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Lactobacillus GG as an immune adjuvant for live-attenuated influenza vaccine in healthy adults: a randomized double-blind placebo-controlled trial



Live-attenuated influenza vaccine (LAIV) protects against influenza by mucosal activation of the immune system. Studies in animals and adults have demonstrated that probiotics improve the immune response to mucosally delivered vaccines. We hypothesized that Lactobacillus GG (LGG) would function as an immune adjuvant to increase rates of seroconversion after LAIV administration.


We conducted a randomized double-blind placebo-controlled pilot study to determine whether LGG improved rates of seroconversion after administration of LAIV. We studied 42 healthy adults during the 2007–2008 influenza season. All subjects received LAIV and then were randomized to LGG or placebo, twice daily for 28 days. Hemagglutinin inhibition titers were assessed at baseline, at day 28 and at day 56 to determine the rates of seroconversion. Subjects were assessed for adverse events throughout the study period.


A total of 39 subjects completed the per-protocol analysis. Both LGG and LAIV were well tolerated. Protection rates against the vaccine H1N1 and B strains were suboptimal in subjects receiving LGG and placebo. For the H3N2 strain, 84% receiving LGG vs 55% receiving placebo had a protective titer 28 days after vaccination (odds of having a protective titer was 1.84 95% confidence interval 1.04–3.22, P=0.048).


Lactobacillus GG is potential as an important adjuvant to improve influenza vaccine immunogenicity. Future studies of probiotics as immune adjuvants might need to specifically consider examining vaccine-naïve or sero-negative subjects, target mucosal immune responses or focus on groups known to have poor response to influenza vaccines.


The trivalent live-attenuated influenza vaccine (LAIV) was licensed in June 2003 as an alternative to inactivated, trivalent influenza vaccine (TIV) for healthy individuals aged between 2 and 49 years. The effectiveness of LAIV in preventing influenza illness varies by age, previous influenza immunization and match between circulating influenza virus and vaccine strains (Nichol and Treanor, 2006). Although studies in children have demonstrated superior or equivalent efficacy between LAIV and TIV in children, recent studies have not consistently demonstrated the same robust results in healthy adults. Monto et al. (2009) reported suboptimal protection against laboratory-confirmed symptomatic influenza from LAIV compared with TIV for the 2007–2008 influenza season. In a study of military personnel aged 17–49 years, over three influenza seasons, receipt of TIV was associated with a significantly lower rate of health-care visits for pneumonia and influenza compared with those who received LAIV (Wang et al., 2009). However, among new recruits being vaccinated for the first time, the incidence of health-care visits was similar. Systemic immune responses to LAIV previously have been found to be lower than TIV (Beyer et al., 2002), and mucosal immunological correlates of protection after receipt of LAIV might be more informative but are difficult to measure (Treanor and Wright, 2003).

The recent outbreak of pandemic H1N1 influenza has heightened the need to improve vaccine responses. Because of its convenient administration and stimulation of mucosal immunity, LAIV remains an important part of the armamentarium to prevent influenza infection. This has led to increasing interest in adjuvants to improve LAIV immunogenicity. Probiotics are defined as live microorganisms that confer a health benefit when administered in adequate amounts. Probiotics may enhance innate and adaptive immunity, and have previously been demonstrated to augment the immune response to mucosally delivered vaccines (Fang et al., 2000). Mice-fed Lactobacillus, before to an influenza virus challenge, had higher levels of influenza-specific immunoglobulin (Ig) G and greater protection against illness (Yasui et al., 2004). Intranasal administration of Lactobacillus GG (LGG) decrease morbidity and mortality in mice infection with H1N1 (Harata et al., 2010). Probiotic Lactobacillus fermentum (CECT5716) (Olivares et al., 2007) and a fermented dairy drink containing Lactobacillus caseii DN-114 (CNCMI-1518) Streptococcus thermophilus and Lactobacillus bulgaricus (Boge et al., 2009) improved the immunogenicity of TIV in two human trials. However, there are no human studies of probiotics as an immune adjuvant for mucosally delivered LAIV. We conducted a proof-of-concept study to evaluate whether the probiotic LGG had any immune-adjuvant effect on serum influenza antibody titers and on increased rates of seroconversion after administration of LAIV to healthy adults during a single influenza season.

