A Phase 2 Extension Study Evaluating the Immunogenicity, Safety, and Tolerability of 3 or 4 Doses of a Clostridioides difficile Vaccine in Healthy US Adults Aged 65 to 85 Years

Abstract

Background

This phase 2 extension explored the long-term antibody persistence of an investigational Clostridioides difficile vaccine and the safety, tolerability, and immunogenicity of dose 4 approximately 12 months post–dose 3.

Methods

One year post–dose 3, healthy US 65- to 85-year-olds (N = 300) were randomized to dose 4 of vaccine at previously received antigen levels (100 or 200 μg) or placebo. Assessments included safety and percentages of participants achieving neutralizing antibody titers above prespecified thresholds (≥219 and ≥2586 neutralization units/mL for toxins A and B, respectively).

Results

In participants previously given three 200-µg doses and placebo in the extension, toxin A and B neutralizing antibodies were above prevaccination levels 48 months post–dose 3 (36 months after placebo); 24.0% and 26.0% had toxin A and B antibodies at or above prespecified thresholds, respectively. Neutralizing antibodies increased post–dose 4 (12 months post–dose 3) and persisted to 36 months post–dose 4. Thirty days post–dose 4, all participants had toxin A and 86.5% to 100% had toxin B titers at or above prespecified thresholds. Local reactions were more frequent in vaccine recipients. Systemic and adverse event frequencies were similar across groups.

Conclusions

C difficile vaccine immune responses persisted 48 months post–dose 3. Dose 4 was immunogenic and well tolerated, supporting continued development.

Clinical Trials Registration. ClinicalTrials.gov NCT02561195.

The persistence of a Clostridioides difficile vaccine in older adults and the immunogenicity, safety, and tolerability of a fourth dose were explored. Immune responses were persistent at 36 months after 3 doses, and a fourth dose was immunogenic, safe, and well tolerated.

Clostridioides difficile (previously Clostridium difficile) is a major cause of health care– and community-associated diarrhea [1, 2]. Symptoms can range from mild to severe outcomes, including colitis, toxic megacolon, intestinal perforation, and death [2, 3]. Risk factors for C difficile infection (CDI) include recent antibiotic treatment, older age, and certain comorbidities [1]. Recurrence is a key challenge to treating CDI, occurring in up to 25% of patients given a traditional course of antibiotics to manage a primary infection [1, 4].

Traditionally associated with health care facilities, the burden of community-associated CDI is increasing, with an estimated US incidence of 121.2 per 100 000 population in 2019 vs 55.3 in 2011 [5, 6]. A highly virulent drug-resistant strain belonging to polymerase chain reaction ribotype 027 emerged from 2000 to 2010; however, incidence of hospital-acquired CDI has decreased since its peak [1, 5]. Antibiotics such as metronidazole, vancomycin, and fidaxomicin are generally the first line of treatment for primary CDI [1]. Additional antibiotic courses may be used to treat recurrent infections but generally with less success than for primary infections [1]. Another approach to treating recurrent CDI is the use of monoclonal antibodies against C difficile toxin B (bezlotoxumab) [7], which is currently licensed for use in adults in the United States and Europe. Another treatment option recently approved in the United States for cases of recurring antibiotic-resistant CDI involves fecal transplants from healthy donors [1, 8–10].

With (1) an increasing burden of community-acquired CDI, (2) widespread environmental contamination and related transmission within health care facilities, and (3) the emergence of highly virulent drug-resistant strains, coupled with an aging population, a preventive approach is fundamental in combating CDI [1, 3, 5, 6, 11, 12]. Current preventive strategies focus on infection control, particularly when patients with CDI are concerned, and include guidelines regarding hand hygiene, use of disposable equipment, and surface disinfecting, among others [13]. However, no vaccine is available to prevent CDI.

