#17,696
Despite decades of research, seasonal influenza continues to serve up surprises, some of which appear in a new study published The Lancet Microbe by CDC and other influenza researchers this week.
Although A/H3N2 has a reputation of producing more severe flu seasons - particularly in the elderly - this study finds that those hospitalized with A/H1N1 or influenza B are more likely to develop severe complications.
It wasn't all that long ago that influenza B was considered a less formidable version of flu (see 2009 Study), a notion that has been repeatedly dispelled over the past decade (see 2019's ERJ: When “B” becomes “A” : The Emerging Threat of Influenza B Virus).
Adding to that revised assessment, we have the following press release from the CDC, which reveals that those hospitalized with H1N1 or Influenza B appear more likely to experience severe outcomes (including death), than those with H3N2.
We've a press release from the CDC and a link and a few excerpts from the study, all of which suggest that the public health impacts of H1N1 and Influenza B should not be underestimated.
I'll have a brief postscript after the break.
New CDC Study Looks at Flu Severity by Virus Type and Subtype
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September 26, 2023 — A new CDC study published in The Lancet Microbe found that more than one in six people hospitalized with flu had severe outcomes such as intensive care unit (ICU) admission, use of invasive mechanical ventilation or extracorporeal membrane oxygenation (MV/ECMO), and death across a subset of people hospitalized with flu over nine seasons in the United States.
Despite a higher number of flu A(H3N2) hospitalizations, patients hospitalized with flu A(H1N1)pdm09 or B viruses had a higher likelihood of in-hospital severe outcomes.
Compared to hospitalizations with flu A(H3N2) virus, people hospitalized with flu A(H1N1)pdm09 virus were:Compared to hospitalizations with flu A(H3N2) virus, people hospitalized with flu B virus were:
- 42% more likely to be admitted to the ICU
- 79% more likely to receive MV/ECMO
- 25% more likely to die
Similar increases in likelihood of ICU admission and MV/ECMO use were seen for people of all ages hospitalized with A(H1N1)pdm09 viruses, although results were variable by age for influenza B. Before this study, little was known about the relative severity of flu by virus type and subtype in hospitalized people in the United States.
- 6% more likely to be admitted to the ICU
- 14% more likely to receive MV/ECMO
- 18% more likely to die
Researchers looked at flu-related hospitalizations from the 2010─2011 through 2018─2019 flu seasons from FluSurv-NET sites in the following 13 states: California, Colorado, Connecticut, Georgia, Maryland, Michigan, Minnesota, Ohio, Oregon, New Mexico, New York, Tennessee, and Utah. FluSurv-NET is a population-based surveillance system that captures laboratory-confirmed flu-related hospitalizations among people of all ages in more than 250 acute-care hospitals in the United States. The FluSurv-NET system represents over 29 million people, or approximately 9% of the U.S. population. Of the of 104,969 flu-related hospitalizations captured by FluSurv-NET system:
- 52% of people were vaccinated against flu.
- 88% of people had at least one comorbid condition.
- 7% of people were hospitalized with flu A(H3N2).
- 6% of people were hospitalized with flu A(H1N1)pdm09.
- 7% of people were hospitalized with flu B.
- ICU admission occurred in 16.7%, MV/ECMO use in 6.5%, and death in 3%.
These findings highlight the importance of annual flu vaccination and early antiviral treatment for patients at increased risk of severe flu, no matter which flu viruses are circulating. More information on the different types of flu viruses is available.