Materials and methods

Study design

In the 2007–2008 influenza season, we conducted a double-blind randomized placebo-controlled clinical trial to assess the safety and immunogenicity to LAIV in healthy subjects of 18–49 years of age while also receiving an oral probiotic—LGG (ATCC 53101, Culturelle) or matching placebo. The study was approved by the Tufts Medical Center Institutional Review Board, the Tufts Clinical Research Center and registered on (NCT00620412). The study was supported in part by the National Institutes of Health grant M01RR00054 and Amerifit Brands, Cromwell, CT, USA. Amerifit Brands had no role in the design, conduct, analysis or interpretation of the results.

Subjects for whom LAIV was contraindicated, who had received the 2007–2008 influenza vaccine or who had used any probiotic in 4 weeks before enrollment were ineligible to participate. Receipt of influenza vaccine in previous influenza seasons and yogurt consumption were not exclusion criteria, but subjects were asked to avoid consumption of any yogurt or probiotic during the first 4 weeks of the study. Subjects were recruited from the local community using Institutional Review Board-approved advertisements. Written informed consent was obtained from all participants before they were screened for eligibility criteria. Screening included a complete medical history, physical examination and routine laboratory tests (including HIV, Hepatitis B and Hepatitis C testing). All subject study visits occurred at the Tufts Medical Center Clinical Research Center. Subjects were recruited until the end of the influenza season (1 April 2008).

After meeting eligibility criteria, all participants received nasally administered LAIV according to the manufacturer's recommendations (FluMist, Medimmune Vaccines, Gaithersburg, MD, USA) at the baseline study visit. Approximately 0.1 ml (that is, half of the total sprayer contents) was sprayed into each nostril while the recipient was sitting in the upright position. The 2007–2008 influenza season vaccine contained A/Solomon Islands/3/2006 (H1N1)-like (new for the 2007–2008 season), A/Wisconsin/67/2005 (H3N2)-like and B/Malaysia/2506/2004-like antigens.


The randomization scheme was generated by the study statistician (Ms Fiorino) using the web randomization site (GE Dallal). Ms Fiorino had no contact with the study subjects. Randomization assignments (1:1 LGG/placebo) were made in permuted blocks of 2 and 4. Block sizes were also randomly assigned. Once the eligibility criteria were met, study participants were randomly assigned to receive capsules of either LGG or matching, identically appearing placebo. Participants were enrolled by the study investigators (PH and LD).


The study participants received either LGG (gelatin capsule containing 1 × 1010 LGG organisms and 295 mg Inulin) or matching, identically appearing placebo (gelatin capsule containing 355 mg Inulin). Capsules were administered twice daily orally for 28 days. The first study capsule was administered under observation, immediately after LAIV administration. Subjects received a 28-day supply of study capsules and were instructed to bring their remaining capsules to all study visits.


All study participants, physicians, nurses and clinical staff were blinded to study assignments. By using the randomization scheme detailed above, it was impossible for research personal to adjust randomization, or determine which groups the participants were assigned to. Because the appearance and inactive substances in the placebo and study treatment were identical, neither the subjects nor researchers knew the identity of the subject's study assignment. All of the above measures led to successful allocation concealment and blinding.


Subjects were given diary cards and asked to record any symptoms that occurred during the 28 days when they took study capsules. Patients were assessed for adverse events at day 14, day 38 and day 56 study visits. At each study visit, subjects were questioned about specific side effects, possibly related to receiving LAIV or probiotics, and symptoms consistent with influenza-like illnesses. All adverse events elicited during the study visits or recorded on the diary cards were graded as mild, moderate or serious. An independent Data Safety Monitoring Board (DSMB) was appointed to monitor this study. The DSMB was in charge of monitoring of subject's safety and overall conduct and quality of study data, and it did not monitor vaccine efficacy because antibody titers were batched and run at the end of the study. The DSMB met before the beginning of subject recruitment and then yearly until the study was completed. In addition, the DSMB reviewed all serious adverse events within 72 h of being reported.

