One dose of an MF59-adjuvanted pandemic A/H1N1 vaccine recruits pre-existing immune memory and induces the rapid rise of neutralizing antibodies

Protective antibody responses to a single dose of 2009 pandemic vaccines have been observed in the majority of healthy subjects aged more than 3 years. These findings suggest that immune memory lymphocytes primed by previous exposure to seasonal influenza antigens are recruited in the response to A/H1N1 pandemic vaccines and allow rapid seroconversion. However, a clear dissection of the immune memory components favoring a fast response to pandemic vaccination is still lacking. Here we report the results from a clinical study where antibody, CD4+ T cell, plasmablast and memory B cell responses to one dose of an MF59-adjuvanted A/H1N1 pandemic vaccine were analyzed in healthy adults. While confirming the rapid appearance of antibodies neutralizing the A/H1N1 pandemic virus, we show here that the response is dominated by IgG-switched antibodies already in the first week after vaccination. In addition, we found that vaccination induces the rapid expansion of pre-existing CD4+ T cells and IgG-memory B lymphocytes cross-reactive to seasonal and pandemic A/H1N1 antigens. These data shed light on the different components of the immune response to the 2009 H1N1 pandemic influenza vaccination and may have implications in the design of vaccination strategies against future influenza pandemics.     

重型甲型H1N1流感患者外周血免疫细胞变化及其临床意义

目的 通过动态观察重型甲型H1N1流感(重症及危重症)患者外周血淋巴细胞的数量变化,探讨患者的免疫病理机制及淋巴细胞亚群检测在甲型H1N1流感诊断中的临床意义.方法 采用流式细胞技术检测2009年10-12月在我院住院治疗的41例重症、危重症甲型H1N1流感患者和同期41例轻症患者的外周血T、B和NK细胞数量.结果 在发病初期,重症及危重症患者外周血总T细胞、CD8+T细胞、NK细胞较轻症甲型H1N1流感患者显著降低(P＜0.01),CD4+T细胞较轻症患者降低(P＜0.05);危重症患者总T细胞、CD4+T细胞、CD8+T细胞、NK细胞低于重症患者(P＜0.01).恢复期重症及危重症甲型H1N1流感患者总T细胞、CD8+T细胞低于轻症患者(P＜0.05),NK细胞低于轻症患者(P＜0.01),B细胞高于轻症患者(P＜0.05).结论 重症及危重症甲型H1N1流感患者存在明显的细胞免疫损伤.     

Vaccination for 2009 pandemic H1N1 influenza A did not induce conserved epitope-specific memory CD8 T cell responses in HIV+ northern Thai children

The influenza virus causes severe illness in susceptible populations, including children and people living with human immunodeficiency virus (HIV). Here, we investigated cell-mediated immune responses (CMI) against influenza CD8 T cell conserved epitopes in HIV-infected (HIV+) northern Thai children following the 2009 pandemic H1N1 influenza A vaccination. Sixty HIV+ children were vaccinated with two doses of the 2009 pandemic influenza vaccine and their CD8 T cell responses were assessed. We found no significant differences in the increase of cytokines-producing and CD107a-expressing CD8+ T cells or CD8+ memory T cells in response to pooled conserved epitopes stimulation in vitro between children with different serologic responses to the vaccine at all time points of the study. Our results suggest that the 2009 pandemic H1N1 vaccine did not induce the conserved epitope-specific immune responses in HIV+ children. Vaccine design and vaccination strategy against influenza in these populations warrant further studies.     

