Interim Influenza Vaccine Effectiveness Against …

Discussion

Analysis of state-level California surveillance data from influenza vaccination and laboratory reporting systems indicates that influenza vaccination provided moderate protection against laboratory-confirmed influenza across all age groups during October 2023–January 2024. Influenza immunization has been demonstrated to avert complications and severe outcomes associated with influenza, including illness, hospitalization, and death (2,5); therefore, annual influenza vaccination is recommended for all persons aged ≥6 months (3). Measured influenza VE in this analysis was highest among children and adolescents and lowest among older adults, a finding that has been observed in previous seasons (6). This finding suggests that influenza vaccination for adults aged ≥50 years is less effective than among other age groups. Vaccination of adults aged ≥50 years remains a high priority given their increased risk for severe influenza, even if estimated VE is lower for older adults compared with younger persons (6).

Influenza vaccination and laboratory reporting requirements allowed for early season California VE estimates using routine surveillance data. Although the outcome of interest in this analysis (positive influenza laboratory test result) is not one used by CDC VE platforms, the results of this study can be interpreted alongside those from established platforms to broaden characterization of VE. In addition, these VE estimates are consistent with current CDC VE estimates and previous season VE calculations of 40%–60% when influenza viruses are well matched to influenza vaccine components.** Integrated community influenza surveillance and immunization data were used to calculate early season VE estimates during the current (2023–24) influenza season in Canada, and this approach might be feasible for jurisdictions that have similar availability of immunization and laboratory surveillance data (7).

In-season early VE estimates that reflect protection against any positive influenza test result can complement established methods that estimate VE against other outcomes or in other settings (8). For example, during a year when mismatched influenza viruses predominate, knowledge of low VE might support communication surrounding additional protective and treatment measures against influenza (e.g., social distancing, promoting proper cleaning and disinfection procedures, and use of antivirals) when indicated. Preparations in response to low VE might be especially important in hospitals and long-term care facilities, including measures such as active surveillance for acute respiratory illness among residents and limiting personal contact (9). Earlier interim VE estimates might also help guide midseason disease forecasting by providing timely updates of a critical modeling parameter.

Limitations

The findings in this report are subject to at least seven limitations. First, 2023 was the first year during which reporting of negative influenza test results to CDPH and influenza vaccination to a centralized IIS was mandated. This limitation might have resulted in incomplete reporting and potential bias in observed VE. Second, it was not possible to determine whether infants and children aged <9 years, for whom 2 doses of seasonal influenza vaccine are recommended during their first influenza season (5), were fully or partially vaccinated. Third, accompanying symptom information was not available for any influenza laboratory results to differentiate VE against asymptomatic infection or symptomatic illness. Estimates might be biased if characteristics of persons who were tested and those who were not differ. Fourth, information on testing setting (e.g., outpatient, inpatient, or intensive care unit) and illness severity was not available to measure VE for specific outcomes (e.g., hospitalization or death). Fifth, these findings are limited to California and might not be generalizable across the entire United States; multiple influenza viruses circulate, and the proportion of circulating viruses and testing practices by area might differ, as might VE against each influenza virus type or subtype. Sixth, subtype information was not available to estimate VE against influenza A(H1N1)pdm09 and A(H3N2) viruses, though >80% of influenza A strains characterized this season at California public health laboratories are H1N1(pdm09), similar to national data.^†† Finally, other sources of confounding and bias were not assessed, including the presence of preexisting conditions in the study population that might affect the likelihood of severe influenza, propensity to seek health care and influenza testing, or influenza vaccination behaviors that are differential with respect to infection status (10).

Implications for Public Health Practice

Surveillance data can be used to calculate interim VE quickly and efficiently at the state level. Early VE estimates might allow for more timely public health action for influenza prevention and treatment recommendations. Interim VE estimates from this analysis indicate that the current seasonal influenza vaccine protects against receipt of a positive influenza laboratory test result among persons aged ≥6 months. Annual influenza vaccination is recommended for all persons aged ≥6 months to prevent illness and adverse complications associated with influenza. Influenza vaccination remains the best way to prevent influenza.