Outcome measures

Serum samples were obtained before administration of LAIV at the baseline visit, at day 14, day 28 and day 56 visits. Sera were stored at −80 °C and sent on dry ice to the Laboratory for Specialized Clinical Studies at Cincinnati Children's Hospital Medical center (Cincinnati, OH, USA). The laboratory was blinded to subject identifiers or treatment group. Antibody titers to each of the vaccine components was measured by the serum hemagglutinin inhibition (HAI) assay using standard methods (Lu et al., 2009). The primary outcome was a protective HAI titer 40 at day 28. Secondary outcomes were geometric mean titers at days 28 and 56, HAI titers after 56 days and seroconversion at day 56. Titers equal to or greater than 1:40 were considered protective. Seroconversion was defined as an increase from <1:40 to 1:40 or at least a fourfold rise in HAI antibody titers at any time after vaccination.

Statistical analysis

As our study is the first to evaluate LGG as an immune adjuvant to LAIV, our sample size of 52 subjects (26 per group) had 80% power to detect a large effect size of 0.8 or greater, using a two-group t-test with a 0.05 two-sided significance level (nQuery Advisor 5.0, Statistical Solutions, Boston, MA, USA). We also based our sample size estimates on data and sample sizes used in other studies of probiotics administered as immune adjuvants (Fang et al., 2000), and on similar studies evaluating the immunogenicity of adjuvants for LAIV and other immune adjuvants including MF59 and CPG7909 (Frey et al., 2003; Muszkat et al., 2003; Cooper et al., 2004). No adjustment was made for multiple comparisons of response to the three vaccine strains. Analyses were completed on a per-protocol basis for this proof-of-concept study. The proportion of patients with protective antibody response and who seroconverted for each vaccine strain were compared using a Fisher's exact test. Serum geometric mean antibody titers pre-vaccination and post-vaccination at days 28 and 56 of each vaccine strain were compared using a two-group t-test. The proportion of subjects who reported adverse events was also evaluated. All statistical analyses were carried out using SPSS (version 17.0, Chicago, IL, USA).


Recruitment and enrollment

A total of 86 subjects were screened between 26 September 2007 and 28 March 2008 (Figure 1). Overall, 44 subjects were excluded: 11 declined to participate and 33 did not meet the inclusion criteria. Of the 42 subjects eligible, 42 attended the baseline visit, received LAIV and were randomized to LGG or placebo. We terminated enrollment after 42 subjects because we reached the end of the influenza season and there was no benefit in continued administration of LAIV. Only 3 of the 42 (7%) were lost to follow-up, all immediately after the baseline visit. Each of these subjects received three phone calls and written notification, but did not respond to repeated attempts to determine why they dropped out from the study.

Figure 1

Participant flow diagram.

Baseline demographics and adverse events

Age, gender and race were similar between groups (Table 1). Approximately 52% of the LGG group and 38% of the placebo group had previously received TIV (P=0.54). No subject had ever received the LAIV vaccine before. In all, 17 subjects in the placebo group (85%) and 14 subjects in the LGG group (74%) reported at least one adverse event at any time during the study (Table 2). All were rated as mild. During the course of the study, more subjects in the placebo than LGG group reported muscle aches (P=0.02) and decreased appetite (P=0.047). One placebo recipient reported a serious adverse event (hospitalization for a sinus infection) occurred on day 58. This event was reviewed by the study investigators and the DSMB, and considered unrelated to administration of study drug (determination was made before knowing that the study drug was placebo) or receipt of vaccine.