HIV/AIDS人群甲型H1N1流感感染状况血清学研究及影响因素探讨

目的了解HIV/AIDS人群甲型H1N1感染状况及影响因素,为HIV/AIDS人群甲型H1N1防控策略提供科学依据。方法于2010年7-10月,在签署知情同意书后,随机抽取262名龙岗区CDC红丝带之家的HIV/AIDS随访人群(15～49岁)进行问卷调查,同时抽血检测甲型H1N1流感抗体和CD4+T细胞,采用χ2检验及Logistic回归分析方法对HIV/AIDS患者甲型H1N1流感的感染状况及影响因素进行单、多因素的分析。结果在调查的262例HIV/AIDS中,甲型H1N1流感抗体阳性(抗体滴度≥1:40)22例,抗体阳性率为8.4%,几何平均滴度(GMT)为7.6。卡方检验结果提示,HIV/AIDS患者的性别、户籍、婚姻、文化程度、工作状况和生活习惯、有无发热咳嗽咽痛病史、有否接种甲型H1N1流感疫苗对感染甲型H1N1流感无影响(P>0.05),而多因素Logistic回归分析显示,年龄越高(OR=4.11)、CD4+T细胞越低(OR=2.579)的HIV/AIDS感染甲型H1N1流感的可能性就越大。结论 HIV/AIDS人群甲型H1N1流感抗体水平较低,是甲型H1N1流感的高危人群,在甲型H1N1流感暴发流行时须重点防控和保护;而HIV/AIDS的年龄和CD4+T细胞水平是感染甲型H1N1流感的危险因素。     

CD8+ T cell immunity to 2009 pandemic and seasonal H1N1 influenza viruses

A novel strain of H1N1 influenza A virus (pH1N1) emerged in 2009, causing a worldwide pandemic. Several studies suggest that this virus is antigenically more closely related to human influenza viruses that circulated prior to 1957 than viruses of more recent seasonal influenza varieties. The extent to which individuals who are naïve to the 2009 pH1N1 virus carry cross-reactive CD8+ T cells is not known, but a certain degree of reactivity would be expected since there is substantial conservation among the internal proteins of the virus. In the present study, we examined the production of multiple cytokines in response to virus from CD8+ T cells in healthy adult subjects, between 18 and 50 years of age (born post 1957), who had no evidence of exposure to the 2009 pH1N1 virus, and had blood collected prior to the emergence of the pandemic in April of 2009. Human peripheral blood mononuclear cells (PBMCs) were stimulated in vitro with a panel of live viruses, and assayed by intracellular cytokine staining and flow cytometry. Although results were variable, most subjects exhibited cytokine positive CD8+ T cells in response to pH1N1. Cytokine producing cells were predominantly single positive (IL2, IFNγ, or TNFα); triple-cytokine producing cells were relatively rare. This result suggests that although many adults carry cross-reactive T cells against the emergent pandemic virus, these cells are in a functionally limited state, possibly because these subjects have not had recent exposure to either seasonal or pandemic influenza strains.     

Immunoinformatic comparison of T-cell epitopes contained in novel swine-origin influenza A (H1N1) virus with epitopes in 2008–2009 conventional influenza vaccine

In March 2009 a novel swine-origin influenza A (H1N1) virus (S-OIV) emerged in Mexico and the Western United States. Vaccination with conventional influenza vaccine (CIV) does not result in cross-reactive antibodies, however, the disproportionate number of cases (37%) occurring among persons younger than 50 years old suggested that adaptive immune memory might be responsible for the relative lack of virulence in older, healthy adults. Using EpiMatrix, a T-cell epitope prediction and comparison tool, we compared the sequences of the three hemagglutinin (HA) and neuraminidase (NA) proteins contained in 2008–2009 CIV to their counterparts in A/California/04/2009 (H1N1) looking for cross-conserved T-cell epitope sequences. We found greater than 50% conservation of T helper and CTL epitopes between novel S-OIV and CIV HA for selected HLA. Conservation was lower among NA epitopes. Sixteen promiscuous helper T-cell epitopes are contained in the S-OIV H1N1 HA sequence, of which nine (56%) were 100% conserved in the 2008–2009 influenza vaccine strain; 81% were either identical or had one conservative amino acid substitution. Fifty percent of predicted CTL epitopes found in S-OIV H1N1 HA were also found in CIV HA sequences. Based on historical performance, we expect these epitope predictions to be 93–99% accurate. This in silico analysis supports the proposition that T-cell response to cross-reactive T-cell epitopes, due to vaccination or exposure, may have the capacity to attenuate the course of S-OIV H1N1 induced disease—in the absence of cross-reactive antibody response. The value of the CIV or live-attenuated influenza vaccine containing the 2008–2009 vaccine strains, as defense against H1N1, could be further tested by evaluating human immune responses to the conserved T-cell epitopes using PBMC from individuals infected with H1N1 and from CIV vaccinees.     