An investigational C difficile vaccine was previously assessed in an initial 3-dose regimen in the primary stage of this study. The safety and immunogenicity of 3 doses of C difficile vaccine were evaluated in healthy adults aged 65 to 85 years at various doses (100 or 200 μg) and dosing schedules (month regimen: 0, 1, and 6 months; day regimen: 1, 8, and 30 days). In this primary stage of the study, the C difficile vaccine was safe and well tolerated, and robust immune responses were reported with the 200-μg month-based regimen. It is important to understand the duration of immunity, as well as the effects of additional doses beyond the primary series; this extension stage examines antibody persistence after the initial 3 doses up to 48 months, as well as responses to a fourth (booster) dose of C difficile vaccine administered 12 months after the 3-dose series.

METHODS

Study Design and Participants

This phase 2 observer-blinded study, based on a placebo-controlled randomized design, was conducted from 12 September 2016 to 13 February 2020 at 14 US sites. In the primary stage of the study, participants were randomized to receive 3 doses of C difficile vaccine at months 0, 1, and 6 (month regimen) or days 1, 8, and 30 (day regimen). Within each regimen, participants were randomized with an interactive response technology system in a 3:3:1 ratio to receive 100-μg C difficile vaccine, 200-μg C difficile vaccine, or placebo. These results have been reported elsewhere [14].

Participants from the month and day regimens who received the first 3 doses of C difficile vaccine (100 or 200 μg) were eligible to enroll in the extension stage; these participants were randomized in a 1:1 ratio to receive, as a fourth dose, either C difficile vaccine at the same antigen level (100 or 200 μg) or placebo 12 months after the third dose of vaccine (Figure 1). Only site staff dispensing and administering the vaccine were unblinded; all other site personnel, including the principal investigator and the participants, were blinded during the vaccination periods of the study. Once data from month 7 (after dose 1) were available from all continuing participants, site personnel were unblinded so that they could identify those who were eligible for the extension stage. Site personnel were blinded to vaccine allocation of the fourth dose until 6 months after the fourth dose. Laboratory personnel performing the immunologic assays remained blinded throughout the study.

Figure 1.

CONSORT diagram with participant dispositions in the (A) month and (B) day regimens during the extension phase. C difficile, Clostridioides difficile.

The study was conducted in accordance with legal and regulatory requirements, the International Council for Harmonisation guideline for good clinical practice, and the Declaration of Helsinki. The study was approved by the institutional review board and/or independent ethics committee for each participating site. Written informed consent was provided by each participant before enrollment and performance of any study-related procedures.

Participants were healthy adults aged 65 to 85 years without a history of CDI, who were available for the duration of the study and able to be contacted by phone. Those with preexisting chronic medical conditions were eligible if the condition was determined to be stable. Women could not be of childbearing potential, and men had to agree to use effective contraception. Participants in the primary stage who were randomized to placebo in either dosing regimen or who completed visit 9 before study unblinding were not eligible for the extension stage. A full list of exclusion criteria can be found in the supplementary appendix.

Interventions

The investigational product was a toxoid-based vaccine, utilizing genetically and chemically inactivated C difficile toxin A and toxin B. The vaccine was administered by intramuscular injection into the upper deltoid muscle, preferably in the nondominant arm. The 0.5-mL injection contained 100 or 200 μg (toxoids A and B combined) of the C difficile vaccine candidate (reconstituted with 1-mg/mL aluminum hydroxide) or placebo (0.9% sodium chloride).

