Influenza A Symptoms More Severe Than Influenza B

Bin Cao, MD

Patients admitted to the hospital with influenza A have more severe illness and slower clinical improvement compared to patients with influenza B, according to a new study.

The new report comes from a team of investigators from China, the United States, and England, who sought to gain more comprehensive insight into the clinical differences between patients with the 2 types of influenza. They noted that existing data is far from conclusive, and many of the studies on the matter have been based either on population influenza surveillance data or clinical studies with a limited number of cases.

In order to gain a higher-definition look at differences between the 2 flu types, investigators decided to use a newly created scale of hospitalized patient status, which classifies patients into 7 mutually exclusive categories. A classification of “category 1” means the patient is discharged from the hospital; “category 7” refers to a patient who has died.

“Though the specific form and utility of the ordinal endpoint scale are yet to be fully validated, it provides a new way to assess the clinical outcomes of in hospitalized patients with influenza virus infection,” wrote corresponding author Bin Cao, MD, of the China-Japan Friendship Hospital, in Beijing.

The investigators enrolled 574 patients in a prospective observational study. Each patient had been admitted to a particular hospital with either influenza A (369 patients) or influenza B (205 patients) during the 2016-2018 flu seasons.

After 28 days, 82.4% of the influenza A patients had seen clinical improvement (defined as improving by at least 2 steps on the scale), while 90.7% of influenza B patients showed clinical improvement. A Cox analysis showed that patients with influenza B had a higher probability of clinical improvement, and patients with influenza A had a “marginally” higher mortality rate.

However, when examined through the lens of the 7-point scale, the clinical realities were illuminated. For instance, at day 1 of hospitalization, 8.2% of patients with influenza A (30 patients) were in category 6 (the highest category except for death), while just 5.9% of patients with influenza B (12 people) had the same severity of flu. Meanwhile, 72.3% of influenza A patients (266 people) were in category 3, which is defined as not being in the ICU and not requiring oxygen. Among the patients with influenza B, 86.3% of patients (177 people) were in category 3.

By day 14, 5.8% of influenza A patients had died (21 people), while 4.4% of patients with influenza B had died (9 people). A total of 55.4% of patients with influenza A had been discharged from the hospital (201 people), while 62.4% of patients with influenza B were discharged (128 people).

“The proportion with clinical improvement up to day 28 was significantly lower and the proportions developing influenza-related pneumonia, ARDS [Acute Respiratory Distress Syndrome] and requiring ICU admission were significantly higher among patients with influenza A virus infection, compared to those with influenza B virus infection,” the investigators wrote. The authors hypothesized that viral virulence and the role of cross-protective cell medicated immunity may be responsible for the gap.

The authors have not yet responded to a request for comment.

The study, “Comparative outcomes of adults hospitalized with seasonal influenza A or B virus infection: application of the seven-category ordinal scale,” was published in Open Forum Infectious Diseases.