2021-2022 Season

Summary

What was the 2021-2022 flu season like?

The timing and severity of the 2021-2022 influenza (flu) season was different than most seasons before the COVID-19 pandemic. Though relatively mild, there was more activity during the 2021-2022 flu season than during the 2020-2021 flu season. Activity remained elevated later in the spring than any flu season on record. Nationally, flu activity during the 2021-2022 season began to increase in November and remained elevated until mid-June with two different waves of influenza A(H3N2) virus activity occurring. The first peaked in late December 2021, and the second in April 2022. Prior to the emergence of SARS-CoV-2, flu activity would most commonly begin to increase in October or November and peak in February. Compared with pre-pandemic seasons, flu activity during the 2021-2022 season was elevated much later into the spring with a historically late second national peak in April. While it's not unusual to have two waves of flu activity during a season, usually the predominant flu virus during each wave is different.

When did the 2021-2022 flu season peak?

The 2021-2022 flu season featured two distinct waves of A(H3N2) viruses. The first wave of activity peaked in mid-December throughout the country, but the timing of peak activity during the second wave varied by region ranging from mid-March through May. Notably, the second wave, which peaked nationally in April, occurred much later than has been previously observed.

How many people were hospitalized from flu during the 2021-2022 season?

During the 2021-2022 influenza season, influenza-associated hospitalizations were monitored in two systems: the Influenza Hospitalization Surveillance Network (FluSurv-NET), which covers approximately 9% of the U.S. population, and HHS Protect Hospitalization Surveillance, which consists of reports from all U.S. hospitals. During October 1, 2021, through June 11, 2022, a total of 5,130 laboratory-confirmed influenza-related hospitalizations were reported by FluSurv-NET sites. Activity occurred in two waves with hospitalization rates first peaking nationally during the week ending January 1, 2022, (week 52) at 1.0 per 100,000 population. The second, slightly higher peak, occurred during the weeks ending April 23 and April 30, 2022 (weeks 16 and 17) with a rate of 1.2 per 100,000 population.

The overall cumulative hospitalization rate was 167.5 per 100,000 population with the highest rate among adults aged ≥65 years (50.8), followed by children aged 0–4 years (21.9), adults aged 50-64 years (16.2), children aged 5-17 years (9.0) and adults aged 18-49 (9.1). The majority (96.7%) of influenza-associated hospitalizations, were due to influenza A viruses (99.2% of those subtyped were A(H3N2) viruses). Among those with information about underlying conditions, 93.7% of adults and 65.3% of children reported at least one underlying medical condition.

For the 2021-2022 season, HHS Protect Influenza-Associated hospitalizations were added as a component to monitor severe illness associated with influenza. A total of 74,181 influenza-associated hospitalizations were reported between October 3, 2021, and October 1, 2022, and hospitalizations occurred during two waves of activity. The total cumulative influenza-associated hospitalization rate in HHS Protect was 22.6 per 100,000 population. Similar to FluSurv-NET, the first wave peaked nationally during late December (week ending January 1, 2022), and the second, higher peak occurred during mid-April (the week ending April 23, 2022). Regionally, the timing of the second wave peak varied; regions 6 and 7 (Central and South Central) peaked in mid-March, regions 2, 3, and 5 (New York/New Jersey, Mid-Atlantic and Midwest) peaked in April; and regions 1, 4, 8, 9 and 10 (New England, Southeast, Mountain, West Coast and Pacific Northwest) peaked in May.

How many people died from flu during the 2021-2022 season?

It was estimated that about 5,000 people died from flu in the United States during the 2021-2022 season. CDC estimates deaths in the United States using mathematical modeling. The model used by CDC uses a ratio of deaths to hospitalizations to estimate the total number of influenza-associated deaths from the estimated number of influenza-associated hospitalizations.

To estimate the number of influenza-associated hospitalizations that occur in a flu season, CDC looks at how many in-hospital deaths are observed in the FluSurv-NET surveillance system. The in-hospital deaths are then adjusted for under-detection of influenza. Not all flu-related deaths occur in the hospital, so CDC uses death certificate data to estimate how likely influenza-associated deaths are to occur outside the hospital. CDC looks at death certificates that have pneumonia or influenza causes (P&I), other respiratory and circulatory causes (R&C), or other non-respiratory, non-circulatory causes of death, because deaths related to flu may not have influenza listed as a cause of death. CDC uses information on the causes of death from FluSurv-NET to determine the mixture of P&I, R&C, and other coded deaths to include in its investigation of death certificate data. Finally, once the proportion of influenza-associated deaths that occurred outside of the hospital is estimated, an estimate of the deaths-to-hospitalization ratio is derived from it.

