CDC: The US 2021-2022 Influenza Season Was Mild and Driven by A(H3N2), a Strain Included in 2022-2023 Vaccines

Lorraine L. Janeczko, MPH

August 08, 2022

During the 2021-2022 flu season, influenza activity in the United States was lower than pre-pandemic levels despite increased reporting and testing, possibly owing to COVID-19 precautions, according to a new report from the US Centers for Disease Control and Prevention (CDC).

For the 2022-2023 season, CDC has updated recommendations for the annual vaccine to include a closer antigenic match to the A(H3N2) subclade that predominated in the 2021-2022 US season, the report notes.

"The 2021-22 influenza season was mild," lead author Angiezel Merced-Morales, MPH, and colleagues write in Morbidity and Mortality Weekly Report. "Influenza activity continued from October 2021 through mid-June 2022, with A(H3N2) viruses predominating throughout the season."

"Since SARS-CoV-2 emerged in the United States in early 2020, influenza activity has been lower than that seen before the pandemic," add the authors, from the CDC's Influenza Division of the National Center for Immunization and Respiratory Diseases.

"The adoption of COVID-19-related mitigation measures might have had an impact on the timing or severity of influenza activity," they write.

Collaborative Influenza Data Collection

The US influenza surveillance system involves collaborations between CDC and state, local, and territorial health departments; clinical and public health laboratories; vital statistics offices; healthcare providers; and hospitals, emergency departments, clinics, and long-term care facilities.

CDC used the cumulative rates of influenza-related hospitalizations reported through FluSurv-NET, as well as a mathematical model to estimate the number of people with symptomatic influenza who had a medi­cal visit, were hospitalized for, or died of influenza.

Using data available from early October 2021 through mid-June 2022, CDC estimated that influenza virus infection resulted in 8-13 million symptomatic illnesses, 3.7-6.1 million medi­cal visits, 82,000-170,000 hospitalizations, and 5000-14,000 deaths.

Virus Surveillance

Clinical laboratories tested more than 2.8 million respiratory specimens for influenza virus. Overall, 4.5% specimens tested positive, including 98.6% for influenza A and 1.4% for influenza B. Each week, between 0.1% and 9.9% of the specimens tested positive for influenza.

Of the nearly 900,000 specimens tested in public health laboratories, 2.8% were positive for influenza virus. Of these, 99.5% were positive for influenza A viruses and 0.5% were positive for influenza B viruses.

Among over 19,000 seasonal influenza A virus specimens that were subtyped, 0.1% were influenza A(H1N1)pdm09 and 99.9% were influenza A(H3N2).

Of the A(H3N2) viruses with age data available, the proportions reported were 10% in people aged 0-4 years, 51% in those aged 5-24 years, 28% in those aged 25-64 years, and 11% those aged 65 years or older. The numbers of reported A(H1N1)pdm09, B/Victoria, and B/Yamagata viruses were too low to analyze by age group.

Influenza Mortality and COVID-19

COVID-19 continued to dominate the deaths from pneumonia, influenza, and/or COVID-19 (PIC). According to the National Center for Health Statistics Mortality Surveillance System, the weekly percentage of PIC deaths remained above the epidemic threshold — set at 1.645 standard deviations above the seasonal baseline — during the entire 2021-2022 season.

Of the 387,112 PIC deaths, 71.6% of death certificates listed COVID-19 as an underlying or contributing cause of death and 0.6% listed influenza. These numbers indicate that PIC-related deaths were due primarily to COVID-19, not to influenza.

All 31 laboratory-confirmed influenza-related pediatric deaths reported to CDC were linked with an influenza A virus infection, and all 13 influenza A viruses with subtyping information were identified as A(H3N2).

Effective Antiviral Agents

All of the influenza viruses collected and tested for antiviral resistance by CDC were susceptible to zanamivir, and over 99% were susceptible to baloxavir, oseltamivir, and peramivir.

2022-2023 Influenza Vaccine Composition

The US Food and Drug Administration's Vaccines and Related Biologic Products Advisory Committee has selected the composition of the 2022-2023 influenza vaccines on the basis of the World Health Organization's recommended Northern Hemisphere 2022-2023 influenza vaccine composition.

The committee updated the recommended A(H3N2) component for the 2022-2023 influenza vaccine to one that belongs to the subclade that predominated in the US during the 2021-2022 season, 3C.2a1b.2a.2. They made no changes to the A(H1N1)pdm09 or the B/Yamagata egg-based, cell-based, or recombinant vaccine recommended components.

Kevin McCarthy, PhD

Commenting on the report to Medscape Medical News, Kevin McCarthy, PhD, assistant professor of microbiology and molecular genetics at the University of Pittsburgh Center for Vaccine Research, said that influenza seasons are notoriously unpredictable.

"Behavioral changes brought about by the SARS-CoV-2 pandemic changed long-standing patterns of influenza circulation," he noted in an email. "This year, the peaks of infection were blunted, the overall severity of the season was diminished, and the season was elongated.

"Our vaccines for the 2022-2023 season have been chosen to match viruses currently circulating during the protracted tail end of the 2021-2022 season," he said. "Perhaps this will help us more closely match the circulating and vaccine viruses.

"As we return to something closer to the pre-pandemic world, will influenza return to its long-held trends?" asked McCarthy, who was not involved in developing the report. "Unfortunately, we will only know after next year's report."

"Regardless of what the future holds," he advises, "receiving the 2022-2023 vaccine is the best way to prepare."

The authors agree.

"Receiving a seasonal influenza vaccine each year remains the best way to protect against seasonal influenza and its poten­tially severe consequences," they write.

Influenza surveillance reports for the US are published online weekly at https://www.cdc.gov/flu/weekly, and more information about influenza viruses, surveillance, vaccines, antiviral medications, and novel influenza A infections is at https://www.cdc.gov/flu.

The authors and McCarthy report no relevant financial relationships.

Morbidity and Mortality Weekly Report. Published online July 22, 2022. Full text HTML and PDF

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