Infectious Diseases I

Influenza is a seasonal viral illness affecting all ages and associated with significant morbidity and

mortality. Seasonal patterns in the United States vary from year to year. For example, the 2008–2009

influenza season started in November 2008 and waned by April 2009, but then, because of influenza A

H1N1 (2009 H1N1), it resurged in May 2009, peaking in mid-October 2009 and persisting to April 2010.

The 2023-2024 season started to significantly increase in October 2023 before peaking in December

2023.

hospitalized with influenza-related illness. During the previous 2020–2021 season, the reported overall

rate (cumulative rate per 100,000 population) of hospitalization for laboratory-confirmed influenza was

0.8, notably lower than previous seasons (67 for 2019–2020 season). However, the rate increased during

the 2021–2022 season to 17.3, which is still notably lower compared with rates prior to the SARS-CoV-2

pandemic. The average weekly rate of hospitalization from influenza was 0.1, with a peak of 8.9 during

the 2023-2024 season. Age-related rates for adult hospitalizations were 50.4 for those 65 years of age

and older, 16.0 for 50–64 years of age, and 9.1 for 18–49 years of age. The rate of ICU admission among

those hospitalized was 14.3%, compared to the the five seasons from 2015–2020 (range 15%–19%).

Risk factors for severe influenza requiring hospitalization include chronic respiratory or metabolic

illness, immunosuppression (disease or pharmacotherapy), pregnancy, and age older than 65 years.

In 2009, re-emergence of the high-virulence strain influenza A H1N1 (pandemic 2009 H1N1) caused

significant morbidity and mortality in young adults and other groups considered at a lower risk of severe

influenza. The pandemic 2009 H1N1 strain continues to be tracked and varies significantly from year-

to-year. In 2023-2024, the 2009 H1N1 was responsible for approximately 49.5% of reported influenza

cases in the United States.

The most common complications of severe influenza include hypoxemic respiratory failure, bacterial

pneumonia, and ARDS. Concomitant sepsis, septic shock, multiple organ failure, encephalopathy, and

rhabdomyolysis also are associated with severe influenza.

Among all causes of deaths in the United States, pneumonia and influenza have surpassed the epidemic

threshold since 2009, with rates close to 10% during the peak of influenza seasons. Severe influenza

caused by the 2009 H1N1 strain has been associated with crude mortality rates of 15%–53%.

Influenza is caused by the RNA viruses influenza A, B, or C, which usually spread through droplet

transmission. Influenza A and B viruses are the predominant causes of clinically significant illness.

Influenza virus subtypes are described by surface proteins hemagglutinin (H) and neuraminidase (N).

2009 H1N1 and H3N2 being the most prevalent influenza A subtypes. Influenza B prevalence has

varied during the same period, but it remains an important cause of illness. Overall for 2023-2024,

influenza A was isolated in 77.3% of cases. The most common specified influenza strain isolated was

influenza A H1N1 at 49.5% of all influenza A and influenza B subtyped specimens.

Influenza viruses can cause a broad range of respiratory tract infections, ranging from mild to moderate

upper respiratory tract infections to severe pneumonia. Influenza infection has been associated with

acute viremia.

Annual vaccination remains the primary tool for influenza prevention.