The findings of this study clearly show that otherwise healthy children can produce a protective immune response when they are infected by the recent pandemic A/H1N1/2009 influenza virus. They are in line with those of authors studying seasonal influenza viruses [10, 11], and confirm that most children, including those aged less than two years, seem to have an immune system that can efficiently face influenza virus infection. More than 90% of the convalescent children in our population had a pandemic A/H1N1/2009 antibody level of ≥40 about four weeks after the onset of their illness. Both the seroconversion rate and the magnitude of antibody responses of our healthy children were similar to those reported by others studying convalescent sera obtained from older patients [2, 12–14].
However, despite these similarities, some of our findings are quite different from those found in adult and elderly subjects: those previously exposed to seasonal A/H1N1 influenza viruses experienced significantly less severe influenza [2–5], whereas the rates of hospitalisation and a diagnosis of pneumonia were significantly higher in our children with protective antibodies against seasonal A/H1N1 influenza virus than in those without. Moreover, on the basis of what has been demonstrated in experimental animals, we expected to find a stronger immune response to the pandemic A/H1N1/2009 influenza virus in children who had previously been affected by seasonal A/H1N1 influenza. It has been found that the previous exposure of ferrets to a contemporary seasonal A/H1N1 influenza virus is capable of priming a greater antibody response to a subsequent dose of a non-adjuvanted monovalent pandemic A/H1N1/2009 influenza vaccine , but we found no significant difference in pandemic A/H1N1/2009 antibody levels between our children who were positive for antibodies against seasonal A/H1N1 infection and those who were negative. It is possible that both these findings can be explained by non-immunological factors. The lack of any correlation between previous immunity against seasonal A/H1N1/2009 influenza virus and severity of influenza disease can simply be the consequence of an age effect. Influenza is per se more severe in younger patients (i.e., those less exposed to seasonal viruses) and these characteristics have been clearly demonstrated also for the recent pandemic [4, 5]. Moreover, data collected in experimental animals cannot be easily compared with those found in humans because these are very little homogeneous by age and sex and the global sample can be potentially affected by a selection bias. However, both these findings can be explained by means of the so-called "antigenic sin phenomenon", according to which the relative absence of exposure to influenza virus variants (as is usual in otherwise healthy children who have not received an influenza vaccination) may promote viral spread, whereas multiple exposures to variant viruses (as is usual in older people or subjects repeatedly vaccinated against seasonal influenza) may engender greater immune protection because of the immune system's capacity for cross-reactivity . This hypothesis is consistent with the findings of Laurie et
al. , who showed that ferrets previously exposed to repeated infections due to seasonal influenza A viruses were significantly more protected against pandemic A/H1N1/2009 influenza virus than those exposed to a single infection.
The antibody responses to pandemic A/H1N1/2009 influenza virus in our children were not influenced by age, gender, previous immunity against seasonal A/H1N1 influenza virus, viral load or the use of oseltamivir, but were significantly greater in the patients with the most severe disease. Our viral load data conflict with those reported by Hung et al. , who found a strict correlation between convalescent neutralising antibody titres and viral load. However, it is difficult to evaluate the effect of viral load on the characteristics of influenza because viral load peaks 1-2 days after symptom onset [14, 18], frequently before the patients attend an Emergency Room and their nasopharyngeal secretions are collected.
Our findings concerning oseltamivir are in line with those of Hung et al. , and what has been demonstrated in subjects suffering from seasonal influenza [18, 19]. These findings clearly show that antiviral treatment does not affect the subsequent humoral immune response to viral infection, and so it is unlikely to be associated with a higher rate of recurrence or reinfection.
Finally, our data regarding the greater immune response of children with more severe disease are quite similar to those found in adults by Mak et al. , and further support the conclusion that the immune system of healthy children seems capable of facing pandemic influenza infection.