Although the 2009 influenza pandemic caused significant concern in Guatemala and helped to increase recognition of influenza as an important public health issue, the data we have presented here suggest that in Guatemala, the clinical presentation of pH1N1 was similar to that of the seasonal influenza viruses that were circulating before and during the pandemic.
Despite reports of significant differences in the age distributions of seasonal influenza A and pH1N1 in both temperate 
and tropical climates 
, we found similar age distributions for both ILI and pneumonia associated with seasonal influenza A and pH1N1. Both seasonal H1N1 and pH1N1 caused pneumonia primarily in children <1 year old; infants account for approximately 3% of the Guatemalan population, but they made up 37% and 39% of the hospitalized pneumonia patients with pH1N1 and seasonal H1N1, respectively. Among ILI patients, school-age children 5 to 14 years old accounted for a third to half of influenza A cases, and they were the predominant age group affected by all three influenza A virus subtypes.
Although not statistically significant, all indicators of severity (i.e., admission to an ICU, mechanical ventilation and CFP) were higher among hospitalized pneumonia patients with pH1N1 as compared with seasonal H1N1. Because our study combined pre-pandemic and pandemic periods, we analyzed whether changes in practices or procedures could have resulted in findings of greater severity for pH1N1 than seasonal influenza. The proportion of hospitalized pneumonia patients admitted to the ICU was similar before and after the pandemic began, and although use of antivirals was rare, treatment with antivirals only occurred during the pandemic period. There was no difference between hospitalized pneumonia or ILI patients in time to presentation at a health facility by influenza A subtype.
There have been three other concurrent comparisons of seasonal influenza and pH1N1 in hospitalized patients that reported on severe outcomes; none found any significant differences in the ICU admission rates or CFP by influenza subtype, but all occurred in well-resourced settings where antivirals would have been available for treatment 
. We report a higher CFP for pH1N1 than has been reported elsewhere, but this is likely due, in part, to a limited supply of antivirals available for treatment, and does not explain the higher CFP for pH1N1 than seasonal H1N1.
One possible explanation for the higher CFP from pH1N1 in Guatemala is the increase in RSV transmission during the pandemic period. Viral coinfection, especially with RSV, has been hypothesized to reduce the T helper cell 1 response, thereby increasing disease severity 
. Among the children <5 years old with pH1N1 who died, 75% were coinfected with RSV. It is not clear whether RSV acted synergistically with pH1N1 to cause more severe disease in these patients, or whether RSV itself might have been the cause of their death. Further investigation in this population of the effect of RSV and influenza coinfection is warranted.
Clinical symptoms of hospitalized pneumonia patients were similar between seasonal H1N1 and pH1N1, except for measured temperature ≥38°C in the first 24 hours after hospital admission, which was significantly less frequent among patients with pH1N1. We looked for differences in treatment seeking behaviors and treatments taken before admission that could explain this finding, but the use of any medication, and antipyretics in particular, did not differ between hospitalized pneumonia patients with seasonal H1N1 and pH1N1. A difference in fever has not been noted in any other concurrent comparison of patients with seasonal influenza A and pH1N1 
Although there have been many reports of a higher proportion of pH1N1 ILI patients with gastrointestinal symptoms 
, we found no difference in the proportions of hospitalized pneumonia or ILI patients with diarrhea by subtype. It is possible that our use of a stringent case definition for diarrhea may have missed an association with more mild gastrointestinal symptoms such as nausea.
A recent comparison of seasonal influenza and pH1N1 cases in Philadelphia found more lower respiratory tract symptoms (i.e., cough and pleuritic chest pain) among pH1N1 than seasonal influenza cases 
; we did not find any significant differences in the prevalence of these symptoms among hospitalized pneumonia patients, but among ILI patients, both difficulty breathing and pleuritic chest pain were significantly more common among seasonal influenza patients, rather than those with pH1N1. It is possible that during the pandemic, patients with lower respiratory tract symptoms were more likely to proceed directly to the hospital for treatment.
This study has several important strengths. Case definitions, laboratory diagnostics and procedures for data collection did not change during the time period covered in this report; this eliminates the possibility that findings were related to changes in surveillance methodology as a result of the pandemic, which can be a problem when using historical controls. A broad case definition permitted inclusion of influenza cases that might otherwise go undetected; for example, requirement of fever in the case definition for severe acute respiratory disease could miss a significant proportion of serious illness associated with both seasonal H1N1 and pH1N1.
The main limitation of this study is the relatively small number of cases of influenza that could be analyzed, which limits the power to detect differences in characteristics and clinical presentation. Because the sample size was small, it is possible that we were not able to identify important differences between seasonal influenza A viruses and pH1N1 influenza that might appear in a larger data set, especially related to signs of severity which were consistently elevated with pH1N1 but were not statistically significant. This limitation has been noted for least one other similar study 
, and should be taken into consideration when evaluating results from our study. Because surveillance for pneumonia in Quetzaltenango was initiated only four months before the pandemic began, and surveillance for ILI was initiated two months after the first case of pandemic influenza, the majority of the seasonal influenza cases come from Santa Rosa and this may have introduced some unmeasured biases in the comparison between seasonal influenza and pH1N1. Although we did not find a difference in the number of days between symptom onset and care seeking at our surveillance clinics and hospitals between seasonal influenza A viruses and pH1N1, we are unable to determine from this dataset whether there was an increase in the probability of healthcare seeking as a result of the pandemic. We used a standard definition for ILI that includes a measured fever and this is likely to have caused us to miss cases of influenza that presented without fever.
In conclusion, the epidemiology of pH1N1 in Guatemala was not significantly different from that associated with the seasonal influenza subtypes circulating locally before and during the pandemic in terms of the age groups most affected and clinical signs and symptoms. In Guatemala, influenza is largely a disease of children, with the most severe disease in infants, and targeted use of influenza vaccine in children may be warranted. The 2009 influenza pandemic raised awareness of the burden of disease caused by influenza in the tropics; increased attention should be extended to monitoring and addressing the morbidity and mortality associated with seasonal influenza.