Oregon Health, Authority Public Health Division, Acute and Communicable Disease Prevention

2021–2022 Influenza Hospitalization Report

Background

Since 2003, the Oregon Emerging Infections Program (EIP) has conducted surveillance for pediatric influenza hospitalizations in collaboration with the Centers for Disease Control and Prevention (CDC) Influenza-Associated Hospitalization Surveillance Network (FluSurv-NET). Surveillance for adult influenza-associated hospitalizations was added in 2005.

The objectives of EIP FluSurv-NET surveillance are:

  • To estimate age-, race-, and ethnicity-specific hospitalization rates

  • To describe temporal trends of laboratory-confirmed influenza hospitalization

  • To describe characteristics of persons hospitalized with influenza

  • To describe clinical features and the course of influenza disease among persons hospitalized with influenza

In Oregon, the EIP surveillance area for influenza hospitalizations comprises the Portland tri-county area (Clackamas, Multnomah, and Washington counties) with its 2021 population of 1,840,7471, which is approximately 43% of the population of Oregon.

This report summarizes FluSurv-NET surveillance in Oregon during the 2021–2022 season (October 1, 2021–June 30, 2022).2

Methods

Cases are defined as laboratory-confirmed influenza hospitalizations among residents of the EIP surveillance area who test positive for influenza within 14 days before admission or 3 days after admission. Cases are reported by hospitals in the tri-county area. Medical records are reviewed by trained personnel, who use the EIP case-report form to collect standardized data regarding demographic characteristics, clinical manifestations, underlying conditions, and outcomes.

Surveillance Results

During October 1, 2021–June 30, 2022, 79 influenza-related hospitalizations were reported in Oregon’s EIP surveillance area—68 adult (≥18 years) cases and 11 pediatric (<18 years) cases. This represents a crude rate of 4.3 cases per 100,000 residents.

Table 1 shows the sex, age, ethnicity, and race of Oregon EIP FluSurv-NET cases in the 2021–2022 season. The median age of hospitalized cases was 59 years (range, 2 months–93 years). 44.3% of hospitalizations occurred in persons aged 65 years and older, 57.0% of cases were female, and 46.8% of cases identified as Non-Hispanic White.

Table 1. Characteristics of Oregon EIP FluSurv-NET hospitalization cases, 2021–2022

Case Characteristics

n = 79 (%)

Sex

Male

34 (43.0%)

Female

45 (57.0%)

Age

<6 months

2 (2.5%)

6-23 months

0 (0.0%)

24-59 months

3 (3.8%)

5-10 years

1 (1.3%)

11-17 years

5 (6.3%)

18-49 years

25 (31.6%)

50-64 years

8 (10.1%)

65+ years

35 (44.3%)

Race

Hispanic

20 (25.3%)

White, Non-Hispanic

37 (46.8%)

Black, Non-Hispanic

5 (6.3%)

Asian/Hawaiian or Pacific Islander

5 (6.3%)

American Indian/Alaska Native

1 (1.3%)

Multiple Races

0 (0.0%)

Unknown

11 (13.9%)

Table 2 shows the number and rates of Oregon EIP FluSurv-NET hospitalizations by age group during the 2021–2022 season.

Table 2. 2021–2022 Oregon EIP FluSurv-NET hospitalization rates by age group

Age Group (years)

n = 79

Rates per 100,000 people

0-4

5

5.2

5-17

6

2.2

18-49

25

2.9

50-64

8

2.4

65+

35

12.4

Figure 1 shows the distribution of Oregon EIP FluSurv-NET hospitalization rates by age group during the influenza season. The peak of influenza-associated hospitalizations occurred during MMWR week 19 of 2022 (May 08, 2022–May 14, 2022). The first case was reported during MMWR week 48 (November 28, 2021–December 04, 2021), and the last case was reported during MMWR week 23 (June 05, 2022–June 11, 2022).

For a data table equivalent of this graph, refer to Appendix A.

