Background

Hemodialysis (HD) patients are a high-risk population with much higher hospitalization and mortality rates due to COVID-19 than the general population [1, 2]. Vaccination of HD patients is recommended to prevent infection and severe disease [3]. However, a number of reports have confirmed that after the usual two doses of COVID-19 vaccine, the antibody responses in HD patients are significantly weaker than in the general population [3,4,5,6,7,8,9,10,11,12]. Moreover, the elevated antibody titers decay over time [9]. We therefore believe that a COVID-19 booster vaccination for populations at high risk for severe disease, including HD patients, is justified. For that reason, in Japan a third dose of the vaccine is administered to populations at high risk of severe disease, including HD patients, with the expectation of a booster effect. In addition, the omicron variant strain of SARS-CoV2 is now the predominant strain in many countries, including Japan. With the omicron variant, immune escape is often seen, making breakthrough infection after vaccination an important issue [13]. And while the omicron variant appears to cause mild disease in the general population, there are reports of more severe cases among HD patients, so caution should be exercised [14].

Study design, setting, and participants

This observational, prospective, single-center study to evaluate humoral responses after mRNA vaccination in Japanese HD patients was conducted at Fujita Memorial Hospital. Overall, 185 HD patients (HD group) and a control group composed of 109 healthcare workers without kidney failure from our hospital (HCW group) were included in the study (Fig. 1). All HD patients over 18 years of age in our dialysis facility were considered for inclusion. Exclusion criteria were vaccination refusal (only one HD patient and two HCWs), a history of SARS-CoV-2 infection prior to vaccination, and positivity for anti-S IgG antibodies (> 50 AU/mL) prior to vaccination. The characteristics of the participants at the beginning of the study are detailed in reference [4]. Between the second and third vaccinations, 18 patients dropped out of the HD group (9 died, 4 were transferred, 1 had a COVID-19 infection after the second vaccination, and 1 refused the third vaccination) and 9 dropped out of the HCW group (8 retired and 1 had a moderate adverse reaction) (Fig. 1). The characteristics of the participants at the time of the third vaccination are shown in Table 1.

Fig. 1
figure 1

Study flowchart. HD hemodialysis, HCWs healthcare workers

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Table 1 Participant profile at the time of the third vaccination
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The mRNA vaccine was administered at our dialysis facility. Two doses of BNT162b2 vaccine (30 μg each) were administered between April 4 and May 22, 2021. As recommended by the manufacturer, there was a 21-day interval between the first and second vaccinations. The third vaccination was administered 7–8 months after the second. Among the remaining 167 HD patients, 162 received the BNT162b2 vaccine and 5 received the mRNA-1273 vaccine. The remaining 100 HCWs all received the BNT162b2 vaccine. Follow-up continued until September 5, 2022.

Humoral response assessment

Postvaccination, serum levels of anti-SARS-CoV-2 spike protein IgG antibodies were assayed using a chemiluminescent microparticle immunoassay (SARS-CoV-2 IgG II Quant assay on an ARCHITECT analyzer; Abbott). The assay detects antibodies against the receptor binding domain of the S1 subunit of the SARS-CoV-2 spike protein and presents a positive predictive agreement of 99.4% (95% confidence interval [95% CI], 96.50–99.97%) and a negative predictive agreement of 99.6% (95% CI, 99.15–99.37%). In addition, the results of this assay are consistent with those obtained using the neutralization method to wild-type SARS-CoV-2 (positive agreement, 100.0%; 95% CI, 95.72–100.00%) [15, 16]. Anti-SARS-CoV-2 spike IgG antibody assays have shown excellent correlation with neutralizing antibodies [17]. A value of 50 arbitrary units per milliliter (AU/ml) was considered evidence of a vaccination response [18]. Antibodies were measured before vaccination and a median of 17 (IQR: 16–19) days after the second and third vaccinations in both the HD and HCW groups. An anti-nucleocapsid IgG assay (Abbott) was also employed with 41 HD patients who had high spike antibody levels after the third vaccination.

