Safety and immunogenicity of a 2009 influenza A (H1N1) vaccine in hemodialysis patients

Outcomes of vaccinations against respiratory diseases in patients with end-stage renal disease undergoing hemodialysis: A systematic review and meta-analysis

Due to the nature of the disease, end-stage renal disease (ESRD) patients suffer from dysfunction of the adaptive immune system, which leads to a poorer response to vaccination. Accordingly, it is crucial to evaluate the efficacy and safety of management strategies, including vaccinations, which could potentially reduce the risk of respiratory diseases, such as pneumonia, influenza, or COVID-19, and its associated outcomes. We searched PubMed, CENTRAL, ScienceDirect, Scopus, ProQuest, and Google Scholar databases using designated MeSH keywords. The risk of bias was assessed using ROBINS-I. The quality of evidence was assessed using the GRADE (Grading of Recommendations, Assessment, Development, and Evaluation) approach. Relative risk (RR) and 95% confidence interval (CI) were calculated. Heterogeneity was investigated using forest plots and I2 statistics. This systematic review included a total of 48 studies, with 13 studies of influenza (H1N1 and H3N2) vaccination and 35 studies of COVID-19 vaccination. H1N1 vaccination in ESRD patients undergoing hemodialysis induced lower seroconversion rates (RR 0.62, 95% CI: 0.56-0.68, p <0.00001) and lower seroprotection rates (RR 0.76, 95% CI: 0.70-0.83, p <0.00001) compared to controls. H3N2 vaccination in ESRD patients undergoing hemodialysis yielded lower seroconversion rates (RR 0.76, 95% CI: 0.68-0.85, p <0.00001) and lower seroprotection rates (RR 0.84, 95% CI: 0.77-0.90, p <0.00001) compared to controls. Twenty-nine studies demonstrate significantly lower antibody levels in ESRD patients undergoing hemodialysis compared to the controls following COVID-19 vaccination. This review presents evidence of lower seroconversion and seroprotection rates after vaccination against viral respiratory diseases in patients with ESRD undergoing hemodialysis. Since hemodialysis patients are more susceptible to infection and severe disease progression, a weakened yet substantial serological response can be considered adequate to recommend vaccination against respiratory diseases in this population. Vaccination dose, schedule, or strategy adjustments should be considered in stable ESRD patients on maintenance hemodialysis. Trial registration: Systematic review registration: https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42021255983, identifier: CRD42021255983.

Safety and efficacy of coronavirus disease-19 vaccines in chronic kidney disease patients under maintenance hemodialysis: A systematic review

Background and aims

Patients on maintenance dialysis are a high-risk, immune-compromised population with 15%-25% coronavirus disease (COVID-19) mortality rate that has been underrepresented in COVID-19 vaccination clinical trials. The aim of study was to review of those studies to determine the safety and efficacy of the COVID-19 vaccination in chronic kidney disease (CKD) patients receiving maintenance hemodialysis systematically.

Methods

The effectiveness was assessed by looking at the humoral and cellular responses. The humoral response is defined as de novo IgG- or IgA-anti-SpikeS1 antibody positivity. The establishment of de novo T-cell immunity after immunization was used to measure cellular response. Adverse results were also reported of the included studies to analyze the safety of COVID-19 vaccines. Eight previous works were included in our study.

Results

Two doses of COVID-19 vaccines were shown to be effective with seroconversion rate of humoral response ranging from 81% to 97% among eight studies. The T-cell response was shown 67% and 100% in two studies. COVID-19 vaccines did not have notable adverse events and hence can be considered safe.

Conclusion

Although a single dosage has not shown to improve humoral immune response in most hemodialysis trials, a double dose has been reported to improve seroconversion rate and humoral immune response. Further research are required to observe if hemodialysis patients generate effective T-cell responses.

Antibody and T Cell Response to SARS-CoV-2 Messenger RNA BNT162b2 Vaccine in Kidney Transplant Recipients and Hemodialysis Patients

BACKGROUND

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is associated with a high rate of mortality in patients with ESKD, and vaccination is hoped to prevent infection.

METHODS

Between January 18 and February 24, 2021, 225 kidney transplant recipients (KTRs) and 45 patients on hemodialysis (HDPs) received two injections of mRNA BNT162b2 vaccine. The postvaccinal humoral and cellular response was explored in the first 45 KTRs and ten HDPs.