一起甲型H1N1和H3N2流感病毒混合感染调查研究

目的 从病例临床特征、抗体变化规律及病原学检测等多角度分析一起由甲型H1N1和H3N2流感病毒混合感染引起暴发疫情的流行特征,为有效控制疾病的发生与蔓延提供科学依据.方法 采用回顾性调查和现场流行病学调查方法进行问卷调查,利用RT-PCR和血凝抑制试验分别进行流感病毒和血清抗体检测.结果 45例流感样病例中,确诊40例,其中22例为甲型H1N1流感、12例为季节性H3N2型流感、6例为H1N1和H3N2病毒混合感染.不同病毒感染病例的临床表现差异无统计学意义.序列比对分析显示,混合病毒感染和单一病毒感染者的甲型H1N1和季节性H3N2病毒基因序列没有差异.同时分析表明,所有流感病例的病毒株均对金刚烷胺耐药,对奥司他韦(达菲)有效.患者的双份血清血凝抑制试验检测表明,暴发由甲型H1N1和H3N2流感病毒混合感染引起.结论 这是一起由甲型H1N1和H3N2流感病毒混合感染引起的疫情,甲型H1N1病毒在传播过程中可能较H3N2更具有优势.     

Assessing the impact of confounding (measured and unmeasured) in a case-control study to examine the increased risk of pandemic A/H1N1 associated with receipt of the 2008-9 seasonal influenza vaccine

Background

This study examines the role of measured and unmeasured confounding in the relationship between the 2008-9 seasonal influenza vaccine and pandemic H1N1 (pH1N1) influenza virus.

Methods

Data were taken from a test-negative case-control study of 462 lab confirmed pandemic A/H1N1 (pH1N1) cases and 484 test-negative controls. Adjusted odds ratios (OR) and 95% confidence intervals (CI) were derived using multivariate logistic regression. The analysis was repeated using propensity matching. A sensitivity analysis was conducted to quantify the impact of a hypothetical unmeasured confounder.

Results

Cases were more likely to have received the seasonal influenza vaccine after adjusting for multiple confounders using multivariate regression (OR 1.82, 95% CI: 1.25-2.65), using propensity matching (OR 1.86, 95% CI: 1.19-2.92) and in subsequent sensitivity analyses. An unmeasured confounder would need a prevalence of 20%, an odds ratio with the vaccine and pH1N1 of ≥3.5 and ≥3.0 (respectively) to result in a non-significant association. Using a prevalence of 40% the respective associations were 3.0 and 2.5.

Conclusion

A significant positive association between the seasonal influenza vaccine and lab confirmed pH1N1 was observed after considering multiple confounders and using different methods for confounder adjustment. This was not likely explained by an unmeasured confounder given the prevalence and strength of association needed to result in a non-significant association.     