Immunogenicity Assessments

The objectives for the extension study were to describe the immunogenicity and antibody persistence after C difficile vaccine in healthy adults up to 36 months after administration of a fourth dose. Exploratory endpoints described (1) immunogenicity and persistence at multiple time points after vaccination and (2) the immune responses of participants in the month regimen who received three 200-µg doses of vaccine and a fourth as placebo to evaluate persistence after 3 doses of C difficile vaccine. Immunogenicity endpoints included the geometric mean concentrations (GMCs), percentage of participants in each vaccine group meeting prespecified thresholds, and geometric mean fold rises (GMFRs) of toxin A– and toxin B–specific neutralizing antibody levels from baseline (before dose 1) to post–dose 4. The prespecified thresholds for neutralizing antibody titers were ≥219 and ≥2586 neutralization units/mL for toxin A– and toxin B–specific antibody, respectively. Prespecified thresholds for neutralizing antibody titers were based on a previously demonstrated correlate of protection from a phase 2 trial [14]. Serum samples were collected for immunogenicity testing before dose 4, after dose 4 at 8 and 30 days, and after dose 4 at 6, 12, 18, 24, 30, and 36 months. Immune responses were evaluated with toxin neutralization assays for toxins A and B as previously described [15]. The evaluable immunogenicity populations for both regimens included eligible participants who received dose 4, had a valid and determinate assay result from blood drawn on day 8 after dose 4 (7–14 days after vaccination), and had no major predefined protocol violations.

Safety Assessments

The primary safety objective was to assess the safety and tolerability of a fourth dose of C difficile vaccine. Safety endpoints included the number and percentage of participants reporting local reactions and systemic events up to 14 days after dose 4, adverse events (AEs) from dose 4 to 28 days after dose 4, and serious AEs (SAEs) from dose 4 to 6 months after dose 4. Immediate AEs were recorded within 30 minutes after vaccination. The safety population included all participants who received ≥1 dose of the investigational product.

Statistical Analyses

Safety and immunogenicity data from the extension stage were analyzed separately for each regimen. All binary data were reported by number and percentage with exact 2-sided 95% Clopper-Pearson CIs. For continuous data (ie, GMCs, GMFRs), 95% CIs were calculated with a Student t distribution on log-transformed data that were then back-transformed to the original scale.

RESULTS

A total of 300 participants were enrolled and randomized in the extension stage of the study (Figure 1), with a completion rate of 88% in the month regimen and 90% in the day regimen. Demographic characteristics were generally similar in both regimens (Table 1).

Table 1.

Baseline Characteristics: Safety Population–Extension Stage

	Month Regimen Vaccine Groupa					Day Regimen Vaccine Groupa
	100-µg C diff / Placebo	100-µg C diff / 100-µg C diff	200-µg C diff / Placebo	200-µg C diff / 200-µg C diff	Total	100-µg C diff / Placebo	100-µg C diff / 100-µg C diff	200-µg C diff / Placebo	200-µg C diff / 200-µg C diff	Total
No. of participantsb	51	52	58	59	220	16	18	24	22	80
Sex
Female	24 (47.1)	25 (48.1)	34 (58.6)	22 (37.3)	105 (47.7)	6 (37.5)	15 (83.3)	12 (50.0)	13 (59.1)	46 (57.5)
Male	27 (52.9)	27 (51.9)	24 (41.4)	37 (62.7)	115 (52.3)	10 (62.5)	3 (16.7)	12 (50.0)	9 (40.9)	34 (42.5)
Race
White	44 (86.3)	41 (78.8)	53 (91.4)	53 (89.8)	191 (86.8)	16 (100.0)	17 (94.4)	23 (95.8)	21 (95.5)	77 (96.3)
Black	3 (5.9)	5 (9.6)	3 (5.2)	1 (1.7)	12 (5.5)	0	0	1 (4.2)	1 (4.5)	2 (2.5)
Asian	4 (7.8)	4 (7.7)	2 (3.4)	5 (8.5)	15 (6.8)	0	0	0	0	0
Other	0	2 (3.8)	0	0	2 (0.9)	0	1 (5.6)	0	0	1 (1.3)
Ethnicity
Non-Hispanic/non-Latino	46 (90.2)	49 (94.2)	56 (96.6)	56 (94.9)	207 (94.1)	14 (87.5)	18 (100.0)	22 (91.7)	20 (90.9)	74 (92.5)
Hispanic/Latino	5 (9.8)	3 (5.8)	2 (3.4)	3 (5.1)	13 (5.9)	2 (12.5)	0	2 (8.3)	2 (9.1)	6 (7.5)
Age,c y
Mean (SD)	72.2 (4.97)	71.3 (5.41)	71.5 (4.81)	72.0 (4.53)	71.7 (4.90)	70.2 (4.35)	69.1 (3.45)	70.7 (4.20)	70.2 (5.31)	70.1 (4.37)
Median	71.0	71.0	71.0	72.0	71.0	70.0	68.0	70.5	69.0	69.0
Range	65–83	65–85	65–82	65–84	65–85	65–79	65–77	65–79	65–83	65–83
Baseline C difficile serostatusd
Seronegative
Toxin A−/toxin B−	6 (11.8)	11 (21.2)	7 (12.1)	4 (6.8)	28 (12.7)	11 (68.8)	3 (16.7)	8 (33.3)	3 (13.6)	25 (31.3)
Seropositive
Toxin A+/toxin B−	1 (2.0)	1 (1.9)	2 (3.4)	1 (1.7)	5 (2.3)	0	0	0	1 (4.5)	1 (1.3)
Toxin A−/toxin B+	10 (19.6)	17 (32.7)	14 (24.1)	16 (27.1)	57 (25.9)	4 (25.0)	8 (44.4)	8 (33.3)	7 (31.8)	27 (33.8)
Toxin A+/toxin B+	34 (66.7)	23 (44.2)	35 (60.3)	38 (64.4)	130 (59.1)	1 (6.3)	7 (38.9)	8 (33.3)	11 (50.0)	27 (33.8)