Table 1 Demographic characteristic of study subjects
Table 2 Characteristics of adverse events in the LGG and placebo groups, and immune response to immunization with live-attenuated influenza vaccine


Antibody titers are presented for the 39 subjects who completed the study (per protocol analysis) (Table 3). Overall, 15% seroconverted for the H1N1 strain, 45% for the H3N2 strain and 41% for the B strain. There was no difference in seroconversion rates between treatment and placebo groups from baseline for the H1N1 and B strains. There was a significant increase in seroprotection in the LGG group vs placebo for the A/Wisconsin/67/2005 (H3N2) vaccine strain on day 28 (84 vs 55%, P=0.048). However, at day 56 the rates of seroconversion were not statistically significant (63% in the LGG vs 33% in the placebo groups, P=0.36).

Table 3 Immune response to immunization with live-attenuated influenza vaccine in the LGG vs placebo groups


Influenza remains a major cause of morbidity and mortality in the United States, resulting in approximately 19 000–36 000 deaths a year, and 200 000 excess hospitalizations (Fiore et al., 2010). Influenza vaccination is the primary means of preventing influenza infection. Two types of vaccine are available—inactivated TIV and LAIV. The Advisory Committee on Immunization Practices currently recommends annual universal influenza vaccination of all adults and children above the age of 6 months (Fiore et al., 2010). Although young children and the elderly suffer the greatest morbidity and mortality, influenza vaccination in healthy adults reduces both direct medical costs, such as physician visits and antibiotics uses and indirect costs such as work absenteeism (Nichol et al., 1999; Wilde et al., 1999; Fiore et al., 2010). The efficacy of influenza vaccine varies from year to year depending on the match between vaccine subtypes and circulating viral strains, patient's age and preexisting immunity. In years with a good match between vaccine and circulating virus, the efficacy of vaccine in healthy adults ranges between 80 and 90% for TIV (Wilde et al., 1999; Bridges et al., 2000; Jefferson et al., 2010). Studies of LAIV in healthy adults have demonstrated a wide range of variability, with rates of clinical efficacy ranging between 40 and 80% (Nichol et al., 1999; Ohmit et al., 2006; Monto et al., 2009). A number of alternatives to improve the efficacy of both LAIV and TIV have been explored in recent years, including increasing to the dose of antibody in the vaccine (Centers for Disease Control, Prevention, 2010), using alternative routes of administration such as intradermal injection (Holland et al., 2008) and using immune adjuvants. Currently there is no Food and Drug Administration-approved immune adjuvant for the influenza vaccine in the United States.

A number of studies in animals and humans have demonstrated the potential of probiotics to function as immune adjuvants. Lactobacillus species and other probiotics stimulate both the humoral and innate immune systems (MacDonald and Bell, 2010). Healthy volunteers who received LGG before oral Salmonella typhi vaccine, developed higher immunoglobulin A antibodies to the vaccine than those who received placebo (Link-Amster et al., 1994; Fang et al., 2000). Infants receiving LGG in conjunction with live, oral rotavirus vaccine had higher rates of Ig A seroconversion and higher numbers of rotavirus-specific Ig M-secreting cells than infants receiving placebo (Isolauri et al., 1995). Similarly, administration of LGG, 1 week before an oral polio booster, was associated with increased poliovirus-neutralizing antibody titers and poliovirus-specific IgA and IgG (De Vrese et al., 2005). A recently published trial in newborns demonstrated that supplementation with a probiotic formula, containing Bifidobacterium longum and Lactobacillus rhamnosus LPR, was associated with a trend toward higher hepatitis B titers in infants immunized at birth (Soh et al., 2010).

Studies in humans and animal support the role of Lactobacillus in the prevention of influenza infection. In a mouse model of influenza infection with an H1N1 strain, 3 days of intranasal exposure to LGG was significantly associated with a lower frequency of accumulated symptoms and a higher survival rate than in control mice (Harata et al., 2010). Using the same mouse model of influenza infection, oral administration of LGG or Lactobacillus TMC0356 for 19 days was associated with lower clinical symptom scores and pulmonary virus titers as compared with control mice (Kawase et al., 2010). In studies of children in day-care centers, administration of LGG was associated with a decrease in upper respiratory infections and duration of infections (Hatakka et al., 2001; Hojsak et al., 2010). Probiotics have been investigated as immune adjuvants for TIV. In a placebo-controlled trial using a supplement containing Lactobacillus paracasei, elderly patients receiving the influenza and pneumococcal vaccines had an increase in the innate immune response and a decreased number of infections when compared with placebo (Bunout et al., 2004). In another randomized placebo-controlled trial, elderly patients who received a yogurt drink containing L. casei had higher influenza-specific antibody titers increased after vaccination (Boge et al., 2009). In a trial of healthy adults, L. fermentum (CECT5716) improved influenza vaccine immunogenicity (Olivares et al., 2007). Taken together, these studies highlight the interaction between Lactobacillus and the respiratory mucosal immune system and the potential of LGG as an immune adjuvant with mucosally administered LAIV.