Other variables

The Kt/V, protein catabolic rate (PCR), normalized protein catabolic rate (nPCR), creatinine (Cr) index, and geriatric nutritional risk index (GNRI) in the HD patients were calculated as described previously [19,20,21]. BMI was defined as dry weight in kilograms divided by height squared, in meters. We used recorded laboratory tests, which were routinely conducted for all HD patients at the beginning of the month prior to their first dose of SARS-CoV-2 vaccine. Details of the patients’ maintenance were obtained from their medical charts.

Adverse events

Using a standardized questionnaire, vaccination-related adverse events were separately assessed in the HD and HCW groups. These included fever, malaise, headache, chills, vomiting, diarrhea, myalgia, and injection site pain. We questioned the HD patients and HCWs about the subjective severity of the adverse events after vaccine administration. The grading was divided into three levels (mild, moderate, and severe) established according to the Food and Drug Administration toxicity grading scale [22].

Breakthrough infection

Diagnosis of a breakthrough infection was made based on the clinical signs of COVID-19 and a positive PCR test for SARS-CoV-2 virus in nasopharyngeal swabs for patients who received 2 or 3 doses of m-RNA vaccination.

Statistical analyses

All data for continuous variables are summarized and displayed as the mean (SD) for each group. For categorical variables, the Chi-square statistic was used to assess the significance of differences between groups. Normally distributed parameters were compared using t tests. Parameters not normally distributed were compared using Kruskal–Wallis/Mann–Whitney U tests. Values of P < 0.05 were considered statistically significant for all analyses. EZR Statistics for Windows and R version 3.4.1 (The R Foundation for Statistical Computing, Japan) were used for all statistical analyses [23].

Results

Participants

The HD group was significantly older, more male-dominated, had a higher rate of diabetes and hematologic complications, and received more steroids than the HCW group (Table 1).

Humoral response

After two doses of mRNA vaccine, the anti-spike IgG level in the HD group (median, 2728.7; IQR, 1024.2–7,688.2) was significantly lower than in the HCW group (median, 10,500; IQR, 9346.1–24,500) (Fig. 2, left panel). However, after three doses of the vaccine, the difference between the anti-spike IgG titers in the HD (median, 20,000; interquartile range, 7729–37,000) and HCW (median, 21,500; IQR, 14,000–32,250) groups (Fig. 2, right panel) groups had disappeared. In the HD group, 164/167 (98.2%) patients showed antibody titers of ≥ 809 AU/mL, the neutralizing antibody titer for the delta variant, while 130/167 (77.8%) patients showed titers of ≥ 5889 AU/mL, the neutralizing antibody titer for the omicron variant [24]. In the HCW group, neutralizing antibody titers for the delta and omicron variants were attained in 100/100 (100%) and 98/100 (98.0%) of participants, respectively.

Fig. 2
figure 2

Anti-spike antibody titers in HD patients and HCWs after the 2nd and 3rd vaccinations. Me median; N.S. not significant; Q1 in HD quartile 1 antibody level in the HD group, Q3 in HD, quartile 3 antibody level in the HD group

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Table 2 shows the involvement of several factors in the low (< Q1) and high (> Q3) antibody responders after the 2-dose and 3-dose vaccinations. As we reported previously [3], factors associated with antibody hyporeactivity after 2 doses of vaccine in HD patients included advanced age, low nutrition (low nPCR, lymphocytes, and Cr index), current smoking, steroid medication, and complications of hematologic diseases. After 3 doses of vaccine, however, all of the factors related to a low antibody response were no longer significant, and even HD patients exhibited a robust antibody response.

