Atlanta—Pharmacists might be facing a difficult sales job trying to immunize Americans who have not yet received flu vaccination.

While the CDC still recommends influenza shots for almost everyone older than age 6 months, it also is reporting relatively low effectiveness of the vaccine, especially against the predominant virus. That information appeared recently in the Morbidity & Mortality Weekly Report.

For the period from November 2, 2017, to February 3, 2018, overall adjusted vaccine effectiveness (VE) against influenza A and influenza B virus infection associated with medically attended acute respiratory infection (ARI) was 36%. During that time, 69% of influenza infections were caused by A(H3N2) viruses, for which VE was estimated to be 25%, according to the CDC. The news was better for A(H1N1)pdm09 viruses, for which the VE was 67%, and influenza B viruses, for which the VE was 42%.

“These early VE estimates underscore the need for ongoing influenza prevention and treatment measures,” public health officials pointed out. “CDC continues to recommend influenza vaccination because the vaccine can still prevent some infections with currently circulating influenza viruses, which are expected to continue circulating for several weeks. Even with current vaccine effectiveness estimates, vaccination will still prevent influenza illness, including thousands of hospitalizations and deaths. Persons aged ≥6 months who have not yet been vaccinated this season should be vaccinated.”

The CDC was able to perform the estimate of interim VE against circulating influenza virus because flu activity has been early and widespread this season. The results weren’t surprising, however.

“These interim estimates reflect ongoing challenges with the A(H3N2) vaccine component since the 2011–2012 season,” the article notes. “The interim estimate of 25% VE against A(H3N2) viruses this season indicates that vaccination provided some protection, in contrast to recently reported, nonsignificant interim estimates of 17% from Canada and 10% from Australia and is similar to final (32%) VE estimates in the United States against A(H3N2) viruses during 2016–2017.”

Researchers also explained that, among children aged 6 months through 8 years, interim VE estimates against any influenza and A(H3N2) virus infection were higher than for adults, emphasizing, “The risk for A(H3N2) associated medically attended influenza illness was reduced by more than half (59%) among vaccinated children.”

In promoting vaccination for those not already immunized, the CDC notes that “vaccination provided substantial protection against circulating A(H1N1)pdm09 viruses, as well as moderate protection against influenza B viruses predominantly belonging to the B/Yamagata lineage, the second influenza type B component included in quadrivalent vaccines.”

Public-health officials caution that several more weeks of influenza activity are likely. “Influenza vaccination has prevented thousands of hospitalizations during previous seasons when influenza A(H3N2) viruses were predominant, including during the 2014–2015 season when interim VE estimates were similar to those reported here,” study authors write. 

“Appropriate use of influenza antiviral medications for treatment of severely ill persons or persons at high risk for complications from influenza who develop influenza symptoms is important, especially among older adults, who currently have the highest hospitalization rates.”
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