Study Finds Influenza A Deadlier and More Severe Than Influenza B

A new study by investigators in China assessing the difference of disease severity between influenza A and B among hospitalized adults has found that patients with influenza A viruses were more ill and had slower recoveries than patients with influenza B.

Although influenza A and B viruses co-circulate each flu season, influenza A viruses H1N1 and H3N2 have tended to predominate during recent flu seasons globally and have accounted for more than 96% of flu illnesses in the current flu season in the United States. In a new prospective observational study published in the journal Open Forum Infectious Diseases, an international research team examined laboratory-confirmed influenza A and B cases to compare illness and outcomes in patients.

In the paper, the investigators cite previous epidemiologic studies that found influenza A (H3N2) infections cause higher rates of influenza-associated hospitalizations and mortality among seasonal viruses, along with other studies finding no difference in factors such as length of hospital stay, mortality, mortality, or death between influenza A and B viruses. “Whether the illness severity caused by these influenza viruses is clinically similar in adults is controversial,” the investigators write. “Therefore, more comprehensive studies are required to evaluate the comparative severity of illness caused by the 2 viruses in those hospitalized.”

For the study, investigators recruited hospitalized adult patients admitted to China-Japan Friendship Hospital in Beijing between October 1, 2016, and June 1, 2018. Patients in the study were treated with neuraminidase inhibitors if they sought treatment or were admitted within 48 hours of illness onset. The primary outcome of the study was the integrated rate of clinical improvement up to 28 days from admission, with clinical improvement defined as the decline of 2 categories on a modified 7-category ordinal scale of clinical status. The secondary outcomes included clinical status assessed on days 7, 14, 21, and 28; time to hospital or intensive care unit discharge alive; time to weaning from supplemental oxygen supplement; incidence of influenza-related pneumonia; and in-hospital mortality.

A total of 574 patients were included in the study, 369 (64.3%) with influenza A and 205 (35.7%) with influenza B. Investigators found that patients with influenza A had worse clinical symptoms on admission. By day 28, 34 (9.2%) influenza A patients and 12 (5.9%) influenza B patients died. Clinical improvement occurred in 90.7% of patients with influenza B, compared with 82.4% of patients with influenza A at up to 28 days. In addition, patients with influenza B had a higher clinical improvement probability overall compared with those with influenza A, and hospital mortality for influenza A was 11.4% compared with 6.8% in patients with influenza B.

In an interview with Contagion®, study co-author Yeming Wang, MD, said the findings could prove useful to health care providers. “The clinical features of patients seeking care for influenza A and B infections were very similar. We found no individual symptom or group of symptoms that consistently distinguished influenza A and B infections in patients,” said Dr. Wang. “Accurate identification of influenza A and B is becoming more important, as the area of novel antiviral agents have been upcoming. The antiviral treatment might be different when evidence of several agents is accumulated. Prompt identification of influenza A (including subtype) and influenza B will be important to guide appropriate use of antiviral agents.”