Methods

Patients

The protocol of this cross-sectional study was approved by the Infectious Diseases and Tropical Medicine Research Center of Isfahan University of Medical Sciences (MUI), Isfahan, Iran. Data collection was performed from all consecutive subjects admitted to the Department of Infectious Diseases and Tropical Medicine, Al-Zahra Hospital, MUI, Isfahan, Iran. Our investigation was conducted from September 2017 to February 2018. The criteria for the diagnosis of influenza were based on the U.S. Centers for Disease Control and Prevention (CDC) and included fever (37.8°C or higher), cough, sore throat, runny or stuffy nose, muscle or body aches, headache, tiredness, vomiting, diarrhea, etc. The demographic characteristics of the patients were recorded in a special form. Associated factors including vaccination, contact with birds, recent travel, and contact with patients were also noted.

Virological examination

For virological examination, in all suspected patients, throat swabs were tested for the presence of specific viral antigens for A-H1N1, B-H1N1, and A-H3N2 by means of real-time reverse transcriptase-polymerase chain reaction (RT-PCR). The respiratory samples were sent to the Tehran National Influenza Center in a viral transport medium (Virocult, Medical Wire & Equipment, UK), where the real-time RT-PCR protocol was applied using reagents supplied by the World Health Organization (WHO).

Imaging procedures

In the present study, the chest computerized tomography (CT) scan with intravenous (IV) contrast was performed to examine the pulmonary involvement of the three mentioned viral strains in patients with influenza. In this procedure, 120 ml (350 mg/ml) of IV iohexol (Omnipaque™; General Electric) was administered, with images being obtained after a 90-s delay. Provided chest scans were assessed for the presence or absence of patchy infiltration, interstitial infiltration, pleural effusion with or without infiltration, and also the site of lung involvement.

Serological and clinical examination

The assessed clinical manifestations included fever, headache, digestive involvement, myalgia, vocal hoarseness, calf pain, and tachycardia. Each variable was investigated as a simple ‘yes or no’ for symptoms. We also recorded the white blood cell (WBC), lymphocyte, neutrophil, platelet, C-reactive protein (CRP), alanine transaminase (ALT), aspartate transaminase (AST), alkaline phosphatase (ALP), blood urea nitrogen (BUN), hemoglobin (Hb), uric acid, lactate dehydrogenase (LDH), erythrocyte sedimentation rate (ESR), creatinine phosphokinase (CPK), total bilirubin (Bil-T), and direct bilirubin (Bil-D) levels; each variable was assessed as a numerical value.

Statistical analysis

Statistical analysis was performed using the SPSS statistical package program, version 22.0 (SPSS, RRID:SCR_002865). The demographic data of the patients were analyzed using the Chi-squared and Kruskal–Wallis tests. The comparison between the clinical and serological features of subjects with various types of influenza was made using the Chi-squared test. Continuous variables were assessed by analysis of variance (ANOVA) to identify differences between groups. Differences were evaluated further using Scheffe’s posthoc test if the ANOVA indicated significance. P-values less than 0.05 were accepted as meaningful differences.