Data are presented as No. (%) unless noted otherwise.

Abbreviations: C diff, Clostridioides difficile vaccine; LLOQ, lower limit of quantitation.

^aThe numerator is the vaccine group assigned in the original planned stage; the denominator is the vaccine group assigned in the extension stage.

^bNumber of participants is used as the denominator for the percentage calculations.

^cAge at randomization in the original planned stage.

^dBaseline serostatus at extension stage (before dose 4) defined by LLOQ value for toxin A– or toxin B–specific neutralizing antibody level, with + indicating neutralizing antibody level ≥LLOQ and – indicating neutralizing antibody level <LLOQ.

Immunogenicity

Month Regimen

For participants who received three 200-μg doses in the primary study and placebo in the extension study (200-μg/placebo group), toxin A–specific GMCs decreased from 249 neutralizing units/mL at 12 months after dose 3 (ie, extension study baseline) to 141 at 48 months after dose 3 (Figure 2A). Toxin B–specific GMCs for the 200-μg/placebo group decreased from 1925 neutralizing units/mL at 12 months after dose 3 to 1021 at 48 months after dose 3. Trends in GMCs in the 100-μg/placebo group were similar to the 200-μg/placebo group; however, GMCs were generally higher for the 200-μg/placebo group. Responses to 3 vaccine doses were persistent at 48 months after dose 3 as compared with before dose 1, as shown by toxin A–specific GMFRs (200 μg/placebo, 1.63; 100 μg/placebo, 1.49) and toxin B–specific GMFRs (200 μg/placebo, 5.45; 100 μg/placebo, 4.68). Thirteen months after dose 3, the percentages of participants achieving prespecified levels of toxin A–neutralizing antibodies in the 200- and 100-μg/placebo groups were 56.9% and 37.5%, respectively (Figure 3A). For toxin B, the percentages were 43.1% and 35.4%.

Figure 2.

GMCs of toxin A– and toxin B–specific neutralizing antibodies in the initial and extension phases for the 200- and 100-μg dose levels. A, Month regimen. B, Day regimen. Arrows indicate days on which doses were administered. Error bars indicate 95% CIs. Blood samples were drawn on day 1 of the extension phase immediately before administration of dose 4. C difficile, Clostridioides difficile; GMC, geometric mean concentration (neutralizing units per milliliter).

Figure 3.