Figure 1. Rates of Oregon EIP FluSurv-NET hospitalizations, by age, 2021–2022

Line chart depicting weekly influenza-associated hospitalization rates across three age groups: under 18 years, 18 to 64 years, and 65 years and older from October 9, 2021, to June 25, 2022. Hospitalization rates were highest among ages 65 years and older

All cases admitted between October 1, 2021–April 30, 2022, 25 adult and 7 pediatric cases, were selected for medical-record review. Medical-record review was not conducted for cases admitted between May 1, 2022 - June 30, 2022.

Table 3 shows selected outcomes and underlying medical conditions for pediatric cases. 71.4% of pediatric cases had an underlying medical condition. The most frequently reported underlying medical conditions among pediatric cases were asthma (42.9%) and obesity (28.6%).

Table 3. Selected outcomes and underlying medical conditions for pediatric Oregon EIP FluSurv-NET cases, 2021–2022

Outcomes and underlying medical conditions

n = 7 (%)

Treatment and intervention

ICU admission

2 (28.6%)

Mechanical ventilation

0 (0.0%)

Treated with antivirals*

6 (85.7%)

Condition†

Asthma

3 (42.9%)

Autoimmune disease

0 (0.0%)

Blood disorders

0 (0.0%)

Cardiovascular disease

0 (0.0%)

Chronic lung disease

0 (0.0%)

Chronic metabolic disease

0 (0.0%)

Gastrointestinal/liver disease

0 (0.0%)

Hypertension

0 (0.0%)

Immunesuppression

0 (0.0%)

Neurologic disease

0 (0.0%)

Obese‡

2 (28.6%)

Renal disease

0 (0.0%)

Vaccine status

Yes

4 (57.1%)

No

6 (85.7%)

Unknown

0 (0.0%)

* Treated with antivirals defined as antiviral treatment during the course of illness.

†Cases may have more than one underlying condition; categories are not mutually exclusive.

‡ Obesity defined as BMI≥30.BMI is not calculated for cases under <2 years.

Table 4 shows selected outcomes and underlying medical conditions for adult cases. 88.0% of adult cases had at least one underlying medical condition prior to admission. The most frequently reported underlying medical conditions among adults were hypertension (44.0%) and cardiovascular disease (40.0%).

Table 4. Selected outcomes and underlying medical conditions for adult Oregon EIP FluSurv-NET cases, 2021–2022

Outcomes and underlying medical conditions

n = 25 (%)

Treatment and intervention

ICU admission

3 (12.0%)

Mechanical ventilation

0 (0.0%)

Treated with antivirals*

17 (68.0%)

Condition†

Asthma

3 (12.0%)

Autoimmune disease

0 (0.0%)

Blood disorders

0 (0.0%)

Cardiovascular disease

10 (40.0%)

Chronic lung disease

5 (20.0%)

Chronic metabolic disease

7 (28.0%)

Gastrointestinal/liver disease

1 (4.0%)

Hypertension

11 (44.0%)

Immunesuppression

0 (0.0%)

Neurologic disease

1 (4.0%)

Obese‡

5 (20.0%)

Pregnancy§

4 (16.0%)

Renal disease

2 (8.0%)

Vaccine status

Yes

20 (80.0%)

No

11 (44.0%)

Unknown

0 (0.0%)

* Treated with antivirals defined as antiviral treatment during the course of illness.

†Cases may have more than one underlying condition; categories are not mutually exclusive.

‡ Obesity among children aged 2 years and older defined as body mass index (BMI) ≥30, calculated using height and weight if known; otherwise, as indicated in medical record.

§ Total number of females ≥15-49 years of age (n=21)

Among all individuals hospitalized for influenza during the 2021–2022 season, 2 (2.5%) died during their hospitalization. All deaths were among adults (2.9% of all influenza-associated adult hospitalizations).

Figure 2 displays rates of influenza-associated hospitalizations by age group and season, 2019–2022. During the 2021–2022 season, the highest rate of hospitalization occurred among persons 65 years and older, at 12.4 cases per 100,000 population, followed by persons 0-4 years of age, at 5.2 cases per 100,000 population. The overall influenza hospitalization rate for the 2021-2022 season was 4.3 cases per 100,000 population. For a data table equivalent of this graph, refer to Appendix B.