Table 2 Comparison of the high (> Q3) and low (< Q1) responders in the HD group after 2 and 3 doses of m-RAN vaccine
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Figure 3 shows the anti-spike antibody levels in HD patients and HCWs with high antibody titers after the third vaccination and the interval between the second and third vaccinations, which may be related to these titers. The median antibody titer in the HD patients was 58,000 AU/mL with an IQR of 49,000–78,000 AU/mL, which was significantly higher than the median titer of 38,000 AU/mL and IQR of 36,000–46,000 AU/mL among HCWs (Fig. 3, left panel). The interval between the second and third vaccinations was significantly shorter in the HD group (median, 217 days; IQR, 215–225 days) than the HCW group (median, 257 days; IQR, 252–260 days) (Fig. 3, right panel). This difference in interval reflects the Japanese government's COVID-19 vaccine policy.

Fig. 3
figure 3

Anti-spike antibody levels after 3 vaccinations in HD patients and HCWs with high antibody titers (left panel) and the interval (days) between the 2nd and 3rd vaccinations (right panel)

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Figure 4 shows the anti-spike antibody titers in the HD patients and HCWs with low antibody titers after the third vaccination and the correlation between the anti-spike antibody titers after the second and third vaccinations. The median antibody titer for HD patients was 3233 AU/mL with an IQR of 1638–4919 AU/mL, which was significantly lower than that for HCWs (9950 AU/mL; IQR, 9031–12,000 AU/mL) (Fig. 4, left panel). In addition, there was a positive correlation between the anti-spike antibody titers after the second and third vaccinations (Fig. 4, right panel).

Fig. 4
figure 4

Anti-spike antibody levels after 3 vaccinations in HD patients and HCWs with low antibody titers (left panel) and the correlation between anti-spike antibody titers after the 2nd and 3rd vaccinations (right panel)

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Adverse reactions to mRNA vaccine

As we previously reported, adverse events were generally milder in the HD group than in the HCW group [4]. Adverse events after the third vaccination were similar to those after the second, and adverse events were again milder in the HD than in the HCW group (Additional file 1: Fig S1, Additional file 1: Fig S2).

Breakthrough infections

Seven HD patients contracted COVID-19 after mRNA vaccination (Table 3). This included five males and two females ranging in age from 50 to 73 years. Two had diabetes as their primary disease, while five were not diabetic. The interval from their last vaccination to breakthrough infection ranged from 70 to 219 days. Their spike antibody titers after two of the three vaccinations ranged from 4919 to 62,000 AU/mL, and the titers just prior to breakthrough infection ranged from 597.9 to 61,000 AU/mL. The disease was mild in all seven patients, thought four required hospitalization; the other three patients were treated as outpatients. Based on the results of these seven cases of breakthrough infection, vaccination against the omicron variant is desirable in the future [25].

Table 3 Breakthrough infections after mRNA vaccination
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Discussion

HD patients with COVID-19 are reported to be at higher risk of severe illness, hospitalization, and death than the general population [1]. Vaccination is a promising means of preventing COVID-19 or reducing its severity. [3]. In our study, the vaccine positivity rate after two doses of BNT162b2 vaccine was 97.8% in the HD group and 100% in the HCW group. However, as in a number of earlier reports, the median antibody titer was significantly lower in the HD than in the HCW group [3,4,5,6,7,8,9,10,11,12,13]. Factors associated with the decreased humoral response after 2 doses of vaccine included older age, lower lymphocyte count, lower nPCR, lower GNRI, lower Cr index, lower BMI, steroid administration, and hematologic disease [4]. However, the HD group also showed a marked increase in anti-spike antibody titer after 3 doses of mRNA vaccine, and the difference from the HCW group disappeared. Our findings are consistent with previous reports, including several reports from Japan, that antibody production in HD patients after three doses of mRNA vaccine and loss of significance of the factors associated with the humoral hyporesponsiveness [26,27,28,29,30,31,32,33,34].

We previously (2003–2004) vaccinated (BimmugenⓇ) 106 HD patients for hepatitis B using a protocol that consisted of three doses of vaccine (10 μg 1 month and 6 months after the initial dose). This resulted in a 66.6% anti-hepatitis B antibody positivity rate after the first course and a 93.4% positivity rate after the second course (details of the data not presented). Those findings are consistent with a recent study showing the importance of repeated hepatitis B vaccinations in a group of HD patients [35]. We therefore hypothesized that booster vaccination would increase anti-SARS-CoV-2 antibody production in HD patients with impaired immune function.

