Children and young adults are likely to be particularly vulnerable to infection during an influenza pandemic and an important source of infection for others. Some 40% of cases have been predicted to occur in individuals aged 19 years or younger 
. Among human cases of highly pathogenic avian influenza A/H5N1 virus infection confirmed over the last decade, a disproportionately high number have been children, although this is possibly due, in part, to the proximity between children and poultry in areas of Asia where most of these cases have occurred 
. A recent study profiled the social contact networks of school age children and teenagers in the USA as a way of assessing the potential for influenza transmission within this population 
. The authors suggested that high school students are likely to form the local transmission backbone for the next influenza pandemic.
Vaccination of children will therefore form an essential element of pandemic influenza vaccination programs. Our study was designed to evaluate the safety and immunogenicity of different formulations of an A/H5N1 vaccine in children. The two full-dose formulations chosen for this study and tested in all age groups of children (the adjuvanted 30 µg formulation and the non-adjuvanted 7.5 µg formulation), had previously been evaluated in a trial in French adults, and found to be well tolerated, immunogenic and able to induce cross-reactive antibodies
. The two half-dose formulations evaluated in the youngest group of children (aged 6–35 months) were chosen in line with recommendations for seasonal influenza vaccination of young children 
, for whom a half of the standard adult vaccine dose can be used. All formulations of the H5N1 vaccine appeared to be well tolerated with notably no evidence of increased reactogenicity after the second vaccination, no serious or significant adverse events and very few severity grade 3 solicited reactions. There were no marked differences in reactogenicity between the higher-dose adjuvanted groups and the lower dose non-adjuvanted groups. In accordance with published data with licensed seasonal influenza, the youngest group of children tended to have a higher incidence of fever 
It has been argued that due to the existence of numerous undetected mild or asymptomatic cases 
, the true human case fatality rate influenza A (H5N1) is considerably lower than the ~60% calculated based only on confirmed cases reported by the WHO 
. Indeed around 10% of a cohort of Hong Kong poultry workers had anti-H5 antibodies after the 1997 outbreak of H5N1 
. The children enrolled to our study in Bangkok showed no evidence of having been previously exposed to H5N1 influenza. They had no detectable antibody responses before vaccination, and the low levels of immune response seen after the first vaccination are concordant with a primary immune response, rather than a booster response. The H5N1-specific CD4 responses seen in some 6–35 month-old children is likely to be due to cross reactive T-cell responses stimulated by prior infection by other influenza strains. Such cross-reactive cellular responses between influenza strains have been described previously 
Antibody responses increased in all groups after the second vaccination. In terms of both geometric mean titres and the HI seroresponse rate, responses were highest among children vaccinated with the adjuvanted 30 µg formulation. Antibody responses to both the adjuvanted 30 µg formulation and the non-adjuvanted 7.5 µg formulation appeared to be at least as good as, if not better than those observed with the same vaccine formulations in a previous study in adults 
. It should be pointed out that vaccines in this study were presented in ready-to-use multi-dose vials, whereas in the adult study, vaccines were presented as single dose vials of vaccine and adjuvant for extemporaneous preparation. Both assays used to document the antibody response are functional assays, nevertheless, in absence of an established correlate of protection, it is unclear how the haemagglutination inhibiting and neutralising antibody responses documented in this study would translate to efficacy against infection or disease in a pandemic context. In their guidance for the licensing of pandemic vaccines, the EMEA acknowledges this uncertainty and requires that mock up vaccines be at least able to meet the three criteria defined for the vaccination of adults or elderly adults against seasonal influenza; i.e., a GMTR of at least 2.5 or 2.0, a seroprotection rate of at least 70 or 60% and a seroconversion or significant titre increase rate of at least 40 or 30% 
. Although in our study, the seroresponse threshold considered was 1
32 instead of the 1
40 in the EMEA criteria, with a GMTR of 11.7 and a seroresponse and seroconversion rate of 79%, the 30 µg+Al formulation in children in this study satisfy all three criteria.
Immune responses to the non-adjuvanted vaccines in Thai children in our study appeared comparable to or higher than those observed in a study among US children, despite a 6–12-fold difference in the amount of antigen: after two doses of 45 µg HA without adjuvant, the 38% had titres >1
. Several factors potentially contribute to this difference, including genetic factors, and the lack of standardization of assay methods between laboratories.
We explored the Th1 and Th2 cytokine secretion profile in subjects before and after vaccination. These analyses were performed in the youngest group of children (6–35 months) as it is in immunologically immature infants that immune responses are most biased towards a Th2 response 
. As expected, we observed low levels of cytokine secretion with a Th2-dominant profile before vaccination. In all groups, vaccination induced a Th2-biased response to the H5 protein. Although the study design did not allow the adjuvant effect to be separated from any antigen dose effect, there was no evidence of increased Th2-dominance in among subjects vaccinated with an adjuvanted vaccine.
In summary, these influenza A/H5N1 vaccines appeared immunogenic and well tolerated in a population of naïve children and infants from the age of 6 months, with the 30 µg+Al vaccine formulation eliciting the greatest immune responses, at least as good as those previously seen with the same vaccine formulation in adults. Given the likely burden of disease in children and their role in disease transmission, the vaccination of children will form an essential part of pandemic vaccination campaigns to control the spread of the disease. Furthermore, as the logistics of vaccine administration during a pandemic are expected to be particularly challenging, the ability to use the same vaccine across age groups will be a major advantage.