CLINICAL MANIFESTATIONS SEEMED MORE SEVERE AMONG PATIENTS WITH ANTIBODY 4-FOLD OR A GREATER INCREASE IN TITER BOTH AGAINST pH1N1 AND AGAINST SEASONAL INFLUENZA THAN THOSE WHOSE ANTIBODY 4-FOLD OR GREATER INCREASE IN TITER WAS ONLY AGAINST ONE TYPE OF SEASONAL INFLUENZA

In 2009, after an emergence of pandemic influenza A(H1N1) (pH1N1), one of our serological investigations showed 23 students whose paired serum antibodies simultaneously presented 4-fold or greater increase in titer both against pH1N1 and against seasonal influenza (SI). This caught our attention and in order to further understand clinical features of these cases, a case control study was conducted. From October 2009 to September 2012, we collected the acute and convalescent phase serum samples from patients whose throat swabs were positive for pH1N1 or SI by real-time reverse transcription polymerase chain reaction (RT-PCR). The patients’ information, including clinical symptoms, selfprotect measures, and social activities after illness, was also collected through inspecting medical records, and interviewing face to face or by telephone.