Percentages of participants achieving prespecified levels of toxin A– and toxin B–specific neutralizing antibodies in the (A) month and (B) day regimens during the extension phase. Blood samples were drawn on day 1 of the extension phase immediately before administration of dose 4. Toxin A– and toxin B–specific thresholds were ≥219 and ≥2586 neutralization units/mL, respectively. Error bars indicate 95% CIs. C difficile, Clostridioides difficile.

For the 200- and 100-μg C difficile groups who received the fourth (booster) dose, toxin A– and toxin B–specific GMCs were highest at day 30 after dose 4 and decreased steadily, reaching the lowest point at month 36. GMCs in the 200-μg C difficile group were generally higher than in the 100-μg C difficile group. Long-term persistence was observed at month 36 in toxin A–specific GMFRs (200-μg C difficile, 4.31; 100-μg C difficile, 2.63) and toxin B–specific GMFRs (200-μg C difficile, 11.78; 100-μg C difficile, 7.51). An anamnestic booster response in toxin A–specific GMFRs after dose 4 (vs baseline [before dose 1]) peaked at day 30 (200-μg C difficile, 55.87; 100-μg C difficile, 38.73). Toxin B–specific GMFRs demonstrated a similar trend, with the highest booster response observed at day 30 (200-μg C difficile, 169.35; 100-μg C difficile, 76.65).

Thirty days after dose 4 for the month regimen, the percentages of participants achieving prespecified levels of toxin A–neutralizing antibodies in the 200- and 100-μg C difficile groups were 100% and 100%, respectively; for toxin B, respective percentages were 96.6% and 86.5%. The percentages achieving prespecified antibody thresholds against toxin A– and toxin B–specific antibodies were highest 30 days after dose 4 and decreased steadily, reaching the lowest level at month 36.

Day Regimen

For the day regimen, GMCs for the 200- and 100-μg/placebo groups followed similar trends as the month regimen but were generally lower (Figure 2B). For participants in the 200- or 100-μg C difficile groups, GMCs were highest at day 30 after dose 4, reaching the lowest points between months 24 and 30, and were generally higher in the 200-μg group as compared with the 100-μg group. Similar to the month regimen, long-term persistence of the day regimen was observed at month 36 (C difficile groups) or 48 months after dose 3 (placebo groups) in toxin A–specific GMFRs (200-μg C difficile, 7.73; 100-μg C difficile, 5.11; 200 μg/placebo, 1.17; 100 μg/placebo, 1.01) and toxin B–specific GMFRs (200-μg C difficile, 22.93; 100-μg C difficile, 9.79; 200 μg/placebo, 3.37; 100 μg/placebo, 1.58). Trends in peak booster response for the day regimen were similar to the month regimen. Toxin A– and toxin B–specific GMFRs after dose 4 (vs baseline) peaked at day 30 (toxin A: 200-μg C difficile, 94.33; 100-μg C difficile, 62.52; toxin B: 200-μg C difficile, 171.15; 100-μg C difficile, 74.42).

The day regimen demonstrated similar trends in the percentages achieving prespecified levels of neutralizing antibodies as compared with the month regimen (Figure 3B). Thirty days after dose 4, 100% achieved prespecified levels of toxin A–neutralizing antibodies in both groups, and 100% and 95% in the 100- and 200-μg groups, respectively, achieved prespecified levels of neutralizing antibodies to toxin B. The percentages achieving prespecified antibody thresholds against toxin A– and toxin B–specific antibodies were highest at day 30 after dose 4, reaching a low between months 18 and 24.

Safety

In the month regimen, local reactions were more common in both C difficile groups when compared with both placebo groups (Figure 4A). The most common local reaction in all groups was pain at the injection site. No local reactions were assessed as grade 3 or 4. The percentages reporting systemic events were similar across all groups in the month regimen (Figure 4A). Fatigue and diarrhea were the most common systemic events. No systemic events were assessed as grade 4. Rates of diarrhea were 13.5% and 11.9% in the 100- and 200-µg C difficile vaccine groups, respectively, while rates in the 100- and 200-µg/placebo groups were 16.0% and 13.8%; reported events were all mild or moderate in severity.

