Study: Impact of Flu Vaccine for 2017-18 Season

Although the 2017-18 influenza season saw a low vaccine efficacy (VE) rating, the flu shot still had a substantial impact, according to a new study.

“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,” researchers wrote in Clinical Infectious Diseases.

The researchers estimated that the flu shot prevented 7.1 million (95% 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. The flu shot prevented 10% of all expected hospitalizations and more than two-fifths (41%) of hospitalizations for children aged 6 months through 4 years.

Among everyone aged six months or older who was eligible for a flu shot, researchers estimated there were about 47.9 million illnesses, 22.1 million medical visits, 953,000 hospitalizations, and 79,400 influenza-associated deaths. Older adults (aged ≥65 years) represented 15% of influenza cases but accounted for 70% of hospitalizations and 90% of deaths.

The highest rates of illness were connected to influenza A(H3N2), which affected 9% of young children (6 months–4 years) and 15% of adults aged 50–64 years. Overall, influenza A(H3N2) was linked to about 28.4 million illnesses, 13 million medical visits, 587,000 hospitalizations, and 49,000 deaths. Influenza B and influenza A(H1N1)pdm09 virus infections were connected to 15.7 million and 4.6 million illnesses, respectively.

The researchers evaluated VE against different influenza strains and found it was 22% (95% CI: 12–31%) against influenza A(H3N2), 62% (95% CI: 50–71%) against influenza A(H1N1)pdm09, and 50% (95% CI: 41–57%) against influenza B.

“Prevented illnesses included 2.3 million illnesses due to A(H3N2) viruses and 1.4 million illnesses due to A(H1N1)pdm09 viruses; 48% and 70% of which, respectively, were prevented among children. Additionally, over 3 million illnesses from influenza B viruses were prevented with vaccination,” the researchers wrote.

The study authors concluded, “At this time, vaccination remains an important component of influenza prevention; and our results indicate that current vaccines prevented a substantial burden of illness during the 2017–2018 influenza season.”

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Source: Clinical Infectious Diseases