Severity of influenza-associated hospitalisations by influenza virus type and subtype in the USA, 2010–19: a repeated cross-sectional study
Kelsey M Sumner PhD a b, Svetlana Masalovich MS a, Alissa O'Halloran MSPH a, Rachel Holstein MPH a, Arthur Reingold MD c, Pam Daily Kirley MPH d, Nisha B Alden MPH e, Rachel K Herlihy MD e, James Meek MPH f, Kimberly Yousey-Hindes MPH f, Evan J Anderson MD g i j, Kyle P Openo DrPH h i j, Maya L Monroe MPH k, Lauren Leegwater MPH l, Justin Henderson MPH l, Ruth Lynfield MD m, Melissa McMahon MPH m, Chelsea McMullen MS n, Kathy M Angeles MPH o, Nancy L Spina MPH p…Shikha Garg MD a
Available online 25 September 2023
Summary
Background
Influenza burden varies across seasons, partly due to differences in circulating influenza virus types or subtypes. Using data from the US population-based surveillance system, Influenza Hospitalization Surveillance Network (FluSurv-NET), we aimed to assess the severity of influenza-associated outcomes in individuals hospitalised with laboratory-confirmed influenza virus infections during the 2010–11 to 2018–19 influenza seasons.
Methods
To evaluate the association between influenza virus type or subtype causing the infection (influenza A H3N2, A H1N1pdm09, and B viruses) and in-hospital severity outcomes (intensive care unit [ICU] admission, use of mechanical ventilation or extracorporeal membrane oxygenation [ECMO], and death), we used FluSurv-NET to capture data for laboratory-confirmed influenza-associated hospitalisations from the 2010–11 to 2018–19 influenza seasons for individuals of all ages living in select counties in 13 US states. All individuals had to have an influenza virus test within 14 days before or during their hospital stay and an admission date between Oct 1 and April 30 of an influenza season. Exclusion criteria were individuals who did not have a complete chart review; cases from sites that contributed data for three or fewer seasons; hospital-onset cases; cases with unidentified influenza type; cases of multiple influenza virus type or subtype co-infection; or individuals younger than 6 months and ineligible for the influenza vaccine. Logistic regression models adjusted for influenza season, influenza vaccination status, age, and FluSurv-NET site compared odds of in-hospital severity by virus type or subtype. When missing, influenza A subtypes were imputed using chained equations of known subtypes by season.
Findings
Data for 122 941 individuals hospitalised with influenza were captured in FluSurv-NET from the 2010–11 to 2018–19 seasons; after exclusions were applied, 107 941 individuals remained and underwent influenza A virus imputation when missing A subtype (43·4%). After imputation, data for 104 969 remained and were included in the final analytic sample. Averaging across imputed datasets, 57·7% (weighted percentage) had influenza A H3N2, 24·6% had influenza A H1N1pdm09, and 17·7% had influenza B virus infections; 16·7% required ICU admission, 6·5% received mechanical ventilation or ECMO, and 3·0% died (95% CIs had a range of less than 0·1% and are not displayed). Individuals with A H1N1pdm09 had higher odds of in-hospital severe outcomes than those with A H3N2: adjusted odds ratios (ORs) for A H1N1pdm09 versus A H3N2 were 1·42 (95% CI 1·32–1·52) for ICU admission; 1·79 (1·60–2·00) for mechanical ventilation or ECMO use; and 1·25 (1·07–1·46) for death. The adjusted ORs for individuals infected with influenza B versus influenza A H3N2 were 1·06 (95% CI 1·01–1·12) for ICU admission, 1·14 (1·05–1·24) for mechanical ventilation or ECMO use, and 1·18 (1·07–1·31) for death.
Interpretation
Despite a higher burden of hospitalisations with influenza A H3N2, we found an increased likelihood of in-hospital severe outcomes in individuals hospitalised with influenza A H1N1pdm09 or influenza B virus. Thus, 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.
Most will no doubt notice that 52% of those hospitalized had been vaccinated against the flu, but that doesn't tell the whole story. According to the research:
In analyses stratified by influenza vaccination status, individuals who did not receive the seasonal influenza vaccine had higher odds of death when infected with influenza A H1N1pdm09 or B virus compared with A H3N2 virus.
This aligns with the CDC's press release last week (see CDC Hopes to `Reset' Flu Vaccine Expectations With New Campaign) which stresses that even when the flu vaccine doesn't prevent infection, it can often reduce the severity of one's illness.
While it's true we need a better flu vaccine (see NIAID's Strategic Plan To Develop A Universal Flu Vaccine), current vaccines can still provide valuable protection, which for some, could be lifesaving.