Figure 4.

Local reactions and systemic events by dose in the (A) month and (B) day regimens during the extension phase. 100P, 100 μg/placebo; 100, 100-μg Clostridioides difficile vaccine; 200P, 200 μg/placebo; 200, 200-μg Clostridioides difficile vaccine.

Similar trends were observed in the day regimen, with local reactions more common in the C difficile groups as compared with the placebo groups (Figure 4B). Systemic events were similar across groups, and fatigue and diarrhea were the most commonly observed events. No systemic events were assessed as grade 4. Rates of diarrhea were 16.7% and 13.6% in the 100-µg and 200-µg C difficile vaccine groups, respectively, while rates in the 100- and 200-µg/placebo groups were 12.5% and 25.0%; most reported events were mild or moderate in severity, with 2 events in the 200-µg/placebo group considered severe. For all systemic events regardless of regimen, there were no notable clinically meaningful differences between placebo and C difficile vaccine groups.

Adverse events occurred in 23% and 10% of participants in the month and day regimens, respectively (Table 2). The most common AEs were infections and infestations in the month regimen and gastrointestinal disorders in the day regimen. An immediate AE of moderate severity was reported by 1 participant in the 200-μg/placebo group in the month regimen. In the month regimen, SAEs were reported by 3 participants in the 200-μg C difficile group, 1 in the 100-μg C difficile group, 1 in the 200-μg/placebo group, and 2 in the 100-μg/placebo group. In the day regimen, SAEs were reported by 1 participant in the 200-μg C difficile group and 1 in the 200-μg/placebo group. Two participants in the 200-μg C difficile month regimen group were withdrawn from the study because of AEs after dose 4 (lymphoma and cerebrovascular accident). One participant in the 200-μg C difficile day regimen died during the study (due to cardiovascular event). No immediate AEs, SAEs, AEs leading to withdrawals, or deaths were considered vaccine related.

Table 2.

AEs in Month and Day Regimens During the Extension Stage

	Month Regimen Vaccine Groupa				Day Regimen Vaccine Groupa
	100-µg C diff / Placebo	100-µg C diff / 100-µg C diff	200-µg C diff / Placebo	200-µg C diff / 200-µg C diff	100-µg C diff / Placebo	100-µg C diff / 100-µg C diff	200-µg C diff / Placebo	200-µg C diff / 200-µg C diff
No. of participantsb	51	52	58	59	16	18	24	22
AEs	10 (19.6)	8 (15.4)	17 (29.3)	16 (27.1)	1 (6.3)	0	4 (16.7)	3 (13.6)
Related	0	1 (1.9)	1 (1.7)	4 (6.8)	0	0	2 (8.3)	2 (9.1)
SAEs	2 (3.9)	1 (1.9)	1 (1.7)	3 (5.1)	0	0	1 (4.2)	1 (4.5)
Related	0	0	0	0	0	0	0	0
Immediate AEsc	0	0	1 (1.7)	0	0	0	0	0
Related	0	0	0	0	0	0	0	0
AEs leading to withdrawal	0	0	0	2 (3.4)	0	0	0	0
Related	0	0	0	0	0	0	0	0
Deaths	0	0	0	0	0	0	0	1 (4.5)
Related	0	0	0	0	0	0	0	0

Data are presented as No. (%).

Abbreviations: AE, adverse event; C diff, Clostridioides difficile vaccine; SAE, serious adverse event.

^aThe numerator is the vaccine group assigned in the original planned stage; the denominator is the vaccine group assigned in the extension stage.

^bNumber of participants is used as the denominator for the percentage calculations.

^cOccurring within 30 minutes after vaccination.

DISCUSSION

Emergence of highly virulent strains of CDI and an increase in community-acquired disease has changed the epidemiology of CDI [1, 5]. Although current treatment strategies rely on the use of antibiotics or fecal transplant, a preventive CDI vaccine is necessary to address unmet medical needs [1, 8, 9]. This study was designed to investigate antibody persistence up to 48 months after a 3-dose primary series of a C difficile vaccine candidate and the immune response and antibody persistence up to 36 months after a fourth dose.