RESULTS

After the second dose, eight HDPs (88.9%) and eight KTRs (17.8%) developed antispike SARS-CoV-2 antibodies (P<0.001). Median titers of antibodies in responders were 1052 AU/ml (IQR, 515-2689) in HDPs and 671 AU/ml (IQR, 172-1523) in KTRs (P=0.40). Nine HDPs (100%) and 26 KTRs (57.8%) showed a specific T cell response (P=0.06) after the second injection. In responders, median numbers of spike-reactive T cells were 305 SFCs per 106 CD3+ T cells (IQR, 95-947) in HDPs and 212 SFCs per 106 CD3+ T cells (IQR, 61-330) in KTRs (P=0.40). In KTRs, the immune response to BNT162b2 seemed influenced by the immunosuppressive regimen, particularly tacrolimus or belatacept.

CONCLUSION

Immunization with BNT162b2 seems more efficient in HDPs, indicating that vaccination should be highly recommended in these patients awaiting a transplant. However, the current vaccinal strategy for KTRs may not provide effective protection against COVID-19 and will likely need to be improved.

The Safety and Immunogenicity of the mRNA-BNT162b2 SARS-CoV-2 Vaccine in Hemodialysis Patients

Background

Hemodialysis patients are at high risk for severe COVID-19. SARS-CoV-2 vaccination related safety and immunogenicity data in these patients are rare.

Methods

In this observational study SARS-CoV-2-seronegative hemodialysis patients were vaccinated with two doses of the Pfizer/BioNTech mRNA-BNT162b2 vaccine (COMIRNATY^® 30 µg) and followed for 90 days. Local and systemic side effects were assessed at every dialysis session during the first post-vaccination week after the first and second vaccine dose. Immunogenicity was determined four weeks after vaccination by quantifying anti-SARS-CoV-2 spike protein IgG antibodies (LIAISON^® SARS-CoV-2-TrimericS IgG chemiluminescent immunoassay) expressed in binding activity units per milliliter (BAU/mL) adapted to the WHO International standard.

Results

Fifty patients (32% women, 68% men) with a mean (SD) age of 67.6 (14.8) years were included. Mild local reactions occurred in 38% after the first injection, and in 29.2% with mild, in 2.1% with moderate and in 2.1% with severe degree after the second injection. Systemic reactive events occurred less often, with diarrhea (4% mild, 4% moderate) and fatigue (8% mild) being the most frequent ones. After the first injection 42% of the patients developed a positive response using the assay specific cut-off value of 33.8 binding activity units per milliliter (BAU/mL) with a median (Q1, Q3) anti-SARS-CoV-2 spike IgG concentration of 20.0 (11.7, 51.0) BAU/mL. After the second injection the percentage of seropositive patients increased to 97.9% with an anti-SARS-CoV-2 spike IgG concentration of 1075 (290.8, 1735) BAU/mL. Higher age and immunosuppression were associated with lower, calcitriol treatment and prior seroconversion to hepatitis B vaccination with significantly higher antibody concentration.

Conclusions

The mRNA-BNT162b2 SARS-CoV-2 vaccine appears to be safe and well-tolerated and shows a high immunogenicity in hemodialysis patients.

Practical Guide to Vaccination in All Stages of CKD, Including Patients Treated by Dialysis or Kidney Transplantation

Infection is a major cause of morbidity and mortality in patients with chronic kidney disease (CKD), including those receiving maintenance dialysis or with a kidney transplant. Although responses to vaccines are impaired in these populations, immunizations remain an important component of preventative care due to their favorable safety profiles and the high rate of infection in these patients. Most guidelines for patients with CKD focus on the importance of the hepatitis B, influenza, and pneumococcal vaccines in addition to age-appropriate immunizations. More data are needed to determine the clinical efficacy of these immunizations and others in this population and define optimal dosing and timing for administration. Studies have suggested that there may be a benefit to immunization before the onset of dialysis or transplantation because patients with early-stage CKD generally have higher rates of seroconversion. Because nephrologists often serve as primary care physicians for patients with CKD, it is important to understand the role of vaccinations in the preventive care of this patient population.

