While the influenza vaccine had relatively low effectiveness in preventing infection during the severe 2017–2018 influenza season in the United States, it dramatically decreased hospitalization and deaths among patients who contracted the flu, according to a new report.
One issue, according to the study in Clinical Infectious Diseases, was a vaccine effectiveness (VE) of only 22% against influenza A(H3N2), which predominated. VE, calculated using medically attended reverse transcription polymerase chain reaction–confirmed influenza virus infection in the ambulatory setting, was 62% (95% CI, 50-71%) against influenza A(H1N1)pdm09 and was 50% (95% CI, 41-57%) against influenza B, according to the CDC researches.
The study team also estimated the number of vaccine-prevented influenza-associated illnesses, medical visits, hospitalizations, and deaths for the 2017–2018 influenza season.
To do that, researchers used national age-specific estimates of 2017–2018 influenza vaccine coverage and disease burden.
The overall VE against outpatient medically attended, laboratory-confirmed influenza was 38% (95% CI, 31-43%), but the vaccine prevented 7.1 million (95% credible interval (CrI): 5.4 million-9.3 million) illnesses, 3.7 million (95% CrI, 2.8 million-4.9 million) medical visits, 109,000 (95% CrI, 39,000-231,000) hospitalizations, and 8,000 (95% CrI, 1,100-21,000) deaths, according to the CDC models.
In fact, researchers reported that vaccination prevented 10% of expected hospitalizations overall, and even more in young children aged 6 months to 4 years, decreasing their expected hospitalizations by 41%.
“Despite 38% VE, influenza vaccination reduced a substantial burden of influenza-associated illness, medical visits, hospitalizations, and deaths in the U.S. during the 2017–2018 season,” the study authors concluded. “Our results demonstrate the benefit of current influenza vaccination and the need for improved vaccines.”
However, the CDC reports that flu vaccination coverage among adults was 37.1% last season, a decrease of 6.2 percentage points from the previous year. While vaccination coverage varied by age group and state, coverage decreased in all age groups and in most states. For children, vaccination coverage with one or more dose of flu vaccine was 57.9%, a decrease of 1.1 percentage point from the previous flu season.
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