Although immune responses in the 200- and 100-μg/placebo groups were similar for the month and day regimens at 48 months after dose 3, higher responses were observed with the 200-µg/placebo month regimen. In the 200-µg/placebo group, which matches the dose and schedule in the pivotal efficacy study (NCT03090191), 24.0% and 26.0% of participants remained above the toxin A– and toxin B–specific thresholds at month 36 (48 months after the last vaccine dose), respectively. Toxin A and B GMCs were above baseline at month 36. GMCs were generally numerically highest for the 200-μg/placebo month regimen as compared with the other placebo regimens, consistent with results from the initial study [14]. In both regimens, the 200- and 100-μg C difficile vaccine recipients demonstrated a highly immunogenic response to a fourth dose of vaccine, which persisted for up to 36 months. At 30 days after dose 4, neutralizing antibody responses greater than or equal to prespecified thresholds were achieved by 100% of participants for toxin A and 86.5% to 100% for toxin B. This persistent booster response was most notable in the 200-μg C difficile month regimen. In both regimens, the percentages of participants achieving toxin A– and toxin B–specific neutralizing antibody levels greater than or equal to the specified threshold increased after dose 4 as compared with the first 3 doses. GMCs and GMFRs were also higher after a fourth dose of vaccine as compared with 3 doses in both regimens.

The C difficile vaccine demonstrated an acceptable safety profile and was generally well tolerated. There were no clinically meaningful differences in local reactions between the 200- and 100-μg dose levels, and the rates of systemic events were similar between placebo and the 2 vaccine dose levels in both regimens. Most local reactions and systemic events were mild to moderate in severity and similar to those seen in the primary stage of the study. Overall, the reported AEs demonstrated an acceptable safety profile, and no untoward safety signals were identified. Most AEs were characteristic acute and chronic medical conditions common in individuals aged 65 to 85 years. None of the reported SAEs were considered by the investigator to be related to the study vaccine.

The present results complement the safety, tolerability, and immunogenicity findings from the primary stage of this study [14], as well as a recent study in older Japanese adults [16]. In addition, although the phase 3 CLOVER trial did not meet its prespecified primary endpoint of prevention of primary CDI, C difficile vaccine efficacy was 100% for CDI requiring medical attention when compared with placebo and was also associated with a 75% to 80% reduction in CDI duration [17]. These studies provide additional support for the investigational C difficile vaccine for the prevention of CDI in adults. However, a general limitation of the extension stage of this study was the small sample sizes, particularly in the day regimen.

Overall, the results from the extension stage demonstrated that antibody levels were maintained above baseline levels up to 48 months after the third dose of C difficile vaccine in older adults, particularly in the 200-μg month regimen selected for phase 3, and that a fourth dose was safe and immunogenic for up to 3 years. This study supports the continued development of this C difficile vaccine for prevention of CDI in older adults.

Data Availability Statement

Upon request and subject to review, Pfizer will provide the data that support the findings of this study. Subject to certain criteria, conditions, and exceptions, Pfizer may also provide access to the related individual deidentified participant data. See https://www.pfizer.com/science/clinical-trials/data-and-results for more information.

Supplementary Data

Supplementary materials are available at The Journal of Infectious Diseases online. Consisting of data provided by the authors to benefit the reader, the posted materials are not copyedited and are the sole responsibility of the authors, so questions or comments should be addressed to the corresponding author.

Notes

Acknowledgments . We thank all of the study participants and the study site staff at the participating institutions for their contributions to participant enrollment and data collection. Editorial/medical writing support was provided by Lindsey Kirkland, PhD, and Allison Gillies, PhD (ICON, Blue Bell, PA, USA) and funded by Pfizer Inc.

Financial support . This work was supported by Pfizer Inc. Funding to pay the Open Access publication charges for this article was provided by Pfizer Inc.