Figure 2. Rates of Oregon EIP influenza-associated hospitalizations by age group and year, 2017–2022

Bar chart comparing cumulative influenza-associated hospitalization rates across the 2017-2018, 2018-2019, 2019-2020, and 2021-2022 seasons across five age groups: 0 to 4 years, 5 to 17 years, 18 to 49 years, 50 to 64 years, and 65 years and older. Cumulative hospitalization rates for the 2021-2022 season are notably lower than previous seasons across all age groups.

Figure 3 shows cumulative rates of influenza-associated hospitalizations by race and ethnicity for pediatric cases during the 2021–2022 season; the highest rate occurred among the Hispanic population, at 100.5 cases per 100,000 people followed by Black, Non-Hispanics at 78.9 cases per 100,000 people. For a data table equivalent of this graph, refer to Appendix C.

Figure 3. Cumulative rates of Oregon EIP influenza-associated hospitalizations by race and ethnicity among children, 2021–2022

Line chart depicting cumulative influenza-associated hospitalization rates among children across five race and ethnicity groups: American Indian/Alaska Native, Asian/Hawaiian or Pacific Islander, Black Non-Hispanic, Hispanic, and White Non Hispanic from October 9, 2021, to June 25, 2022.

Figure 4 displays age-adjusted cumulative rates of influenza-associated hospitalization by race and ethnicity among adults during the 2021–2022 season. The highest rate occurred among the Hispanic population, at 15.9 cases per 100,000 people followed by Black, Non-Hispanics at 7 cases per 100,000 people. For a data table equivalent of this graph, refer to Appendix D.

Figure 4. Age-adjusted cumulative rates of Oregon EIP influenza-associated hospitalizations by race and ethnicity among adults, 2021–2022

Line chart depicting cumulative influenza-associated hospitalization rates among adults across five race and ethnicity groups: American Indian/Alaska Native, Asian/Hawaiian or Pacific Islander, Black Non-Hispanic, Hispanic, and White Non Hispanic from October 9, 2021, to June 25, 2022.

Table 5 shows the frequency of virus types and subtypes detected among Oregon EIP FluSurv-NET cases. The primary subtype during the 2021–2022 season was influenza A H3.

Table 5. Influenza virus types and subtypes among Oregon EIP FluSurv-NET cases, 2021–2022.

Virus

n = 79 (%)

Influenza A

2009 H1N1

3 (3.8%)

H3

12 (15.2%)

Influenza A, No Subtype

64 (81.0%)

Influenza B

Victoria Lineage

0 (0.0%)

Yamagata Lineage

0 (0.0%)

Influenza B, No Subtype

0 (0.0%)

Influenza-Associated Pediatric Mortality

During the 2021–2022 influenza season there were 0 reported cases of influenza-associated pediatric mortality in Oregon. More information on influenza-associated pediatric mortality is available from the CDC.

Appendix A

Rates of Oregon EIP FluSurv-NET hospitalizations, by age, 2021–2022
Rates per 1000,0000 Population
Week Under 18 years 18–64 years 65+ years
October 09, 2021 0.0 0.0 0.0
October 16, 2021 0.0 0.0 0.0
October 23, 2021 0.0 0.0 0.0
October 30, 2021 0.0 0.0 0.0
November 06, 2021 0.0 0.0 0.0
November 13, 2021 0.0 0.0 0.0
November 20, 2021 0.0 0.0 0.0
November 27, 2021 0.0 0.0 0.0
December 04, 2021 0.0 0.0 0.4
December 11, 2021 0.0 0.0 0.4
December 18, 2021 0.0 0.3 0.0
December 25, 2021 0.0 0.0 0.4
January 01, 2022 0.0 0.0 0.0
January 08, 2022 0.0 0.0 0.0
January 15, 2022 0.0 0.0 0.0
January 22, 2022 0.0 0.0 0.0
January 29, 2022 0.0 0.0 0.0
February 05, 2022 0.0 0.0 0.0
February 12, 2022 0.0 0.0 0.0
February 19, 2022 0.0 0.0 0.0
February 26, 2022 0.0 0.1 0.0
March 05, 2022 0.0 0.0 0.0
March 12, 2022 0.0 0.2 0.0
March 19, 2022 0.0 0.0 0.7
March 26, 2022 0.0 0.2 0.0
April 02, 2022 0.0 0.0 0.4
April 09, 2022 0.3 0.2 0.0
April 16, 2022 0.5 0.0 0.4
April 23, 2022 0.8 0.1 1.8
April 30, 2022 0.3 0.1 0.7
May 07, 2022 0.3 0.3 0.7
May 14, 2022 0.3 0.3 2.5
May 21, 2022 0.0 0.5 1.1
May 28, 2022 0.0 0.2 0.7
June 04, 2022 0.3 0.3 1.4
June 11, 2022 0.3 0.1 1.1
June 18, 2022 0.0 0.0 0.0
June 25, 2022 0.0 0.0 0.0
July 02, 2022 0.0 0.0 0.0