High-Dose Seasonal Influenza Vaccine in Patients Undergoing Dialysis

BACKGROUND AND OBJECTIVES

High-dose influenza vaccine, which contains fourfold more antigen than standard dose, is associated with fewer cases of influenza and less influenza-related morbidity in the elderly general population. Whether the high-dose influenza vaccine benefits patients on dialysis, whose immune response to vaccination is less robust than that of healthy patients, is uncertain.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We compared hospitalizations and deaths during the 2015-2016 and 2016-2017 influenza seasons by vaccine type (standard trivalent, standard quadrivalent, and high-dose trivalent influenza vaccine) administered within a national dialysis organization. The association of vaccine type with outcomes was estimated using Cox proportional hazards regression with adjustment for patient factors and "center effect." Analyses were stratified by age and dialysis modality.

RESULTS

Between September 1 and December 31, 2015, standard dose trivalent, standard dose quadrivalent, and high-dose trivalent influenza vaccines were administered to 3057 (31%), 5981 (61%), and 805 (8%) patients, respectively. The adjusted rates of first hospitalizations by vaccine type during the influenza season were 8.43, 7.88, and 7.99 per 100 patient-months, respectively, and the adjusted rates of death were 1.00, 0.97, and 1.04, respectively. These differences were not significant. In 2016, 3614 (39%) received quadrivalent vaccine, and 5700 (61%) received high-dose trivalent vaccine. The adjusted rates of first hospitalization by vaccine type were 8.71 and 8.04 per 100 patient-months, respectively, and the adjusted rates of death were 0.98 and 1.02, respectively. Receipt of high dose was associated with a significant reduction in hospitalization (hazard ratio, 0.93; 95% confidence interval, 0.86 to 1.00; P=0.04); there was no significant association with death. There was no significant heterogeneity of either association by age group or dialysis modality.

CONCLUSIONS

Receipt of high-dose compared with standard dose influenza vaccine in 2016-2017 was associated with lower rates of hospitalization in patients on dialysis, although that was not seen in 2015-2016.

Clinical epidemiology of infectious disease among patients with chronic kidney disease

Infectious disease is recognized as an important complication among patients with end-stage renal disease, contributing to excess morbidity and health care costs. However, recent epidemiological studies have revealed that even mild to moderate stages of chronic kidney disease (CKD) substantially increase risk of infection. Regarding underlying mechanisms, evidence suggests various aspects of altered immune response in patients with CKD including impaired function of T cells, B cells and neutrophil. Multiple conditions surrounding CKD, such as older age, diabetes, and cardiovascular disease are important contributors in the increased susceptibility to infection in this population. In addition, several mechanisms impairing immune function have been hypothesized including accumulated uremic toxins, increased oxidative stress, endothelial dysfunction, low-grade inflammation, and mineral and bone disorders. In terms of prevention strategies, influenza and pneumococcal vaccines are most feasible and important. Nevertheless, the extent of vaccine utilization in CKD has not been well documented. In addition, antibody response to vaccination may be reduced in CKD patients, and thus a vaccine delivery strategy (e.g., dose and frequency) may need to be optimized among patients with CKD. Through this review, we demonstrate that infection is a major but underrecognized complication of CKD. As CKD is recognized as a serious public health issue, dedicated research is needed to better characterize the burden of infectious disease associated with CKD, understand the pathophysiology of infection in patients with CKD, and develop effective strategies to prevent infection and its sequela in this high risk population.

Vaccination practices in dialysis patients: A narrative review

In a period of turmoil concerning vaccination practices, there is a serious conflict between scientifically reasonable, evidence-based guidelines and the far-fetched rumors or misconceptions concerning the vaccination practices in the general population. When a significant portion of the medical and paramedical personnel may be deliberately unvaccinated against common biological agents, achieving effective vaccination rates in the dialysis population may be complicated. Vaccination rates are unacceptably low in dialysis patients and seroconversion rates are even lower; further, serological follow-up is generally poor. The particularly anergic immune system of the advanced chronic kidney disease patients is partly a cause of both high rates of infection and low rates of seroconversions. This narrative review is an effort to summarize current knowledge concerning the vaccination practices in dialysis patients with some specific recommendations based on these facts. Of particular interest is a new vaccine, the Zoster Recombinant, Adjuvanted Vaccine (Shingrix), which we will include in our discussion.

Clinical efficacy of seasonal influenza vaccination: characteristics of two outbreaks of influenza A(H1N1) in immunocompromised patients

BACKGROUND

Influenza A(H1N1) causes serious complications in immunocompromised patients. The efficacy of seasonal vaccination in these patients has been questioned.

