A Centers for Disease Control and Prevention (CDC)-led study finds that more than 1 in 6 US patients infected with the influenza A(H1N1)pdm09 or influenza B virus had severe in-hospital outcomes such as intensive care unit (ICU) admission or death during nine recent flu seasons.
Published this week in The Lancet Microbe, the study used Influenza Hospitalization Surveillance Network (FluSurv-NET) data to assess the severity of flu-associated outcomes in 104,969 patients aged 6 months and older who were hospitalized in certain counties in 13 states during the 2010–2011 to 2018–2019 flu seasons (October through April).
The states included California, Colorado, Connecticut, Georgia, Maryland, Michigan, Minnesota, Ohio, Oregon, New Mexico, New York, Tennessee, and Utah.
"The burden of influenza varies greatly from season to season because of differences in circulating influenza virus types and subtypes as well as differences in influenza vaccine coverage and effectiveness, with some years seeing lower vaccine effectiveness and higher hospitalisation and mortality rates when when influenza A H3N2 viruses are predominant," the researchers wrote.
"Despite the effect of virus type or subtype on the yearly influenza burden, little is known about the relative severity of influenza by virus type and subtype in hospitalised individuals," they added.
A(H1N1)pdm09 patients 25% more likely to die
On average, after weighting, 57.7% of patients had influenza A(H3N2), 24·6% had influenza A(H1N1)pdm09, and 17·7% had influenza B. A total of 16·7% required ICU admission, 6.5% needed mechanical ventilation or extracorporeal membrane oxygenation (ECMO), and 3.0% died (95% confidence intervals all had a range of less than 0.1%).
Patients infected with A(H1N1)pdm09 had a higher likelihood of poor in-hospital outcomes than those with A(H3N2). Specifically, they were 42% more likely to be admitted to an ICU, 79% more likely to need mechanical ventilation or ECMO, and 25% more likely to die.
Patients who had influenza B versus influenza A(H3N2) were 6% more likely to be admitted to an ICU, 14% more likely to need mechanical ventilation or ECMO, and 18% more likely to die.
Children aged 6 months to 17 years and adults 18 to 49 years had higher odds of mechanical ventilation or ECMO. Children aged 6 months to 17 years and adults 65 years and older were more likely to die when comparing those with influenza B versus A(H3N2). No link, however, was found between likelihood of death in patients aged 6 months to 17 years and those 65 and older who had A(H1N1)pdm09 versus A (H3N2), but the adjusted odds ratio was also significant for death in those 18 to 49 and 50 to 64 years.
It is important for individuals to receive an annual influenza vaccine and for health-care providers to provide early antiviral treatment for patients with suspected influenza who are at increased risk of severe outcomes.
Of the of 104,969 hospitalized flu patients, 52% were vaccinated against flu, and 88% had at least one underlying illness. Analyses stratified by flu vaccination status showed that unvaccinated patients were more likely to die if they had influenza A(H1N1)pdmo9 or B virus than those infected with A(H3N2).
Vaccine, early antiviral treatment
The authors said that lower flu vaccine effectiveness and subsequent higher hospitalization rates are typically tied to influenza A(H3N2)-predominant seasons. "Although we noted a higher number of hospitalisations from infections with influenza A H3N2 virus in our study, we found that once individuals were hospitalised, those with influenza A H1N1pdm09 and B virus infections were more likely to have severe outcomes."
They called for future research into how the probability of hospitalization and in-hospital outcomes differs by flu type and subtype.
"It is important for individuals to receive an annual influenza vaccine and for health-care providers to provide early antiviral treatment for patients with suspected influenza who are at increased risk of severe outcomes, not only when there is high influenza A H3N2 virus circulation but also when influenza A H1N1pdm09 and influenza B viruses are circulating," the researchers wrote.
"Early season detection of influenza type and subtype can inform public health messaging and vaccination campaigns to reduce influenza-associated hospitalisations and severe in-hospital outcomes; this is especially important now with co-circulation of other respiratory viruses, such as SARS-CoV-2 and respiratory syncytial virus, adding to the respiratory disease burden in hospital settings," they concluded.