^[17][18][43][17,18,51]. However, only a handful of studies have evaluated their efficacy and adverse effects in PWLC. In contrast with the study from John BV et al., antibody responses after adenovirus-based vaccines were poorer when compared with mRNA-based vaccines, especially in patients with decompensated cirrhosis, in two other studies ^[35][47][43,58]. More specifically, Thuluvath P., et al. showed poorer antibody responses in patients with cirrhosis receiving the Ad.26.COV2.S vaccine when compared with those receiving the mRNA vaccines; however only seven PWLC received the Ad.26.COV2.S vaccine in this study ^[35][43]. Likewise, in a study by Kulkarni A et al., patients with decompensated cirrhosis showed suboptimal humoral and cellular immune responses after ChAdOx1 vaccination ^[47][58]. More specifically, 34% of patients with decompensated cirrhosis were non-responders, while CD4-naïve, CD4 effector, B- and B-memory cells were lower in patients with decompensated cirrhosis ^[47][58]. In agreement with the previous studies, no severe adverse events were recorded, confirming the safety of viral vector-based vaccines in PWLC. Overall, the results from these studies show antibody responses similar to that expected from previous experience with vaccination in PWLC, such as hepatitis B or pneumococcal vaccines, with adequate levels of seropositivity but lower levels of serum antibodies ^[48][27]; possible differences in various studies most probably reflect endpoints used in each one. Fortunately, viral vector-based vaccines were successful in protecting from severe infection in all studies irrespective of serum antibody levels.

4. Whole Virion Vaccines

Inactivated whole virion vaccines use an inactivated (by chemicals, heat, or radiation) form of the disease-causing virus, incapable of causing disease but capable of generating an immune response ^[43][44][51,52]. To date, three different inactivated COVID-19 vaccines are commercially available; namely, the Sinopharm BBIBP-CorV and WIBP-CorV vaccines, the Sinovac PiCoVacc vaccine, and the Bharat Biotech BBV152 COVAXIN vaccine ^[49][59]. Various studies have shown that inactivated anti-SARS-CoV-2 vaccines are both safe and effective in healthy adults ^[45][50][51][53,60,61]. On the other hand, data concerning PWLC are rather scarce, with only a few studies evaluating the impact of these vaccines in COVID-19-related hospitalization and mortality. In one of the largest studies, by Diaz LA et al., comprising 2050 PWLC and COVID-19, vaccination led to substantial decreases in hospitalization rates; in this study 79.4% of the whole cohort was vaccinated with the PiCoVacc vaccine ^[52][62]. In another large study, a prospective, multicenter study, by Wang J et al., a total of 553 patients with cirrhosis received two doses of inactivated whole-virion COVID-19 vaccines and adverse events and neutralizing antibodies were assessed ^[53][63]. Overall vaccination was found to be safe and well tolerated for patients with both compensated and decompensated liver cirrhosis. The most common local adverse reaction was injection site pain followed by swelling and erythema and the most common systematic adverse reaction was fatigue and fever; all adverse events were mild and resolved spontaneously ^[53][63]. Interestingly, decompensated cirrhosis was correlated with vaccine hyporesponsiveness, with Child-Pugh grades B and C being independent risk factors for absence of neutralizing antibodies ^[53][63]. In another prospective, multicentered, open label, study by Ai J et al., 153 patients with cirrhosis received two doses of inactivated whole virion SARS-CoV-2 vaccines; the overall positive rate of neutralizing antibodies was significantly lower when compared with healthy participants, with similarly low immunogenicity in compensated and decompensated patients (78.9% and 76.7% respectively). Adverse events among vaccine recipients were mild and transient, with injection site pain being the most common; only three patients exhibited severe transaminasemia post-vaccination, with one of them being judged as severe and needing hospitalization ^[54][64]. Likewise, in another prospective observational study, by Chen Z et al., comparing healthy controls with patients with severe liver disease, PWLC showed inferior antibody responses that also waned faster; no statistically significant differences were found among patients with compensated and decompensated liver cirrhosis ^[55][65]. Another interesting finding of this study was the fact that the overall incidence of adverse events within seven days post-vaccination in PWLC was significantly higher than that of healthy controls (33,3% vs. 12,0%); the majority of these adverse events were mild and non-severe ^[55][65].

5. Protein Subunit Vaccines

Protein subunit vaccines are based on the use of an antigenic protein part, commonly combined with an adjuvant, to enhance immunogenicity ^[56][57][66,67]. Multiple protein subunit vaccines against SARS-CoV-2 are currently under trial ^[58][68], even though up to date only two protein subunits vaccines are approved for use, namely the NVX-CoV2373 vaccine, sold under the names of Nuvaxovid from Novavax, and Covovax from Serum Institute of India ^[59][69]. These vaccines seem to elicit both B-lymphocyte and T-lymphocyte immune responses to the S protein of SARS-CoV-2, while they seem to offer protection against most variants of the virus ^[60][61][70,71]. Moreover, the protein subunit vaccines have showed minimal side effects in clinical trials; the observed side effects were mainly local, such as injection site pain and swelling, redness, and pruritus ^[60][61][62][70,71,72]. Unfortunately, up until now, no trials exist regarding the use of these vaccines in PWLC, so no safe consumptions can be reached, even though the safety and efficacy profile of these vaccines make them good candidates for use in this population.