The 2009 A (H1N1) influenza virus pandemic: A review

In March and early April 2009 a new swine-origin influenza virus (S-OIV), A (H1N1), emerged in Mexico and the USA. The virus quickly spread worldwide through human-to-human transmission. In view of the number of countries and communities which were reporting human cases, the World Health Organization raised the influenza pandemic alert to the highest level (level 6) on June 11, 2009. The propensity of the virus to primarily affect children, young adults and pregnant women, especially those with an underlying lung or cardiac disease condition, and the substantial increase in rate of hospitalizations, prompted the efforts of the pharmaceutical industry, including new manufacturers from China, Thailand, India and South America, to develop pandemic H1N1 influenza vaccines. All currently registered vaccines were tested for safety and immunogenicity in clinical trials on human volunteers. All were found to be safe and to elicit potentially protective antibody responses after the administration of a single dose of vaccine, including split inactivated vaccines with or without adjuvant, whole-virion vaccines and live-attenuated vaccines. The need for an increased surveillance of influenza virus circulation in swine is outlined.     

Immunogenicity and safety of inactivated monovalent 2009 H1N1 influenza A vaccine in immunocompromised children and young adults

Background

Influenza vaccination is recommended for immunocompromised patients.

Methods

Children (6 months to 21 years) with cancer, HIV infection, or sickle cell disease (SCD) received 1 or 2 doses of pandemic 2009 H1N1 monovalent influenza vaccine (H1N1 MIV). Safety and tolerability, hemagglutination inhibition (HI) and microneutralization (MN) antibody titers were measured against 2009 H1N1 influenza A virus after each dose. Seroprotection (SP) and seroconversion (SC) rates were determined.

Results

103 participants were enrolled and 99 were evaluable (39 with HIV, 37 with cancer and 23 with SCD). Mean age (±SD) was 7.9 (±5.4) years for cancer participants, 18.0 (±3.5) for HIV, and 13.3 (±4.2) for SCD. 54% were males; 65% black; and 96% had received seasonal influenza vaccine. HIV-infected participants had a median CD4 count of 625 cells/mm³ (range, 140-1260). 46% had an undetectable HIV viral load and 41% were perinatally infected. No participant had vaccine-related serious adverse events. None developed influenza A proven illness during the 6 months after the vaccine. Local injection reactions were reported in 29% and systemic reactions in 42% after the first dose of vaccine. SC and SP were achieved after the last dose in 48% and 52%, respectively, of participants with leukemia or lymphoma, 50% and 75% of participants with solid tumors, 63% and 92% of HIV-infected participants, and 74% and 100% of participants with SCD.

Conclusion

H1N1 MIV was safe and well tolerated. H1N1 MIV resulted in an adequate immune response in children with SCD. It was only modestly immunogenic in cancer or HIV participants.     

The novel adjuvant CoVaccineHT™ increases the immunogenicity of cell-culture derived influenza A/H5N1 vaccine and induces the maturation of murine and human dendritic cells in vitro

A candidate influenza H5N1 vaccine based on cell-culture-derived whole inactivated virus and the novel adjuvant CoVaccineHT™ was evaluated in vitro and in vivo. To this end, mice were vaccinated with the whole inactivated influenza A/H5N1 virus vaccine with and without CoVaccineHT™ and virus-specific antibody and cellular immune responses were assessed. The addition of CoVaccineHT™ increased virus specific primary and secondary antibody responses against the homologous and an antigenically distinct heterologous influenza A/H5N1 strain. The superior antibody responses induced with the CoVaccineHT™-adjuvanted vaccine correlated with the magnitude of the virus-specific CD4+ T helper cell responses. CoVaccineHT™ did not have an effect on the magnitude of the CD8+ T cell response. In vitro, CoVaccineHT™ upregulated the expression of co-stimulatory molecules both on mouse and human dendritic cells and induced the secretion of pro-inflammatory cytokines TNF-α, IL-6, IL-1β and IL-12p70 in mouse- and IL-6 in human dendritic cells. Inhibition experiments indicated that the effect of CoVaccineHT™ is mediated through TLR4 signaling. These data suggest that CoVaccineHT™ also will increase the immunogenicity of an influenza A/H5N1 vaccine in humans.     