How many children died from flu during the 2021-2022 season?

During October 3, 2021, through October 1, 2022, 44 laboratory-confirmed flu-associated deaths in children were reported to CDC. Forty-two were associated with an influenza A virus infection and, of the 22 influenza A viruses that were subtyped, all were A(H3N2) viruses. Additionally, there were two cases of influenza B virus infection among the reported pediatric deaths. The average age was 6 years, and 25 (57%) children died after admission to a hospital. Among the 41 children with a known medical history, 25 (61%) had at least one underlying medical condition associated with increased risk for flu-related complications. The number of deaths reported to CDC each year prior to the COVID-19 pandemic ranged from 37 (2011-2012 season) to 199 deaths (2019-2020 season). These data have been reported to CDC since 2004, when deaths in children associated with influenza virus infection became nationally notifiable. It is important to note that the actual number of flu-associated deaths in children is likely to be higher than what is reported by states to CDC because not all flu deaths in children are detected/reported.

Flu Activity

What flu viruses circulated during the 2021-2022 season?

The majority of influenza viruses detected during the 2021-2022 flu season were A(H3N2) viruses. Laboratory testing showed the H3N2 viruses that circulated were genetically closely related to the 2021-2022 vaccine virus. While the number of B/Victoria viruses that circulated during the 2021-22 season was small, the majority of the B/Victoria viruses characterized were antigenically similar to the vaccine reference virus.

During the 2021-2022 influenza season (October 3, 2021 – October 1, 2022), 13 human infections with novel influenza A virus were identified. Six A(H1N2)v virus infections were identified (one each in California, Georgia, Michigan, Ohio, Oregon and Wisconsin); five A(H3N2)v virus infections were identified (one each in Michigan and Ohio, and three in West Virginia); and one A(H1)v virus infection was identified (Oklahoma). One avian A(H5N1) virus infection was identified in a person in Colorado with exposure to birds infected with a highly pathogenic avian influenza A(H5N1) virus. The A(H5N1) identification was the first positive test for avian influenza A(H5) virus in a human in the United States.

Flu Vaccines

What flu viruses did the 2021-2022 flu vaccines protect against?

For 2021-2022, recommendations were made for egg-based, cell-based, and recombinant flu vaccines as listed below.

Egg-based vaccine composition recommendations:

• An A/Victoria/2570/2019 (H1N1) pdm09-like virus
• An A/Cambodia/e0826360/2020 (H3N2)-like virus
• A B/Washington/02/2019- like virus (B/Victoria lineage)
• A B/Phuket/3073/2013-like virus (B/Yamagata lineage)

Cell- or recombinant-based vaccine composition recommendations:

• An A/Wisconsin/588/2019 (H1N1) pdm09-like virus
• An A/Cambodia/e0826360/2020 (H3N2)-like virus
• A B/Washington/02/2019- like virus (B/Victoria lineage)
• A B/Phuket/3073/2013-like virus (B/Yamagata lineage)

These recommendations included two updates compared with 2020-2021 U.S. flu vaccines. Both the influenza A(H1N1) and the influenza A(H3N2) vaccine virus components were updated. Compared with the Southern Hemisphere flu vaccine recommendation, this recommendation represented one update and that was to the influenza A(H3N2) component.

What flu vaccines were recommended during the 2021-2022 season?

For the 2021-2022 flu season, there was no recommended preference for any one vaccine over another, and providers could administer any licensed, age-appropriate flu vaccine (IIV, RIV4, or LAIV4).

Vaccine options included:

• Standard dose flu shots.
• High-dose shots for people 65 years and older.
• Shots made with adjuvant for people 65 years and older.
• Shots made with virus grown in cell culture. No eggs are involved in the production of this vaccine.
• Shots made using a vaccine production technology (recombinant vaccine) that do not require having a candidate vaccine virus (CVV) sample to produce. No eggs are involved in the production of this vaccine.
• Live attenuated influenza vaccine (LAIV) – A vaccine made with attenuated (weakened) live virus that is given by nasal spray.