Our study is the first randomized placebo-controlled study to examine the effect of LGG on immune response to LAIV in normal healthy adults. The strengths of our study include the 93% completion rate, use of standardized end points and completion during a single influenza season. Although subjects receiving probiotics did not achieve greater seroprotection after administration of LAIV for the H1N1 and B strains, there was a significant improvement in those subjects receiving LGG for the H3N2 strain. The seroconversion achieved for any of the vaccine strains using LAIV was sub-optimal and was particularly low (17%) for the H1N1 component that was new for the 2007–2008 season. Our sub-optimal seroconversion rates are similar to those of De Villiers et al. (2009), who published the results of a large Phase III study of LAIV in the elderly. Overall seroconversion for all subjects receiving LAIV was 33.9% for the H3N2 strains, and seroconversion rates and efficacy against infection with B strains were also low.

The study drug and LAIV were well tolerated in our study subjects. Although there were significantly more myalgias and decreased appetite in the placebo group, adverse events were similar to those observed in other LAIV studies (De Villiers et al., 2009; Ohmit et al., 2009). One serious adverse event occurred in one placebo recipient who was hospitalized for a sinus infection on study day 58. Before this visit, the subject had not reported any sinus or upper respiratory symptoms.

Our study had several limitations. The sample size was small for this proof-of-concept study, and we had insufficient power to detect small and moderate effects on vaccine responsiveness. Subjects previously vaccinated with TIV might have lower antibody responses to subsequent LAIV (Sasaki et al., 2008), and this may have affected the immune responses in the 49% of our subjects who had previously received TIV. We used traditional HAI assay to evaluate antibody titers. Several studies of pandemic H1N1 influenza in 2009 suggested that use of microneutralization assays or virus-free Enzyme-linked immunosorbent assay methods might be more accurate in measuring antibody responses (Clark et al., 2009; Alvarez et al., 2010).

Several important questions remain for future studies. LAIV is administered intranasally and it might be more relevant to measure the local mucosal response to evaluate the role of immune adjuvants targeting the mucosal surface. It might also be necessary to administer probiotics before LAIV rather than concurrently (Boge et al., 2009). In addition, a more profound immune adjuvant effect might be demonstrated in groups, which traditionally have a poor response to the influenza vaccine such as the elderly. The data generated from this study will be of critical importance to power future studies, investigating the use of probiotics as immune adjuvants for mucosal vaccines.


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We gratefully acknowledge Irina Andreyeva, Leah Morey, Katie Jors and Dr Ambarish Athavale for their assistance in patient recruitment and study visits. This study was supported by NIH grant M01RR00054 and from Amerifit Brands, Cromwell, CT, USA. Please note that the Laboratory for Specialized Clinical Studies, located at the Cincinnati Children's Hospital Medical Center, was contracted to run the HAI assays on the sera. The lab was blinded to the treatment and vaccine given to study subjects. This lab does contract work with a number of sponsors, but has no conflict of interest and received no financial gain because of the results of this study.

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Davidson, L., Fiorino, AM., Snydman, D. et al. Lactobacillus GG as an immune adjuvant for live-attenuated influenza vaccine in healthy adults: a randomized double-blind placebo-controlled trial. Eur J Clin Nutr 65, 501–507 (2011).

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  • live-attenuated influenza vaccine
  • antibody responses
  • probiotics
  • lactobacillus GG

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