Discussion

Early and widespread influenza activity during the 2017–18 influenza season provided the opportunity to estimate interim VE against several circulating influenza viruses, including the predominant A(H3N2) virus. These interim estimates reflect ongoing challenges with the A(H3N2) vaccine component since the 2011–12 season. 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 (4,5) and is similar to final (32%) VE estimates in the United States against A(H3N2) viruses during 2016–17§ (6). However, among children aged 6 months through 8 years, the interim estimates against any influenza and A(H3N2) virus infection were higher; the risk for A(H3N2) associated medically-attended influenza illness was reduced by more than half (59%) among vaccinated children. Also, with interim VE estimates of 67% and 42% against influenza A(H1N1)pdm09 and B viruses, respectively, 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. CDC continues to recommend influenza vaccination while influenza viruses are circulating in the community; 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 2014–15 season when interim VE estimates were similar to those reported here. 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 (3).

VE estimates against A(H3N2) viruses have been lower than estimates against A(H1N1)pdm09 and B viruses for several years (7). Although there is no definitive evidence for antigenic drift of viruses circulating this season compared with cell culture–propagated reference viruses representing the A(H3N2) vaccine component (3), challenges with antigenic characterization of recent A(H3N2) viruses, many of which could not be characterized using traditional hemagglutination inhibition assays, have required the use of additional virus neutralization assays to assess antigenic characteristics. Multiple factors might be contributing to the reported VE against A(H3N2) viruses this season. Immune responses to vaccination differ by age and previous infection or vaccination history and can affect vaccine protection; higher VE against A(H3N2) viruses among young children suggests that vaccination might provide better protection against circulating A(H3N2) viruses to this age group. Also, genetic changes in the vaccine virus hemagglutinin protein that arise during passage in eggs might result in a vaccine immune response that is less effective against circulating viruses (8,9). Human serologic data indicate decreased inhibition of circulating cell culture–propagated A(H3N2) viruses compared with egg-propagated viruses among persons vaccinated with egg-based vaccines.¶ Additional studies are needed to assess whether VE against circulating A(H3N2) viruses varies by vaccine type, including comparisons between egg-based and non–egg-based vaccines. CDC will continue to monitor VE through the remainder of the season and is investigating these factors. In addition, many efforts are under way to improve selection and development of candidate vaccine viruses that are optimal for vaccine production and provide protection against a majority of circulating viruses.

These interim VE estimates underscore the need for influenza antiviral treatment for any patient with suspected or confirmed influenza who is hospitalized, has severe or progressive illness, or is at high risk for complications from influenza, regardless of vaccination status or results of rapid, point-of-care influenza diagnostic tests.** CDC recommends antiviral medications as an adjunct to vaccination, and their potential public health benefit is increased in the context of low VE. A CDC health update issued December 27, 2017, regarding treatment with antiviral medications is available at https://emergency.cdc.gov/han/han00409.asp. Clinicians should be aware that influenza activity is widespread, and influenza should be considered as a possible diagnosis in all patients with acute respiratory illness.

The findings in this report are subject to at least four limitations. First, vaccination status included self-report at four of five sites. End-of-season VE estimates based on updated documentation of vaccination status might differ from interim estimates. Second, information from medical records and immunization registries is needed to evaluate VE by vaccine type and for fully vaccinated versus partially vaccinated children, as well as to evaluate the effects of previous season vaccination and timing of vaccination; end-of-season analysis of VE by vaccine type and effects of partial or previous season vaccination is planned. Third, an observational study design has greater potential for confounding and bias relative to randomized clinical trials. However, the test-negative design is widely used in VE studies and has been used by the U.S. Flu VE Network to estimate VE for previous influenza seasons. Finally, small sample sizes in some age groups resulted in wide confidence intervals, and end-of-season VE estimates could change as additional patient data become available or if there is a change in circulating viruses late in the season. It is also important to note that the VE estimates in this report are limited to the prevention of outpatient medical visits rather than more severe illness outcomes, such as hospitalization or death; data from studies measuring VE against more severe outcomes will be available at a later date.

Annual monitoring of VE supports ongoing efforts to improve influenza vaccines. Although more effective vaccines are needed, vaccination prevents a substantial burden of influenza-related illness annually. During the 2014–15 season, when VE against medically attended illness caused by any influenza virus was less than 20%, vaccination was estimated to prevent 11,000–144,000 influenza-associated hospitalizations and 300–4,000 influenza-associated deaths (https://www.cdc.gov/flu/about/disease/2014-15.htm). Small increases in VE can substantially affect the number of hospitalizations prevented during a severe season (10). Although interim estimates suggest that vaccination has prevented some influenza-related illness this season, influenza vaccines with improved effectiveness are needed to substantially reduce the incidence of disease.