Protection of trivalent inactivated influenza vaccine against hospitalizations among pandemic influenza A (H1N1) cases in Argentina

The aim of this study was to estimate the effectiveness of 2009 seasonal trivalent inactivated vaccine in reducing hospitalizations due to the novel influenza A H1N1 virus among positive cases. Data collected from Argentina's national epidemiological surveillance system were analyzed. All patients had a clinical diagnosis and underwent positive serological tests for pandemic influenza A H1N1. Logistic regression was used to estimate vaccine effectiveness to prevent severe cases of the disease, measured as hospitalizations. The adjusted effectiveness of the vaccine was 50% (95% CI: 40–59%). Vaccination was significantly associated with hospitalizations in all age groups, and within groups that had and had not received antiviral treatment. These results suggest that seasonal influenza vaccine might have conferred partial protection against severe cases due to the novel pandemic influenza.     

AF03-adjuvanted and non-adjuvanted pandemic influenza A (H1N1) 2009 vaccines induce strong antibody responses in seasonal influenza vaccine-primed and unprimed mice

Pandemic influenza vaccines have been manufactured using the A/California/07/2009 (H1N1) strain as recommended by the World Health Organization. We evaluated in mice the immunogenicity of pandemic (H1N1) 2009 vaccine and the impact of prior vaccination against seasonal trivalent influenza vaccines (TIV) on antibody responses against pandemic (H1N1) 2009. In naïve mice, a single dose of unadjuvanted H1N1 vaccine (3 μg of HA) was shown to elicit hemagglutination inhibition (HI) antibody titers >40, a titer associated with protection in humans against seasonal influenza. A second vaccine dose of pandemic (H1N1) 2009 vaccine strongly increased these titers, which were consistently higher in mice previously primed with TIV than in naïve mice. At a low immunization dose (0.3 μg of HA), the AF03-adjuvanted vaccine elicited higher HI antibody titers than the corresponding unadjuvanted vaccines in both naïve and TIV-primed animals, suggesting a potential for antigen dose-sparing. These results are in accordance with the use in humans of a split-virion inactivated pandemic (H1N1) 2009 vaccine formulated with or without AF03 adjuvant to protect children and young adults against influenza A (H1N1) 2009 infection.     

广州市某区2009年甲型H1N1流感病例流行病学特征分析

[目的]了解广州市某区2009年甲型H1N1流感的流行病学特征,为防控提供依据。[方法]从中国疾病预防控制系统下载某区上报的甲型H1N1流感确诊病例的相关信息,用SPSS软件进行统计分析。[结果]2009年5月29日～12月31日,共报告315例甲型H1N1流感确诊病例,4例死亡,病死率为1.3%;病例主要分布在10～49岁的儿童和青壮年;职业以学生为主;平均每周发病10例,并出现3个发病高峰;大多数病例临床症状温和,50岁及以上年龄段病死率较高;所有死亡病例均患有基础疾病。[结论]甲型H1N1流感出现小范围聚集性疫情的危险依然存在,重点关注学生、年老体弱者、婴幼儿、孕妇等特殊人群及抵抗力差合并有其他慢性或先天性疾病者。     

甲型H1N1流感患者淋巴细胞亚群检测结果分析

目的探讨甲型H1N1流感患者淋巴细胞亚群动态变化的意义。方法检测80例甲型H1N1流感患者淋巴细胞在疾病初期和恢复期变化,以正常人群为对照。结果普通甲流患者淋巴细胞略低于正常人群,差异无统计学意义（P〉0.05）;重症甲流患者在疾病初期明显低于普通患者和正常人群,差异有统计学意义（P〈0.05）。结论重症甲型H1N1流感患者急性期体液淋巴细胞变化明显,有助于早期鉴别。     

Cold-adapted X-31 live attenuated 2009 pandemic H1N1 influenza vaccine elicits protective immune responses in mice and ferrets

