Preliminary Estimated Influenza Illnesses, Medical Visits, Hospitalizations, and Deaths in the United States — 2022–2023 Influenza Season

Introduction

This web page provides estimates of the burden of influenza (flu) in the United States for the 2022-2023 flu season.  For the past several years, CDC has used a mathematical model to estimate the numbers of influenza illnesses, medical visits, hospitalizations, and deaths every year (1-4). The methods used to calculate the estimates have been described previously (1-2). CDC uses the estimates of the burden of flu in the population to inform policy and communications related to influenza prevention and control.

2022-2023 Burden Estimates

During the 2022-2023 flu season, influenza activity returned to levels that were seen prior to the emergence of SARS-CoV-2, the virus that causes COVID-19. Influenza activity in the United States during the 2022-23 season (October 2, 2022 – September 9, 2023) was classified as having moderate severity and was characterized by activity that returned to pre-COVID-19 levels but occurred earlier than is usual (5). Among children, the season was classified as high severity (5,6).  Influenza A(H3N2) viruses were the predominant virus subtype circulating during the single wave of activity that peaked in late November and early December; however, influenza A(H1N1)pdm09 and influenza B/Victoria viruses also were reported (7).

CDC estimates that the burden of illness during the 2022–2023 season was similar to some seasons before the pandemic, with an estimated 31 million people sick with flu, 14 million visits to a health care provider for flu, 360,000 hospitalizations for flu, and 21,000 flu deaths (Table 1). The number of cases of influenza-associated illness, medically attended illnesses, hospitalizations, and deaths were similar to the 2013-2014 season (8,9), which was also classified as a moderate severity season.

CDC’s estimates of hospitalizations and deaths associated with the 2022–2023 influenza season are a reminder that influenza viruses can cause severe disease and death. Older adults accounted for only 8% of medical visits for influenza but 72% of deaths and 52% of hospitalizations, which is similar to seasons before the COVID-19 pandemic. These findings continue to highlight that older adults are particularly vulnerable to severe disease with influenza virus infection and that influenza prevention measures such as vaccination are important to reducing the impact of the seasonal epidemics on the population and healthcare system.

Among children and adolescents, the percentages of influenza symptomatic illness, medically attended illnesses, and hospitalizations were higher during the 2022-2023 season compared to pre-COVID-19 pandemic seasons.  Children aged 5-17 years accounted for 28% of symptomatic illnesses, 31% of medically attended illnesses, and 7% of hospitalizations.  These percentages were 5-10% higher than the 2019/20 season.  In addition, deaths in children with laboratory-confirmed influenza virus infection have been a reportable disease in the United States since 2004; 174 deaths were reported for the 2022-23 season as of October 31, 2023 (5). However, flu-related pediatric deaths are likely under-reported, as not all children whose death was related to a flu virus infection may have been tested for flu (10,11). We estimated that about 500 additional deaths associated with flu occurred during the 2022-2023 season among children aged <18 years.

Taken together, these findings highlight the importance of influenza prevention measures among children.

Conclusion

During the 2022-2023 influenza season, CDC estimates that influenza was associated with 31 million illnesses, 14 million medical visits, 360,000 hospitalizations, and 21,000 deaths. The influenza burden was similar to the burden observed and the assessed severity during the 2013-2014 season.

Tables

†Uncertainty interval: Adjusted estimates are presented in two parts: an uncertainty interval [UI] and a point estimate. The uncertainty interval provides a range in which the true number or rate of flu illnesses, medical visits, hospitalizations, or deaths would be expected to fall if the same study was repeated many times, and it gives an idea of the precision of the point estimate. A 95% uncertainty interval means that if the study were repeated 100 times, then 95 out of 100 times the uncertainty interval would contain the true point estimate. Conversely, in only 5 times out of a 100 would the uncertainty interval not contain the true point estimate.

*Some of the data used to calculate burden estimates are incomplete or not yet available. These estimates will change as those data become available and the estimates are updated.

Chart

Burden Estimates Limitations

These estimates are subject to several limitations.

First, rates of influenza-associated hospitalizations include data reported to the Influenza Hospitalization Surveillance Network (FluSurv–NET) through September 30, 2023. Final case counts may differ slightly as further data cleaning from the 2022-2023 season is conducted by FluSurv–NET sites throughout the year. The most updated unadjusted rates of hospitalization for FluSurv-NET sites will be available on FluView Interactive (10).

Second, national rates of influenza-associated hospitalizations and in-hospital death were adjusted for the frequency of influenza testing and the sensitivity of influenza diagnostic assays, using a multiplier approach (3). However, data on testing practices during the 2022–2023 season were not available at the time of estimation. To be conservative when estimating these initial disease burden estimates, we adjusted rates using the lowest multiplier from any season from 2010–2011 through 2019–2020. These burden estimates from the 2022–2023 season will be updated later when data on contemporary testing practices become available.

Third, estimates of influenza-associated illness are made by multiplying the number of hospitalizations by the ratio of illnesses to hospitalizations; estimates of medical visits are made by a similar process. These adjustments are based on data from a prior season; therefore, the burden estimates provided here may not be accurate if patterns of care-seeking changed and the multipliers used are not valid.

Fourth, our estimate of influenza-associated deaths relies on information about location of death from death certificates to calculate ratios of deaths occurring in the hospital to deaths occurring outside of the hospital by categories of causes of death.  However, death certificate data during the 2022–2023 season were not available at the time of estimation. We used death certification data from all influenza seasons from 2010–2011 through 2019–2020 where these data were available from the National Center for Health Statistics. To calculate these ratios, first we calculated the frequency of flu-related deaths reported from our FluSurv-NET surveillance system that have cause of death identified as pneumonia or influenza (P&I), other respiratory or cardiovascular (other R&C), or other non-respiratory, non-cardiovascular (non-R&C).  Next, to account for deaths occurring outside of a hospital, we used information from national death certificates to calculate the proportion of deaths from these causes that occur in and out of the hospital. Data to generate these frequencies were not available from the 2022–2023 season at the time of estimation, so we used the average frequencies of location of death for each of the cause categories from previous seasons, 2010–2011 through 2019–2020.

Fifth, estimates of burden were derived from rates of influenza-associated hospitalizations, which is a different approach than the statistical models used in older CDC published influenza disease burden reports. These differences make it difficult to directly compare our estimates for seasons since 2009 to those older reports, though the estimates from our current method are largely consistent with estimates produced with statistical models for similar years (11-14).  It is useful to keep in mind that direct comparisons to influenza disease burden decades ago are complicated by large differences in the age of the US population and the increasing number of adults aged ≥65 years.

References:

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