AIM

To describe two outbreaks of influenza A(H1N1) in immunocompromised patients.

METHODS

Two outbreaks of influenza A(H1N1) occurred in our institution: on the kidney transplant ward in 2014 including patients early after kidney or simultaneous pancreas-kidney transplantation, and on the oncology ward in 2016 including patients receiving chemotherapy for malignant tumours. Factors leading to these outbreaks and the clinical efficacy of seasonal influenza vaccination were analysed.

FINDINGS

Altogether 86 patients were exposed to influenza A(H1N1) during the outbreaks, among whom the seasonal influenza vaccination status was unknown in 10. Only three out of 38 vaccinated patients were infected with influenza A(H1N1), compared with 20 out of 38 unvaccinated patients (P = 0.02). The death of one out of 38 vaccinated patients was associated with influenza, compared with seven out of 38 unvaccinated patients (P = 0.06). Shared factors behind the two outbreaks included outdated facilities not designed for the treatment of immunosuppressed patients. Vaccination coverage among patients was low, between 40% and 70% despite vaccination being offered to all patients free of charge. Vaccination coverage of healthcare workers on the transplant ward was low (46%), but, despite high coverage on the oncology ward (92%), the outbreak occurred.

CONCLUSION

Seasonal influenza vaccination was clinically effective with both a reduced risk of influenza infection and a trend towards reduced mortality in these immunocompromised patients. Several possible causes were identified behind these two outbreaks, requiring continuous awareness in healthcare professionals to prevent further outbreaks.

Outbreak of Influenza A(H1N1) in a Kidney Transplant Unit-Protective Effect of Vaccination

Seasonal influenza vaccination is recommended for patients with end-stage renal disease (ESRD), despite suggested inferior efficacy among these patients. We characterize an outbreak of influenza A(H1N1) in a kidney transplant unit. Altogether 23 patients were treated on the ward for postoperative care after kidney transplantation during the outbreak. After the first positive case, all patients were tested with nasopharyngeal swab tests and 7 patients were diagnosed with influenza A(H1N1). Altogether 17/23 patients had received adequate seasonal influenza vaccination, of whom 2/17 tested positive for influenza (one asymptomatic, one with mild cough). Five of six unvaccinated patients were diagnosed with influenza A(H1N1); 3/5 suffered from severe respiratory failure and were treated with ventilator support in the ICU, but all died due to acute respiratory distress syndrome, whereas 2/5 suffered from mild viral pneumonitis and recovered fully. The risk of influenza infection and mortality was significantly increased in unvaccinated patients (odds ratio 37.5 [95% CI 2.7-507.5, p = 0.01] and 6.7 [95% CI 2.3-18.9, p = 0.003], respectively). Influenza A(H1N1) had a high mortality in our cohort of nonvaccinated immunosuppressed patients early after kidney transplantation. None of the vaccinated patients developed serious disease, supporting the role of vaccination also for ESRD patients.

The pandemic influenza A (H1N1) 2009 vaccine does not increase the mortality rate of idiopathic interstitial pneumonia: a matched case-control study

Background

Evidence regarding the mortality rate after administration of the pandemic influenza A (H1N1) 2009 vaccine on patients with underlying diseases is currently scarce. We conducted a case-control study in Japan to compare the mortality rates of patients with idiopathic interstitial pneumonia after the vaccines were administered and were not administered.

Methods

Between October 2009 and March 2010, we collected clinical records in Japan and conducted a 1∶1 matched case-control study. Patients with idiopathic interstitial pneumonia who died during this period were considered case patients, and those who survived were considered control patients. We determined and compared the proportion of each group that received the pandemic influenza A (H1N1) 2009 vaccine and estimated the odds ratio. Finally, we conducted simulations that compensated for the shortcomings of the study associated with adjusted severity of idiopathic interstitial pneumonia.

Results

The case and control groups each comprised of 75 patients with idiopathic interstitial pneumonia. The proportion of patients who received the pandemic influenza A (H1N1) 2009 vaccine was 30.7% and 38.7% for the case and control groups, respectively. During that winter, the crude conditional odds ratio of mortality was 0.63 (95% confidence interval, 0.25–1.47) and the adjusted conditional odds ratio was 1.18 (95% confidence interval, 0.33–4.49); neither was significant. The simulation study showed more accurate conditional odds ratios of 0.63–0.71.