Flu Vaccine Availability

How much flu vaccine was produced and distributed during the 2021-2022 season?

Flu vaccine is provided by private manufacturers, so supply depends on manufacturers. For the 2021-2022 flu season, manufacturers distributed 179.4 million doses of flu vaccine to the U.S. market as of February 25, 2022. CDC provided weekly updates on total flu vaccine doses distributed throughout the 2020-2021 flu season.

Were there delays in the availability of flu vaccine?

Flu vaccine manufacturers did not report any significant delays in national flu vaccine supply or distribution during 2020-2021.

How effective was the 2021-2022 flu vaccine?

Preliminary end-of-season vaccine effectiveness estimates showed that flu vaccines reduced people's risk of mild to moderate flu illness caused by flu A(H3N2)–the most common flu virus this season–by about one-third (35% overall). The U.S. Flu VE Network collected outpatient data on enrolled children and adults aged 6 months and older with acute respiratory illness (fever and a cough) from October 4, 2021, to April 30, 2022. The study used a test-negative design that compared vaccination odds among influenza positive cases and influenza negative controls that did not include people testing positive for SARS-CoV-2. Vaccine effectiveness among people 6 months and older against any influenza A virus infection was 34% with a 95% confidence interval of 19% to 46%.

• Vaccine effectiveness among people 6 months and older against influenza A(H3N2) virus infection was 35% with a 95% confidence interval of 19% to 47%.
• Vaccine effectiveness among people 6 months to 17 years against influenza A(H3N2) virus infection was 44% with a 95% confidence interval of 22% to 60%.
• Vaccine effectiveness among people 18 to 49 years of age against influenza A(H3N2) virus infection was 27%, but not statistically significant with a 95% confidence interval of -3% to 48%.
• Vaccine effectiveness in people older than 50 years could not be measured. Vaccine effectiveness by vaccine type or against H1N1 or influenza B viruses could not be measured.

These estimates were higher than initial estimates published in "Interim Estimates of 2021– 22 Seasonal Influenza Vaccine Effectiveness — United States, February 2022". Preliminary early-season vaccine effectiveness estimates from March 2022 suggested flu vaccines were providing no measurable protection against mild to moderate illness caused by the most common influenza A(H3N2) virus during the 2021-2022 flu season. The earlier estimates were lower than these end-of-season estimates because of the small number of early-season flu cases and bias related to COVID-19 illnesses. In the past, vaccine effectiveness against H3N2 viruses has often been lower than against A(H1N1) and influenza B viruses. These updated preliminary end-of-year vaccine effectiveness estimates are similar to what we have seen against H3N2 viruses in the past.

How many antiviral resistant viruses were detected during the 2021-2022 season?

Antiviral resistance means that a virus has changed in such a way that antiviral drugs are less effective or not effective at all in treating or preventing illnesses with that virus.

CDC conducts ongoing surveillance and testing of influenza viruses for reduced antiviral susceptibility and resistance among seasonal and novel influenza A viruses, and guidance is updated as needed.

CDC assesses susceptibility of influenza viruses to antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir, using next-generation sequence analysis supplemented by other laboratory assays.

The use of adamantanes (amantadine and rimantadine) is not recommended for treatment and prevention of influenza A virus infection because of persistently high prevalence of resistance among circulating influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). During the 2021-2022, 0.2% of the viruses tested had markers associated with reduced susceptibility to the antiviral drugs recommended for treatment of influenza by CDC. Resistance to the adamantane class of antiviral drugs remained widespread among influenza A viruses.

Flu Vaccine Coverage

What vaccine uptake estimates did CDC provide during the 2021-2022 season?

CDC's Weekly Flu Vaccination Dashboard provides preliminary, within-season, weekly influenza vaccination estimates, which are updated during the season as more data become available. Overall, estimates suggested flu vaccination coverage was lower during the 2021-2022 flu season compared with last season, especially among certain groups at increased risk of flu complications, such as pregnant people and children. A detailed summary of flu vaccination coverage for the 2021-2022 season is available.

The dashboard included information on the number of flu vaccine doses distributed in the United States, weekly flu vaccination coverage rates for children ages 6 months to 17 years old, monthly flu vaccination coverage rates among pregnant persons, vaccine coverage data by race and ethnicity, and information on how many flu vaccines were administered in pharmacies and doctor's offices.

