The association of lymphopenia with the clinical severity in the Korean children admitted to the hospital with pandemic (H1N1) 2009 infection

Background: Critical illness due to pandemic (H1N1) 2009 is an emerging threat to global health. In this study, lymphopenia was focused on as a major risk factor for a critical clinical course of pandemic (H1N1) 2009 infection. We investigated the association of lymphopenia at the time of admission with the clinical severity of the admitted children with pandemic (H1N1) 2009 infection. Material and Methods: We performed a retrospective study on the patients who were younger than 15 years of age and who were admitted to Wonju Christian Hospital due to pandemic (H1N1) 2009 infection between August 20, 2009 and February 20, 2010. Pandemic (H1N1) 2009 infection was confirmed by reverse transcriptase polymerase chain reaction (RT-PCR) in all patients. We divided the study period into two periods as August 20 -November 30 2009 (pre-vaccination period) and December 1 2009- February 20 2010 (post-vaccination period). The clinical differences between two periods were analyzed. To define the role of lymphopenia, we examined the differences of clinical manifestations between the H1N1 patients with lymphopenia and those without lymphopenia. Results: Among the 2,399 children who had H1N1 infection, 149 patients (6.2%) were admitted under the following diagnoses: pneumonia (67.1%), bronchiolitis/ asthma (18.8%), croup (6%) and febrile convulsion (8.7%). The median age of the patients was signifi cantly different between during the pre-vaccination period and the post-vaccination period (6 years of age [range: 0.25-14] vs. 3 years of age, [range: 0.1-14], P<0.05). The proportion of patients who had lymphopenia was signifi cantly different between two periods (39.5% vs. 20%, P<0.05). When we compared the clinical severity between the patients with lymphopenia and those without lymphopenia, age (P<0.0001), the length of hospital stay (P<0.0001) and the serum levels of C-reactive protein (P<0.01) were signifi cantly different. Conclusion: Our data support that lymphopenia may be a major determining factor that could cause a critical clinical course during pandemic period among children in the Republic of Korea.