Conclusions

In our study, we detected no evidence that the influenza A (H1N1) 2009 vaccine increased the mortality rate of patients with idiopathic interstitial pneumonia. The results, however, are limited by the small sample size and low statistical power. A larger-scale study is required.

Effectiveness of influenza vaccine in patients on hemodialysis--a review

The influenza virus is one of the most common causes of viral respiratory tract infections. Some chronic diseases predispose to a severe course of the disease and increase the risk of complications and death. To minimize the risk of infection and complications, care of patients with increased risk should include prophylactic measures such as the administration of a seasonal influenza vaccine. An influenza vaccine is the best and cheapest method of influenza prevention. It is indicated for patients with chronic kidney disease, both during conservative treatment and renal replacement therapy.

Many studies that have assessed the efficacy of an influenza vaccine in patients on hemodialysis have found that immune deficiency predisposes these patients to infection and a severe course of the disease. Because the immune response to a standard influenza vaccine in this population is weak, the studies covered many aspects of vaccination, including the need for a booster dose.

Unlike in a healthy population, the efficacy of an influenza vaccine in patients on hemodialysis might be insufficient; however, the vaccine is still able to induce immunity in a significant number of patients. Considering the latest data and the results of studies described above, the recommendation of a seasonal influenza vaccine should be obligatory in all hemodialysis patients.

This paper is based on original articles available from Medline database. The most recent and most significant literature on the influenza vaccine in patients on hemodialysis has been reviewed.

Immunization in end-stage renal disease: opportunity to improve outcomes

Infection is the second most common cause of death in patients with end-stage renal disease (ESRD), following cardiovascular causes. Immunization is a fairly simple, but underutilized, strategy for prevention of infectious morbidity and mortality in patients with kidney failure. It is imperative for nephrologists and primary care providers to have an understanding of immunization as an essential component of preventive healthcare measures in this high-risk population. Patients with ESRD represent a unique population due to their immunosuppressed state, dialysis-related exposures and suboptimal response to routine vaccines. While the Advisory Committee on Immunization Practices (ACIP) provides guidelines for vaccination of patients with renal disease against Hepatitis B, influenza and pneumococcal disease, the data on immunization against other commonly preventable infectious diseases are lacking. This article reviews the recent evidence on immunization in the ESRD population and synthesizes the related implications for maximizing prevention of infectious diseases in this high-risk population.

Risk factors affecting seroconversion after influenza A/H1N1 vaccination in hemodialysis patients

BACKGROUND

Hemodialysis (HD) patients have multiple causes of immune dysfunction and poor immune response to influenza vaccination. We investigated the antibody response rate to a pandemic H1N1/2009 influenza vaccination and clinical parameters influencing the induction of antibody responses in HD patients.

METHODS

A total of 114 HD patients were vaccinated with a monovalent adjuvanted H1N1 inactivated influenza vaccine. Titers of neutralizing antibodies were evaluated by hemagglutination inhibition (HI) assay at pre- and 4 weeks after vaccination. Seroconversion was defined as either a pre-vaccination HI titer < 1:10 and a post vaccination HI titer > 1:40 or a pre-vaccination HI titer ≥ 1:10 and a minimum four-fold rise in post-vaccination HI antibody titer. Seventeen out of 114 HD patients (14.9%) tested positive for antibodies against influenza A/H1N1/2009 before vaccination. The remaining 97 baseline sero-negative patients were included in the analysis.

RESULTS

Only 30 (30.9%) HD patients had seroconversion 4 weeks after vaccination. The elderly patients, those over 65 years of age, showed significantly lower seroconversion rate compared to younger HD patients (20.5% vs. 39.6%, p = 0.042). Furthermore, patients with hemoglobin values less than 10 g/dL had a significantly lower seroconversion rate compared to those with higher hemoglobin values (20.0 vs. 38.6%, p = 0.049). By multivariate logistic regression analysis, only age ≥65 years (OR = 0.336, 95% confidence interval (CI) 0.116-0.971, p = 0.044) and hemoglobin levels <10 g/dL (OR = 0.315, 95% CI 0.106-0.932, p = 0.037) were independently associated with seroconversion after vaccination.