Notably, in the present study, comparison between HD patients and HCWs with high anti-spike antibody titers after the third vaccination showed that antibody titers were significantly higher in the HD than in the HCW group.

Yoshifuji et al. reported that both cellular and humoral immunity were significantly higher in the HD group than in the control group at 3 weeks and 3 months after the booster dose, which was considered helpful for the prevention of severe disease. Panizo et al. [31] showed that most hemodialysis patients develop SARS-CoV-2-S antibody responses comparable to that of healthy controls at 15 days and 3 months after complete vaccination schedule (two doses) [36]. Also, Espi et al. found that the percentage of circulating spike-specific CD8 + T cells was more heterogeneous and slightly reduced in patients on maintenance hemodialysis patients as compared with in healthy volunteers after the second vaccination [34]. One reason for this is likely the significantly shorter interval between the second and third vaccinations in the HD group (Fig. 3, left panel). Bensouna et al. reported that patients with a greater increase in anti-spike antibody titers after three doses of BNT162b2 vaccine had a shorter interval between the second and third doses [26]. When they divided their patients into three groups based on the ratio of anti-spike antibody titers after the second and third vaccinations, they found that the interval between the second and third doses was a factor involved in the ratio. They then examined groups with second and third vaccine antibody titer ratios of ≥ 43 and ≤ 7.3 and found that the interval between the second and third vaccinations was 74 [IQR, 72–79] days for the high antibody titer ratio group and 59 [IQR, 36–67] days for the low titer ratio group (P < 0.001). Our results show that the HD group, which had a shorter vaccination interval, had significantly higher antibody titers after the third vaccination than the HCW group, which had a longer interval. In the context of the abovementioned studies, this suggests the effect of the vaccination may be weakened if the vaccination interval is shorter or longer than optimal. On the other hand, Yoshifuji et al. reported that the hemodialysis group had significantly higher antibody titers than the control group, but there was no difference in the vaccination interval from the primary vaccine to the booster vaccine in the two groups [31].

Comparison between groups with low antibody titers after the third vaccination showed significantly lower titers in the HD than HCW group (Fig. 4, left panel), and a positive correlation was observed between the antibody titers after the second and third vaccinations (Fig. 4, right panel). Although the data are not shown, a positive correlation was also observed between the HD and HCW groups as a whole. The cause of this low antibody response may be a disorder of the genetic or acquired immune response. With regard to disorders of the genetic immune response, Higuchi et al. found a relationship between antibody titer and HLA polymorphism in Japanese rheumatoid arthritis patients after two doses of BNT162b2 vaccine [37]. We therefore suggest that a genetic factor may contribute to determining the magnitude of the increase in antibody titer after vaccination. As for acquired immune response disorders in hemodialysis patients, this is as previously reported.

Seven HD patients experienced COVID-19 breakthrough infections after the second or third vaccination. These breakthroughs occurred even when antibody titers measured just prior to COVID-19 infection were high. We suggest this may be due to the fact that the mRNA vaccine used was less compatible with the omicron variant or the exposure to SARS-CoV-2 virus was high.

This study has several limitations. First, the study was carried out at a single institution, and the number of participants in the study was relatively small. Second, the anti-spike antibodies measured in this study were not neutralizing antibodies, nor did we examine antibodies against the omicron variant, which is currently the dominant strain in Japan. Third, in this study, cellular immunity to the vaccine was not examined.

Conclusions

Anti-SARS-CoV-2 spike antibody titers were significantly lower in HD patients than in healthy controls after two doses of mRNA vaccine. However, after three doses, the HD patients also showed a very good increase in antibody titer, which no longer differed from that of healthy controls. The interval between the second and third vaccinations may have contributed to this result. Seven breakthrough infections occurred, and future vaccination against the omicron variant would be desirable.