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In the U.S., 2022-2023 influenza activity began earlier than usual, increasing in October 2022, and has been associated with high rates of hospitalizations among children.

Influenza A(H3N2) represented most influenza viruses detected and sub-typed during the 2022-2023 Seasonal Influenza Vaccine Effectiveness (VE) period, but A(H1N1) pdm09 viruses co-circulated as well.

“Most viruses characterized were in the same genetic sub-clade as and antigenically similar to the viruses included in the 2022-2023 Northern Hemisphere influenza vaccine. Effectiveness of the influenza vaccine varies by season, influenza virus sub-type, and antigenic match with circulating viruses,” the CDC report sent to www.sursesiresurse.ro reveals.

The latest CDC Interim Estimates of 2022-2023 Seasonal Influenza Vaccine Effectiveness report used data from two concurrent studies conducted at Marshfield Clinic Health System (MCHS) in Wisconsin from October 23, 2022–February 10, 2023, to estimate influenza vaccine effectiveness (VE). Overall, VE was 54% against medically attended outpatient acute respiratory illness (ARI) associated with laboratory-confirmed influenza among patients aged 6 months–64 years. In a community cohort of children and adolescents aged <18 years, VE was 71% against symptomatic laboratory-confirmed influenza A virus infection. These interim analyses indicate that influenza vaccination substantially reduced the risk for medically attended influenza among persons aged <65 years and for symptomatic influenza in children and adolescents.

“Annual influenza vaccination is the best strategy for preventing influenza and its complications. CDC recommends that health care providers continue to administer annual influenza vaccine to persons aged ≥6 months as long as influenza viruses are circulating,” according to the CDC document.

Summary

Influenza activity for the 2022–23 winter season increased earlier than usual, with high rates of influenza-associated hospitalizations among children. The interim estimates of 2022–23 influenza VE (influenza vaccine effectiveness) from two concurrent studies in Wisconsin suggest that the current season’s influenza vaccines provide substantial protection against influenza.

“These findings are consistent with estimates reported in the Southern Hemisphere for the 2022 season and Canada for the current season, where similar viruses predominated. However, influenza vaccination coverage in the United States this season has been lower than during pre–COVID-19 pandemic seasons, particularly among children, pregnant women, and rural areas. Increased vaccination coverage is needed to realize the full potential of seasonal influenza vaccines,” the CDC report shows.

The interim estimates reported reflect early season VE and might differ from end-of-season VE estimates with additional enrollments, or if a change in circulating viruses would occur later in the season.

“Influenza activity was low nationally through the week ending February 4, 2023. However, CDC continues to monitor influenza activity through routine surveillance for any indications that activity might increase again; two waves of influenza activity have occurred during many previous seasons,” the CDC experts wrote.

Seasonal influenza vaccines protect against influenza A and B viruses, both of which might continue or begin to circulate later in the season, resulting in potentially serious complications.

Limitations

The findings in this report are subject to at least four limitations, CDC experts say.

First, the studies were restricted to participants from a single geographic area (central Wisconsin). However, viruses that predominated in the study population were similar to those that predominated across the U.S.

Second, older adults aged ≥65 years were excluded from the test-negative study. Age-specific VE estimates against influenza virus infection caused by A(H3N2) viruses are generally lower for older adults.

Third, sample sizes were small for the interim analysis, which limited the precision of VE estimates, and VE against illness associated with A(H1N1) pdm09 virus infections and the age-specific forecast could not be determined.

Finally, confounding and bias are of concern with observational studies; however, estimates were comparable across two study designs, and the test-negative study design yields valid estimates of influenza VE in most scenarios.

“Annual influenza vaccination is the best strategy for preventing influenza and its complications. During the 2022–23 season to date, influenza A viruses that predominated are genetically and antigenically similar to current vaccine components. Interim VE estimates from this report indicate that the current season’s influenza vaccine substantially reduces the risk for medical visits among persons aged 6 months–64 years and symptomatic illness associated with influenza A virus infection among children and adolescents aged <18 years. Influenza vaccination is recommended for all persons aged ≥6 months for as long as influenza viruses are circulating in the community,” the CDC report concludes.