Influenza virus is one of the most important viruses that cause the respiratory infection seasonally. In April 2009, H1N1 was detected in America and Mexico and then there was pandemic in Korea. We investigated the difference of clinical and laboratory findings between the infections of H1N1 and Influenza B.

We have retrospectively studied the patients under age of 15 years who visited Inje University Sanggye Paik Hospital from August 2009 to April 2010. Evaluation for influenza infection was performed by rapid antigen test or multiplex reverse transcriptase polymerase chain reaction. Complete blood count with differential counts, C-reactive protein and chest X-ray were checked.

Enrolled patients were 2,226 in H1N1-infected group and 288 in influenza B-infected group. Seasonal variation was that H1N1 in autumn and winter but influenza B in spring. The male-to-female sex ratio was same as 1.23 in each group. The mean age of H1N1-infected group was higher than influenza B-infected group (P<0.001). Fever was developed similarly in both groups (P=0.114). However, cough, sputum, rhinorrhea, vomiting, diarrhea, and headache were more prevalent in influenza B infection compared to H1N1 infection (P<0.001). Pneumonia development and admission rate were higher in influenza B infection compared to H1N1 infection (P<0.001, respectively).

Although H1N1 infection spread rapidly, H1N1 caused not so severe symptoms than influenza B. Because of the possibility that influenza epidemic will develop repeatedly in the future, we need to evaluate more about different characteristics depending on the virus subtype and prepare for them.