The data were updated weekly or monthly, depending on the source, throughout the 2021-2022 influenza season.

How did CDC track weekly flu vaccination coverage among children 6 months – 17 years old?

Flu vaccination coverage among children was assessed through the National Immunization Survey-Flu (NIS-Flu), which provided weekly flu vaccination coverage estimates for children ages 6 months to 17 years old. NIS-Flu is a national random-digit-dialed cellular telephone survey of households conducted during the flu season (October-June). Additional information about NIS-Flu methods and estimates from 2019-2020 season are available.

How did CDC track monthly flu vaccination coverage among pregnant people?

Monthly flu vaccination coverage estimates among pregnant people are based on electronic health record (EHR) data from the Vaccine Safety Datalink (VSD), a collaboration between CDC's Immunization Safety Office and nine integrated health care organizations. Of note, because these estimates are based on data from nine integrated health care systems, they may not be representative of all pregnant people in the United States.

How did CDC track the number of flu vaccines administered at pharmacies and doctor's offices?

CDC tracked the number of flu vaccines administered at pharmacies and doctor's offices by using new sources of vaccination data, including IQVIA data for vaccinations administered in retail pharmacies (e.g., chain, mass merchandise, food stores, and independent pharmacies) and doctor's offices.

When were the first flu vaccine uptake estimates provided during the 2021-2022 season?

The first estimates of flu vaccine uptake for the 2021-22 season were posted on Weekly Flu Vaccination Dashboard on October 7, 2021.

Was this the same kind of vaccine uptake information that has been provided in the past?

For each flu season since 2009-2010, CDC has estimated annual flu vaccination coverage for the United States by using data from several nationally representative surveys: the Behavioral Risk Factor Surveillance System (BRFSS), the National Health Interview Survey (NHIS), and the National Immunization Survey-Flu (NIS-Flu). Internet panel surveys of adults, health care personnel, and pregnant people are also used. CDC will continue to provide end of season estimates of influenza vaccination coverage from these data sources.

For the 2021-22 flu season, CDC provided weekly updates on the number of flu vaccine doses distributed, vaccination coverage estimates for children, and the number of doses administered in pharmacies and doctor's offices. Coverage estimates for pregnant people were updated monthly.

Flu Surveillance Data Updates

Were there any updates in the methods for flu surveillance for 2021-2022?

During the 2021-2022 flu season, there were a few changes to CDC's network of influenza surveillance systems compared with the 2020-2021 season.

CDC added another surveillance system, the HHS Protect Hospitalization Surveillance System, that tracks flu hospitalizations in the United States. Hospitals in all 50 states and U.S. territories are reporting on laboratory testing, capacity and utilization, and patient flows for COVID-19 and number of influenza admissions to the hospital and the ICU. This system is designed to facilitate the public health response to the 2019 Novel Coronavirus (COVID-19) pandemic and gathers information on influenza positive admissions.

CDC also added a surveillance system that tracks influenza in long-term care facility (LTCF) residents through the NHSN Long-term Care Facility COVID-19 Module. CMS-certified LTCFs from all 50 states and U.S. territories report COVID-19 and influenza positive test results among residents and staff and personal protective equipment (PPE) supply data to CDC's NHSN. Influenza data elements include the number of residents and number of staff/personnel with new laboratory-confirmed influenza. The number of LTCFs reporting at least one influenza case among residents to CDC's NHSN and the number of facilities reporting each week are reported at the national and HHS region level.

Finally, CDC retired the map depicting "State and Territorial Epidemiologists Reports of Geographic Spread of Flu." This map had been used to capture an assessment of the geographic spread influenza within each state and jurisdiction. This weekly estimate was suspended for the 2020-2021 flu season because the COVID-19 pandemic impacted the data systems used to generate those estimates. Testing practices and health care seeking behaviors were so different that the previous seasons methods/definitions would not have worked. After discussions with public health partners during the summer of 2021, the decision was made to permanently retire this surveillance component due in part to the fact that the systems used to determine the level of spread remain significantly altered by changes in healthcare seeking behavior and testing changes brought on by the COVID-19 pandemic, and a measure of geographic spread was not necessary anymore given improvements in national influenza surveillance systems in recent years. These improvements include more testing, new surveillance system components, and presenting more data at the state level and sub-state level.