As flu activity remains elevated in all regions of the United States for the fifth week in a row, a new study quantifies the impact of flu vaccination last season.
As flu activity in the United States hits its highest levels of the season, investigators on a new study report that influenza vaccination during the 2017-2018 flu season prevented an estimated 8000 deaths.
The US Centers for Disease Control and Prevention’s (CDC) weekly FluView update for week 5 ending February 2, 2019, reports that the percentage of people visiting their health care provider for influenza-like illnesses (ILI) was at 4.3%, up from 3.8% the previous week. While the percentage is still below last season’s peak of 7.5%, the CDC’s situation update notes that flu activity remains elevated overall across the United States.
For the fifth week in a row, all 10 geographic regions in the nation reported ILI levels at or above region-specific baselines. During week 5 there were also 4 additional flu-related pediatric deaths reported to the CDC, bringing the total so far for the 2018-2019 flu season to 28 pediatric flu deaths. Although influenza A H1N1 viruses have been most common across the nation, H3N2 viruses are predominating in the Southeast.
With more than 7 million flu cases in the United States so far this season, health officials continue to recommend the annual flu shot as the best way to protect against influenza. In a new study published in the journal Clinical Infectious Diseases, investigators examine the impact of flu vaccination during the 2017-2018 season, in which 42% of the US population received a flu shot. Last season, investigators note, the end-of-season flu vaccine effectiveness rate was only 38% overall, which included rates of 22% effectiveness against influenza A(H3N2), 62% against influenza A(H1N1)pdm09, and 50% against influenza B.
“We should continue to explore influenza vaccines that are more effective, but I think this study quantifies and directly shows that current flu vaccines, despite the need for improved VE, are preventing significant numbers of flu illnesses, hospitalizations, and deaths,” study co-author Melissa A. Rolfes, PhD, told Contagion® in an interview.
To calculate influenza burden estimates, the investigators used data on illnesses, medical visits, and deaths from hospitalized cases that were reported through the Influenza Hospitalization Surveillance Network, with a focus on patients aged 6 months and older.
From there, the investigators estimated the effect of seasonal influenza vaccination on disease burden using a mathematical compartmental model, stratified by age group. The model estimated that influenza vaccination prevented 7.1 million illnesses, 109,000 hospitalizations, and 8000 deaths related to influenza in the United States during the 2017-2018 flu season. Flu vaccination prevented an estimated 10% of expected hospitalizations overall, though in young children aged 6 months to 4 years the benefits of vaccination were greatest, with 41% of all expected hospitalizations prevented by vaccination.
The 2017-2018 flu season was the most severe since the 2009 pandemic, and the investigators stressed the role of the flu vaccine in reducing the impact. “We are currently finalizing a paper to submit for publication that looks at how many more illnesses, medical visits, and hospitalizations could have been prevented if vaccine coverage had been 5% higher or whether we had met the Healthy People 2020 goal of achieving 70% coverage in each age group,” said Dr. Rolfes, noting that the study team hopes to submit the paper to a journal for peer-review in the coming month or 2. “Perhaps not surprising, but what we’re finding is that vaccinating more people would prevent much more flu, but the impact of increasing coverage is a bit different by age. Vaccinating more older adults (those aged 65 years and older) would prevent more hospitalizations and vaccinating working age adults (aged 18-49 years) and children would prevent more illnesses and medical visits.”