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Appendix B

Rates of Oregon EIP influenza-associated hospitalizations by age group, 2017-2022
Rates per 100,000 Population
Season 0-4 years 5-17 years 18-49 years 50-64 years 65+ years
2017-18 66.5 12.9 21.5 122.2 285.4
2018-19 49.0 18.5 20.9 84.8 255.3
2019-20 57.2 13.6 19.0 53.9 93.1
2021-22 3.5 1.5 1.2 0.6 4.7

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Appendix C

Cumulative rates of Oregon EIP influenza-associated hospitalizations by race and ethnicity among children, 2021-2022
Rates per 100,000 Population
Week American Indian/Alaska Native Asian/Hawaiian or Pacific Islander Black, Non-Hispanic Hispanic White, Non-Hispanic
October 09, 2021 0.0 0.0 0.0 0.0 0.0
October 16, 2021 0.0 0.0 0.0 0.0 0.0
October 23, 2021 0.0 0.0 0.0 0.0 0.0
October 30, 2021 0.0 0.0 0.0 0.0 0.0
November 06, 2021 0.0 0.0 0.0 0.0 0.0
November 13, 2021 0.0 0.0 0.0 0.0 0.0
November 20, 2021 0.0 0.0 0.0 0.0 0.0
November 27, 2021 0.0 0.0 0.0 0.0 0.0
December 04, 2021 0.0 0.0 0.0 0.0 0.0
December 11, 2021 0.0 0.0 0.0 0.0 0.0
December 18, 2021 0.0 0.0 0.0 0.0 0.0
December 25, 2021 0.0 0.0 0.0 0.0 0.0
January 01, 2022 0.0 0.0 0.0 0.0 0.0
January 08, 2022 0.0 0.0 0.0 0.0 0.0
January 15, 2022 0.0 0.0 0.0 0.0 0.0
January 22, 2022 0.0 0.0 0.0 0.0 0.0
January 29, 2022 0.0 0.0 0.0 0.0 0.0
February 05, 2022 0.0 0.0 0.0 0.0 0.0
February 12, 2022 0.0 0.0 0.0 0.0 0.0
February 19, 2022 0.0 0.0 0.0 0.0 0.0
February 26, 2022 0.0 0.0 0.0 0.0 0.0
March 05, 2022 0.0 0.0 0.0 0.0 0.0
March 12, 2022 0.0 0.0 0.0 0.0 0.0
March 19, 2022 0.0 0.0 0.0 0.0 0.0
March 26, 2022 0.0 0.0 0.0 0.0 0.0
April 02, 2022 0.0 0.0 0.0 0.0 0.0
April 09, 2022 0.0 0.0 78.9 0.0 0.0
April 16, 2022 0.0 0.0 78.9 24.2 8.6
April 23, 2022 0.0 0.0 78.9 50.4 23.8
April 30, 2022 0.0 0.0 78.9 50.4 23.8
May 07, 2022 0.0 0.0 78.9 74.3 23.8
May 14, 2022 0.0 0.0 78.9 100.5 23.8
May 21, 2022 0.0 0.0 78.9 100.5 23.8
May 28, 2022 0.0 0.0 78.9 100.5 23.8
June 04, 2022 0.0 0.0 78.9 100.5 31.3
June 11, 2022 0.0 48.7 78.9 100.5 31.3
June 18, 2022 0.0 48.7 78.9 100.5 31.3
June 25, 2022 0.0 48.7 78.9 100.5 31.3
July 02, 2022 0.0 48.7 78.9 100.5 31.3

