#17,766
Although it is still early in our winter respiratory season there are early indications that we may see more of an A/H1N1 dominated flu season - as opposed to A/H3N2 - which would be the first since the COVID-truncated 2019-2020 flu season.
We should see an updated CDC FluView report later today.
While neither subtype is benign, H3N2 often produces the more severe flu seasons - particularly for older adults - while H1N1 tends to impact young adults, adolescents and children the most.
Between H1N1's 3 year absence - and its inevitable antigenic drift over time - it is likely that community immunity to this subtype has dwindled. Seasonal flu vaccines can help offset this immune deficit, but in recent years we've seen a decline in their uptake, particularly in younger (6 mos - 17 yrs) age groups.
The effectiveness of the seasonal flu vaccine (VE) varies from year-to-year, and is largely dependent upon the dominant subtype, how well matched the vaccine viruses are to the dominant strain, and the age, and immune response of the recipient.
Generally speaking, vaccine matching and effectiveness have been better against the less volatile H1N1 subtype (compared to the rapidly evolving H3N2 subtype), and younger people (who tend to have a more robust immune response) are generally better protected than the elderly.
Despite last year being dominated by the more problematic H3N2 virus, this CDC lead research found that receipt of the seasonal flu vaccine reduced the need for Urgent Care, ER, or hospital encounters by nearly 50%.Today we have an analysis of the effectiveness of last year's seasonal flu shot in preventing Influenza-A related visits to Urgent Care, Emergency Departments, and Hospitalizations by children and adolescents < 18.
Vaccine Effectiveness Against Pediatric Influenza-A-Associated Urgent Care, Emergency Department, and Hospital Encounters During the 2022–2023 Season, VISION Network
Katherine Adams, MPH, Zachary A Weber, PhD, MS, Duck-Hye Yang, PhD, Nicola P Klein, MD, PhD, Malini B DeSilva, MD, MPH, Kristin Dascomb, MD, PhD, Stephanie A Irving, MHS, Allison L Naleway, PhD, Suchitra Rao, MBBS, MSCS, Manjusha Gaglani, MBBS ... Show more
Clinical Infectious Diseases, ciad704, https://doi.org/10.1093/cid/ciad704
Published: 16 November 2023 Article history
Abstract
Background
During the 2022–2023 influenza season, the United States experienced the highest influenza-associated pediatric hospitalization rate since 2010–2011. Influenza A/H3N2 infections were predominant.
Methods
We analyzed acute respiratory illness (ARI)-associated emergency department or urgent care (ED/UC) encounters or hospitalizations at three health systems among children and adolescents aged 6 months–17 years who had influenza molecular testing during October 2022–March 2023. We estimated influenza A vaccine effectiveness (VE) using a test-negative approach. The odds of vaccination among influenza-A-positive cases and influenza-negative controls were compared after adjusting for confounders and applying inverse-propensity-to-be-vaccinated weights. We developed overall and age-stratified VE models.
Results
Overall, 13,547 of 44,787 (30.2%) eligible ED/UC encounters and 263 of 1,862 (14.1%) hospitalizations were influenza-A-positive cases. Among ED/UC patients, 15.2% of influenza-positive versus 27.1% of influenza-negative patients were vaccinated; VE was 48% (95% confidence interval [CI], 44%–52%) overall, 53% (95% CI, 47%–58%) among children aged 6 months–4 years and 38% (95% CI, 30%–45%) among those aged 9–17 years. Among hospitalizations, 17.5% of influenza-positive versus 33.4% of influenza-negative patients were vaccinated; VE was 40% (95% CI, 6%–61%) overall, 56% (95% CI, 23%–75%) among children ages 6 months–4 years and 46% (95% CI, 2%–70%) among those 5–17 years.
Conclusions
During the 2022–2023 influenza season, vaccination reduced the risk of influenza-associated ED/UC encounters and hospitalizations by almost half (overall VE 40–48%). Influenza vaccination is a critical tool to prevent moderate-to-severe influenza illness in children and adolescents.
While the current numbers will undoubtedly go up, the CDC's most recent update on vaccination trends (see below) shows that vaccine uptake remains low across all age groups.
Reported on Monday, November 13th, 2023.
- The percent of the population reporting receipt of COVID-19, influenza, and RSV vaccines increased for children and adults since the previous week but remains low.
- The percent of the population reporting receipt of the updated 2023-24 COVID-19 vaccine is 4.9% (95% confidence interval: 3.6-6.1%) for children and 13.9% (12.8-15.0%) for adults 18+, including 30.4% (26.9-33.9%) among adults 65+.
- The percent of the population reporting receipt of a flu vaccine is 32.6% (30.7-34.5%) for children and 34.8% (33.2-36.3%) for adults 18+, including 57.6% (53.7-61.5%) among adults 65+.
- The percent of adults 60+ that report receiving an RSV vaccine is 13.5% (11.4-15.6%).
Although we won't know what kind of flu season 2023-2024 will deliver until sometime next spring, early indications are that it will at least start out as H1N1 dominant, and that suggests that children and adolescents may be disproportionately impacted.
But the vaccine only works if people take it. And and since peaking in 2019-2020 - following a decade of steady yearly gains - we've seen a substantial decline in uptake (see CDC chart below).The good news is that the seasonal flu vaccine is generally more effective against the H1N1 subtype, and that the immune response to vaccination in children and adolescents is usually superior to older adults.
