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Flu vaccine was 38% effective last season, 22% effective against H3N2

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February 6, 2019

Melissa A. Rolfes, PhD, MPH
Melissa A. Rolfes

The influenza vaccine was 38% effective overall during the 2017-2018 influenza season in the United States, and 22% effective against the predominant circulating virus, influenza A(H3N2), researchers reported.

Last year’s influenza season was severe, especially among older adults and young children. The CDC reported that 48.8 million illnesses, 22.7 million medical visits, 959,000 hospitalizations and 79,400 deaths — including 183 confirmed pediatric deaths — were caused by influenza in 2017-2018.

Writing in Clinical Infectious Diseases, Melissa A. Rolfes, PhD, MPH, epidemiologist for the CDC’s influenza division, and colleagues noted that the 2017-2018 influenza season had the “highest morbidity and mortality since the 2009 pandemic.” They noted that only a reported 42% of the U.S. population received an influenza vaccination.

According to their estimates, influenza vaccination prevented an estimated 7.1 million illnesses, 3.7 million medical visits, 109,000 hospitalizations and 8,000 deaths last season.

There are indications that this season’s vaccine may be more effective. In January, researchers in Canada reported that the vaccine is 72% effective against this season’s predominant circulating virus, influenza A(H1N1)pdm09. In the U.S., interim estimates of the vaccine’s effectiveness should be available sometime in mid-February, a CDC spokeswoman previously told Infectious Disease News.

Using data from the U.S. Influenza Vaccine Effectiveness Network, Rolfes and colleagues estimated vaccine effectiveness against medically attended, RT-PCR-confirmed influenza virus infection. Influenza type/subtype-specific burden was estimated using multipliers applied to population-based rates of influenza-associated hospitalizations, and estimates of influenza-associated outcomes prevented by vaccination were determined using a compartmental model, the researchers explained.

According to the study, H3N2 was associated with the highest rates of illness, with the virus type affecting 9% of children aged 6 months to 4 years and 15% of adults aged 50 to 64 years.

“After applying these rates to the U.S. population, influenza A(H3N2) was associated with an estimated 28.4 million illnesses, 13.0 million medical visits, 587,000 hospitalizations, and 49,000 deaths overall,” Rolfes and colleagues wrote.

According to study results, the vaccine reduced the risk for medically attended influenza by 38% (95% CI, 31%-43%). Broken down by type, vaccine effectiveness was estimated to be 22% (95% CI, 12%-31%) against H3N2, 62% (95% CI: 50%-71%) against H1N1 and 50% (95% CI: 41%–57%) against influenza B.

The vaccine prevented 10% of expected hospitalizations, Rolfes and colleagues estimated. When assessed by age group, vaccination prevented 41% of expected hospitalizations among children aged 6 months to 4 years, the highest of all the age groups.

“While public health could benefit from improved influenza vaccines, increases in coverage could also offer significant public health benefits,” Rolfes told Infectious Disease News. “Getting patients vaccinated is the most important way to protect them from flu and its potentially serious complications. We are currently in the 2018-19 influenza season and it is not too late to get an influenza vaccine. As long as influenza is circulating, we encourage clinicians to continue to recommend influenza vaccination to their patients who haven’t already been vaccinated.”

In a related editorial, Infectious Disease News Editorial Board member Kathleen M. Neuzil, MD, MPH, professor of medicine and pediatrics and director of the Center for Vaccine Development at the University of Maryland School of Medicine, and Meagan C. Fitzpatrick, PhD, research associate at the University of Maryland School of Medicine, evaluated the performance of the influenza vaccine using the expression, “Is the glass half empty or is the glass half full?”

They explained that for vaccine performance to be correctly interpreted, the “size of the glass” — or the total amount of disease attributable to influenza — must be known. The findings of Rolfes and colleagues indicated that although the vaccine demonstrated partial effectiveness, the burden of disease was high. Therefore, 38% vaccine effectiveness had a “substantial impact,” according to Neuzil and Fitzpatrick.

“Whether your viewpoint on influenza vaccine effectiveness estimates is that of an optimist or a pessimist, the data provided by Rolfes et al. provide further compelling evidence for the benefits of influenza vaccine, as even imperfect vaccines save lives,” they concluded. – by Marley Ghizzone


CDC. Estimated influenza illnesses, medical visits, hospitalizations, and deaths in the United States — 2017-2018 influenza season. Accessed February 5, 2019.

Disclosures: Fitzpatrick, Neuzil and Rolfes report no relevant financial disclosures. Please see the study for all other authors’ relevant financial disclosures.

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