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Appendix D

Age-adjusted cumulative rates of Oregon EIP influenza-associated hospitalizations by race and ethnicity among adults, 2021-2022
Rates per 100,000 Population
Week American Indian/Alaska Native Asian/Hawaiian or Pacific Islander Black, Non-Hispanic Hispanic White, Non-Hispanic
October 09, 2021 0.0 0.0 0.0 0.0 0.0
October 16, 2021 0.0 0.0 0.0 0.0 0.0
October 23, 2021 0.0 0.0 0.0 0.0 0.0
October 30, 2021 0.0 0.0 0.0 0.0 0.0
November 06, 2021 0.0 0.0 0.0 0.0 0.0
November 13, 2021 0.0 0.0 0.0 0.0 0.0
November 20, 2021 0.0 0.0 0.0 0.0 0.0
November 27, 2021 0.0 0.0 0.0 0.0 0.0
December 04, 2021 0.0 0.0 0.0 2.1 0.0
December 11, 2021 0.0 0.0 0.0 2.1 0.1
December 18, 2021 0.0 0.0 0.0 4.0 0.2
December 25, 2021 0.0 0.0 0.0 4.0 0.3
January 01, 2022 0.0 0.0 0.0 4.0 0.3
January 08, 2022 0.0 0.0 0.0 4.0 0.3
January 15, 2022 0.0 0.0 0.0 4.0 0.3
January 22, 2022 0.0 0.0 0.0 4.0 0.3
January 29, 2022 0.0 0.0 0.0 4.0 0.3
February 05, 2022 0.0 0.0 0.0 4.0 0.3
February 12, 2022 0.0 0.0 0.0 4.0 0.3
February 19, 2022 0.0 0.0 0.0 4.0 0.3
February 26, 2022 0.0 0.6 0.0 4.0 0.3
March 05, 2022 0.0 0.6 0.0 4.0 0.3
March 12, 2022 0.0 1.1 0.0 4.4 0.3
March 19, 2022 0.0 1.1 0.0 7.2 0.4
March 26, 2022 0.0 1.1 0.0 7.2 0.5
April 02, 2022 0.0 1.1 0.0 7.2 0.6
April 09, 2022 6.3 1.1 0.0 7.2 0.7
April 16, 2022 6.3 1.1 0.0 7.2 0.7
April 23, 2022 6.3 1.1 0.0 12.7 1.0
April 30, 2022 6.3 1.1 0.0 13.1 1.2
May 07, 2022 6.3 1.1 0.0 13.5 1.3
May 14, 2022 6.3 2.4 1.7 13.5 1.9
May 21, 2022 6.3 3.5 6.0 14.3 2.3
May 28, 2022 6.3 3.5 6.0 14.6 2.6
June 04, 2022 6.3 3.5 7.0 15.5 3.0
June 11, 2022 6.3 3.5 7.0 15.9 3.3
June 18, 2022 6.3 3.5 7.0 15.9 3.3
June 25, 2022 6.3 3.5 7.0 15.9 3.3
July 02, 2022 6.3 3.5 7.0 15.9 3.3

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Footnotes

  1. National Center for Health Statistics. Vintage 2020 postcensal estimates of the resident population of the United States (April 1, 2010, July 1, 2010–July 1, 2020), by year, county, single-year of age [0, 1, 2, .., 85 years and over],bridged race, Hispanic origin, and sex. Prepared under a collaborative arrangement with the U.S. Census Bureau.Available from: /nchs/nvss/bridged_race.htm as of September 22, 2021, following release by the U.S. Census Bureau of the unbridged Vintage 2020 postcensal estimates by 5-year age group on June 17, 2021↩︎

  2. The influenza season traditionally runs from October 1 to April 30 of each season but was extended to June 30, 2025 for the 2021–2022 season.↩︎