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Influenza A causes more severe illness, delayed clinical improvement

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February 27, 2019

In hospitalized patients, influenza A is associated with more severe illness and delayed clinical improvement compared with influenza B, according to recently published study findings.

“Seasonal influenza is a common acute respiratory tract infection leading to about 291,243 to 645,832 respiratory deaths globally each year. Currently, strains from two subtypes of influenza A (H3N2, H1N1) and two lineages of influenza B viruses (Yamagata, Victoria) are the major causes of seasonal epidemics,” researchers wrote in Open Forum Infectious Diseases. “However, whether the illness severity caused by these influenza viruses is clinically similar in adults, is controversial.”

In another recent study examining ICU admissions and associated severity of influenza A vs. influenza B during two influenza seasons in Jerusalem, researchers found that patients with influenza B had similar rates of associated disease severity and ICU admission compared with patients with influenza A. They also found that patients with influenza B had shorter stays in the ICU, although they concluded this was statistically significant for only the 2017-2018 season.

In the current study, researchers investigated the difference of severity between influenza A and B among hospitalized adults using a novel seven-category ordinal scale and existing clinical outcome endpoints.

“The primary outcome was the rate of clinical improvement, defined as the decline of two categories from admission on a seven-category ordinal scale, which ranges from 1 (discharged with normal activity) to 7 (death), or hospital discharge up to 28 days,” the researchers explained.

The prospective, observational study was conducted during the 2016-2017 and 2017-2018 influenza seasons at the China-Japan Friendship Hospital. In total, 574 eligible patients participated in the study, 369 with influenza A and 205 with influenza B.

According to the results, the proportion of patients with worse findings on the ordinal scale at admission were those with influenza A. Clinical improvement up to 28 days occurred in 82.4% of patients with influenza A compared with 90.7% of patients with influenza B, the researchers reported. Modeling showed that patients with influenza B had a higher probability of clinical improvement (adjusted HR, 1.285; 95% CI, 1.03-1.603) and that in-hospital mortality for influenza A was higher than influenza B, 11.4% vs. 6.8%.

“Our findings indicated influenza A infection may result in a worse clinical improvement than influenza B among hospitalized patients with influenza virus infection,” the researchers concluded. “The rate of clinical improvement assessed by ordinal scale might be a reasonable endpoint for patients hospitalized with influenza infection.” – by Caitlyn Stulpin

Disclosures: The authors report no relevant financial disclosures.

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