CDC Presents Updated Estimates of Flu Vaccine Effectiveness for the 2014-2015 Season

Flu vaccine did not protect against drifted H3N2 viruses, but protected against vaccine-like H3N2 and B viruses

On February 26, 2015, updated interim influenza (flu) vaccine effectiveness (VE) estimates for the current 2014-2015 season were presented to the Advisory Committee on Immunization Practices (ACIP). The updated VE estimate against influenza A H3N2 viruses was 18% (95% confidence interval (CI): 6%-29%).This result is similar to the VE point estimate of 23%, which was reported in a January 16 Morbidity and Mortality Weekly Report (MMWR) and confirms reduced protection against H3N2 viruses this season. The VE estimate against influenza B viruses this season was 45% (95% CI: 14% – 65%).

How well the flu vaccine works can vary depending on a number of factors, including the similarity between circulating influenza viruses and vaccine viruses, and the age, health or immune status of the person vaccinated. The findings for VE against H3N2 viruses this season are about one-third of the VE expected when the flu vaccine is well matched to circulating influenza viruses. The VE against influenza B viruses this season is similar to the effectiveness observed when vaccine viruses and most circulating viruses are well matched.

Reduced protection against H3N2 viruses this season has been attributed to the fact that more than two-thirds of circulating H3N2 viruses analyzed at CDC are drifted from the H3N2 vaccine virus recommended for vaccine production. The proportion of drifted viruses at the U.S. VE study sites was even higher (>80%).

These updated estimates were derived from data collected from the U.S. Flu VE Network from November 10, 2014, through January 30, 2015, and include an additional four weeks of data in comparison to CDC’s early VE estimates released in mid-January.

When VE against all influenza viruses was combined, the overall VE estimate was 19% (95% CI: 7%– 29%). In practical terms, this means the flu vaccine reduced a person’s risk of having to seek medical care at a doctor’s office for flu illness by 19%.

None of the VE estimates by age for this season are statistically significant at this time. Possible explanations for this include: the flu vaccine is having a small effect or there are insufficient samples sizes at this point to produce estimates by age group. Final estimates will be published at the conclusion of the season. It is possible that estimates will change as the season progresses. Influenza activity is declining but remains elevated in the United States and an increasing proportion of influenza B viruses has been detected in recent weeks.