CONCLUSIONS

Our data show that HD patients, especially who are elderly with low hemoglobin levels, are at increased risk for lower seroconversion rate after influenza A/H1N1 vaccination. Further studies are needed to improve the efficacy of vaccination in these high risk patients.

TNF blockers show distinct patterns of immune response to the pandemic influenza A H1N1 vaccine in inflammatory arthritis patients

OBJECTIVE

To evaluate the immunogenicity of the anti-influenza A H1N1/2009 vaccine in RA and spondyloarthritis (SpA) patients receiving distinct classes of anti-TNF agents compared with patients receiving DMARDs and healthy controls.

METHODS

One hundred and twenty patients (RA, n = 41; AS, n = 57; PsA, n = 22) on anti-TNF agents (monoclonal, n = 94; soluble receptor, n = 26) were compared with 116 inflammatory arthritis patients under DMARDs and 117 healthy controls. Seroprotection, seroconversion (SC), geometric mean titre, factor increase in geometric mean titre and adverse events were evaluated 21 days after vaccination.

RESULTS

After immunization, SC rates (58.2% vs 74.3%, P = 0.017) were significantly lower in SpA patients receiving anti-TNF therapy, whereas no difference was observed in RA patients receiving this therapy compared with healthy controls (P = 0.067). SpA patients receiving mAbs (infliximab/adalimumab) had a significantly lower SC rate compared with healthy controls (51.6% vs 74.3%, P = 0.002) or those on DMARDs (51.6% vs 74.7%, P = 0.005), whereas no difference was observed for patients on etanercept (86.7% vs 74.3%, P = 0.091). Further analysis of non-seroconverting and seroconverting SpA patients revealed that the former group had a higher mean age (P = 0.003), a higher frequency of anti-TNF (P = 0.031) and mAbs (P = 0.001) and a lower frequency of MTX (P = 0.028). In multivariate logistic regression, only older age (P = 0.015) and mAb treatment (P = 0.023) remained significant factors for non-SC in SpA patients.

CONCLUSION

This study revealed a distinct disease pattern of immune response to the pandemic influenza vaccine in inflammatory arthritis patients receiving anti-TNF agents, illustrated by a reduced immunogenicity solely in SpA patients using mAbs.

TRIAL REGISTRATION

ClinicalTrials.gov, www.clinicaltrials.gov, NCT01151644.

Immune response to 2009 H1N1 vaccine in HIV-infected adults in Northern Thailand

BACKGROUND

In late 2009, the Thai Ministry of Public Health provided two million doses of the monovalent pandemic influenza H1N1 2009 vaccine (Panenza®, Sanofi Pasteur), which was the only vaccine formulation available in Thailand, to persons at risk of more severe manifestations of the disease including HIV infection. Several studies have shown poorer immune responses to the 2009 H1N1 vaccines in HIV-infected individuals. There are limited data in this population in resource-limited countries.

RESULTS

At day 28 post-vaccination, seroconversion was found in 32.0% (95%CI 24.5 - 40.2) of the HIV-infected group and 35.0% (95%CI 15.4- 59.2) of the healthy controls (p = 0.79). Seroprotection rate was observed in 33.3% (95%CI 25.8-41.6) and 35.0% (95%CI 15.4-59.2) of the HIV-infected group and the control group, respectively (p = 0.88). Among HIV-infected participants, the strongest factor associated with vaccine response was age 42 y or younger (p = 0.05).

METHODS

We evaluated the immunogenicity of a single, 15µg/0.5ml dose of a monovalent, non-adjuvanted 2009 H1N1 vaccine in 150 HIV-infected Thai adults and 20 healthy controls. Immunogenicity was measured by hemagglutination inhibition assay (HI) at baseline and 28 d after vaccination. Seroconversion was defined as 1) pre-vaccination HI titer < 1:10 and post-vaccination HI titer ≥ 1:40, or 2) pre-vaccination HI titer ≥ 1:10 and a minimum of 4-fold rise in post-vaccination HI titer. Seroprotection was defined as a post-vaccination HI titer of ≥ 1:40.

CONCLUSIONS

A low seroconversion rate to the 2009 H1N1 vaccine in both study groups, corresponding with data from trials in the region, may suggest that the vaccine used in our study is not very immunogenic. Further studies on different vaccines, dosing, adjuvants, or schedule strategies may be needed to achieve effective immunization in HIV-infected population.