The 2009 pandemic influenza H1N1 (pdmH1N1) is characterized by rapid transmission among humans and disproportionate infection to children and young adults. Although the pdmH1N1 demonstrated less lethality than initially expected and has now moved into its post-pandemic period, it remains highly possible that through antigenic shift or antigenic drift the pdmH1N1 might re-emerge in the future as a more virulent strain than before, underscoring the need for vaccination prior to an outbreak. Using X-31 ca as a backbone strain, we generated a live attenuated pdmH1N1 vaccine and evaluated its potential as a safe and effective vaccine using mouse and ferret models. Despite an acceptable level of attenuation phenotypes, single dose of immunization with the vaccine efficiently stimulated both systemic and mucosal antibody responses and provided complete protection against lethal challenge with wild type pdmH1N1 virus, even at the lowest immunization dose of 10³ PFU. The promising results of safety, immunogenicity, and protective efficacy of the vaccine not only contribute to expanding the repertoire of live vaccines as a judicious choice for pandemic H1N1 preparedness, but also suggest the great potential of X-31 ca donor strain to serve as reliable platform for generating diverse live vaccine constructs against seasonal influenza viruses and other pandemic strains.     

Safety and immunogenicity of a trivalent single dose seasonal influenza vaccine containing pandemic A(H1N1) antigen in younger and elderly subjects: a phase III open-label single-arm study

Background

During the pandemic of the 2009 A(H1N1) influenza virus strain, 20-40% of the population in some areas were infected. Infection with A(H1N1) may be mild, with an average case fatality rate below 0.25%, but severe disease is not limited to patients with underlying medical conditions. Since A(H1N1) is expected to continue to circulate it is included in the seasonal influenza vaccines for the 2010-2011 winter season. We investigated the immunogenicity and safety of a preservative-free non-adjuvanted seasonal trivalent influenza vaccine.

Methods

We conducted a single center single-arm study involving 142 subjects (77 adults of 18-60 years and 65 subjects 61 years and above) to test the immunogenicity, safety, and tolerability of a trivalent split influenza vaccine. The vaccine contained 15 μg of hemagglutinin of each of the virus strains recommended for the 2010-2011 northern hemisphere winter season (A/California/7/2009 (H1N1)-like strain; A/Perth/16/2009 (H3N2)-like strain; B/Brisbane/60/2008-like strain) in a non-adjuvanted preservative-free formulation. Antibody response to each antigen was measured by hemagglutination inhibition (HI) 21 days after immunization. Subject diary cards and additional telephone interviews were used to assess the safety profile.

Results

By day 21 after the vaccination, seroconversion, or a 4-fold antibody increase in HI antibody titers, was detectable against A(H1N1) in 84% and 75% of younger and older adults, against A(H3N2) in 80% and 57%, and against the B influenza strain in 61% and 33%. HI antibody titers of 40 or more were observed against A(H1N1) in 99% and 90% of younger and older adults, against A(H3N2) in 100% and 90%, and against the B influenza strain in 91% and 78%. Pre-vaccination antibody titers were protective against A(H1N1), A(H3N2), and B in 26%, 44% and 33%, respectively of the adults below 61 years and in 27%, 54% and 44% of the subjects of 61 years and above. Local and systemic reactions were more common in younger than in older subjects and the most frequently reported reactions were pain at the injection site (36%), myalgia (24%), and fatigue (15%). Five percent elderly subjects and 1% of younger subjects had mild or moderate unsolicited adverse events such as prolonged ecchymosis or night sweats that resolved within 7 days after vaccination.

Conclusions

This single dose trivalent seasonal influenza vaccine generated protective antibodies to all three viral strains and had an acceptable safety profile in both younger and older adults (ClinicalTrials.gov identifier: NCT01147081).     

Pre-vaccination immunity and immune responses to a cell culture-derived whole-virus H1N1 vaccine are similar to a seasonal influenza vaccine

Background

Immune responses to novel pandemic influenza vaccines may be influenced by previous exposure to antigenically similar seasonal strains.

