1. Data presented on influenza-associated hospitalizations collected through FluSurv-NET are preliminary and subject to change as additional data are collected. Figures are based on varying denominators as selected variables may require more time to be collected. Data are refreshed and updated weekly.
2. Case counts in Virus Type graph reflect cases hospitalized during MMWR Week 40 through MMWR Week 17. Cases hospitalized outside these designated weeks but within the October 1 – April 30 FluSurv-NET surveillance period are not included.
3. The denominator for each underlying medical condition is the total number of patients with non-missing data for that condition. Pregnancy percentage is calculated using the number of female cases of childbearing age (15 to 44 years) as the denominator.
4. Starting in 2016-17, for outcomes including intensive care unit admission, mechanical ventilation, pneumonia diagnosis, and death, data in figures are produced based on the number of cases that have been discharged from the hospital and have non-missing data for each outcome.
5. During the 2017-18 season, seven FluSurv-NET sites (CA, GA, MN, NM, NYA, OH, OR) conducted random sampling to select cases ≥50 years for medical chart abstraction, while still performing full chart abstractions of all cases <50 years. During the 2018-19 season, six sites (CA, GA, NM, NYA, OH, OR) conducted random sampling of cases ≥65 years for medical chart abstraction. All other sites performed full chart abstractions on all cases. Data on age, sex, admission date, in-hospital death, and influenza test results were collected for all cases. Due to the sampling methodology, sample sizes for all variables for the 2017-18 and 2018-19 seasons are suppressed and weighted proportions are presented for the following clinical characteristics: underlying conditions, ICU admission, mechanical ventilation, pneumonia diagnosis and antiviral treatment.
6. Data have been collected on underlying medical conditions since 2003-04. In 2019-20, substantial changes were made to how underlying medical conditions were collected on the case report form. Observed changes in prevalence of underlying medical conditions beginning in 2019-20 may be partly due to the change in methodology.