Methods

An open-label, randomized, phase I/II study was conducted to assess the immunogenicity and safety of a non-adjuvanted, inactivated whole-virus H1N1 A/California/07/2009 vaccine. 408 subjects were stratified by age (18–59 and >60 years) and randomized 1:1 to receive two vaccinations with either 3.75 or 7.5 μg hemagglutinin antigen 21 days apart. Safety, immunogenicity and the influence of seasonal influenza vaccination and antibody cross-reactivity with a seasonal H1N1 strain was assessed.

Results

A single vaccination with either dose induced substantial increases in H1N1 A/California/07/2009 hemagglutination inhibition (HI) and neutralizing (MN) antibody titers in both adult and elderly subjects. A single 7.5 μg dose induced seroprotection rates of 86.9% in adults and 75.2% in elderly subjects. Two 7.5 μg vaccinations induced seroprotection rates in adult and elderly subjects of 90.9% and 89.1%, respectively. The robust immune response to vaccination was confirmed by analyses of neutralizing antibody titers. Both HI and MN antibodies persisted for ≥6 months post-vaccination. Between 34% and 49% of subjects had seroprotective levels of H1N1 A/California/07/2009 antibodies at baseline. Higher baseline HI titers were associated with receipt of the 2008–09 or 2009–10 seasonal influenza vaccine. High baseline A/California/07/2009 neutralizing antibody titers were also associated with high baseline titers against A/New Caledonia/20/99, a seasonal H1N1 strain which circulated and was included in the seasonal vaccine from 2000–01 to 2006–07. Pre-adsorption with A/H1N1/New Caledonia/20/99 antigen reduced A/H1N1/California/07/2009 baseline titers in 55% of tested sera. The vaccine was well tolerated with low rates of fever.

Conclusions

A whole-virus H1N1 A/California/07/2009 vaccine was safe and well tolerated and a single dose induced substantial immune responses similar to seasonal influenza vaccines, probably due to immunological priming by previous seasonal influenza vaccines or infections.     

Immunogenicity and safety of monovalent influenza A (H1N1) 2009 in HIV-infected Thai children

To evaluate the immunogenicity and safety of the monovalent pandemic influenza A (H1N1) 2009 (pH1N1) vaccine in HIV-infected Thai children, 2 doses, 28 days apart, of non-adjuvant monovalent pH1N1 vaccine (Panenza^® by Sanofi Pasteur, 15 μg/dose) provided by the National Health Promotion Program of the Thai Ministry of Public Health were given to HIV-infected children. Immunogenicity was measured by hemagglutination inhibition test (HAI) using two antigens, pH1N1 (A/Thailand/104/09) and seasonal influenza A H1N1 (A/Brisbane/59/07-like), at baseline, and 28 days after each dose. Serologic response was defined as four-fold rising of HAI titer or HAI titer ≥1:40 for those with baseline titer ≤1:10. Adverse events were recorded for 7 days after each vaccination. Of the 119 HIV-infected children enrolled, 60 (50.4%) were female with a median (IQR) age of 10.4 (7.2-13.7) years. All but 2 (98.3%) children were receiving antiretroviral therapy. At baseline, the median CD4 cell count was 782 (570-1149) cells/mm³, 91 (80.5%) children had HIV RNA level <40 copies/ml. The baseline HAI titer ≥1:40 for pH1N1 and seasonal H1N1 were 45.4%, and 39.5%, respectively. At 28 days after doses 1 and 2, the serologic response rates for pH1N1 were 54.2% and 67.8% with the geometric mean titer of 109.9 and 141.8; and serologic response rate when tested with seasonal H1N1 were 2.5% and 3.5%, respectively. The presence of baseline HAI titer for pH1N1 or seasonal H1N1 was found to be associated with serologic response. The vaccine was well tolerated. The results suggested that monovalent pH1N1 vaccine was immunogenic and safe in well controlled HIV-infected children with low level of cross reacting antibody to seasonal H1N1.