Feb 21, 2013 (CIDRAP News) – Influenza vaccine over the past few months has provided moderate protection overall but didn't have a significant impact on protecting seniors, a study from the US Centers for Disease Control and Prevention (CDC) revealed today.
The new estimate is based on adjusted results from a study network from Dec 3 through Jan 19, which showed an overall vaccine effectiveness (VE) of 56%, which was similar to an estimate earlier in the season of 62% and a level found in a recent careful meta-analysis of randomized controlled trials. The findings appear in today's issue of Morbidity and Mortality Weekly Report (MMWR).
The latest report is the first of the season to look at VE by age-group, and it found that for adults age 65 and older VE was not statistically significant.
The ongoing study to track the vaccine's effectives involves five centers in the CDC's US Influenza Vaccine Effectiveness Network, located in Wisconsin, Michigan, Pittsburgh, Texas, and Washington. The latest estimate includes data from 2,697 children and adults who were seen in doctor's offices for acute respiratory illnesses and asked if they had been vaccinated against flu.
A total of 1,115 (41%) of the study participants tested positive for flu by polymerase chain reaction; 751 (67%) were infected by type A and 366 (33%) by type B. Researchers found that 98% of influenza A infections were caused by H3N2, which has been the dominant strain in the United States and is typically linked to more serious illnesses.
Investigators found that 32% of the patients with lab-confirmed flu had received their flu vaccine, compared with 50% of those who tested negative. Those numbers produce an overall VE of 56% (95% confidence interval [CI], 47% to 63%).
A more detailed look at how the vaccine performed against influenza A and B found varied results. The vaccination rate among type A case-patients was 37%, versus 25% among type B case-patients. The VE for type A was 47%, compared with 67% for type B.
The authors concluded that the estimates confirm moderate effectiveness in preventing doctor's visits for flu and are similar to the earlier US estimates, plus similar findings from Canada and Europe. They found that protection against influenza was consistent across all age-groups, but protection against influenza A (H3N2) dropped off for older people, a pattern also detected in a recent interim VE report from Europe.
VE in those 65 and older against both influenza A and B was 27% (95% CI, -31% to 59%), and against H3N2 it was 9% (95% CI, -84% to 55%), but both numbers are statistically not significant.
The United States had an early and vigorous flu season that has been especially hard on seniors, who have had high levels of hospitalization from the disease. Concerns about the disease in seniors this year have renewed reminders from health officials about usefulness of early antiviral treatment.
One possible reason for the findings in seniors is that some may have not mounted an effective immune response to the vaccine's H3N2 component, the group wrote. "Nonetheless, this finding should not discourage future vaccination by persons aged ≥65 years, who are at greater risk for more severe cases and complications from influenza."
They cautioned that more data are needed to fully evaluate VE in older adults.
"Although imperfect, influenza vaccines remain the best tool currently available for preventing illness from influenza," the authors emphasized.
Mark Thompson, PhD, who coauthored the report and presented the VE findings today to members of the CDC's Advisory Committee on Immunization Practices (ACIP), said the CDC is confirming what clinicians and state labs have been seeing: flu infections in immunized people, especially seniors.. Thompson is with the CDC's National Center for Immunization and Respiratory Diseases (NCIRD).
"We clearly need better and more effective vaccine and strategies," he said.
Flu expert Michael T. Osterholm, PhD, MPH, said the findings aren't surprising and are consistent with patterns elsewhere, as well findings of a recent meta-analysis in Lancet Infectious Diseases that he coauthored. Osterholm is director of the University of Minnesota's Center for Infectious Disease Research and Policy, which publishes CIDRAP News,
A larger report, the CIDRAP Comprehensive Influenza Vaccine Initiative (CCIVI) report, released last October, also highlighted the vaccine protection gaps and called for a major effort to develop better flu vaccines.
He said now is the time to take a serious look at flu vaccine shortcomings, such as VE issues and waning protection. "All these new pieces of information need to be incorporated into our recommendations and looked at in terms of how to best use the tool that we have," Osterholm said.
"So far there is no national or international strategy for bringing a better vaccine forward," he said. "There won't be a game-changing vaccine by wishing, public relations, or with the current funding mechanisms."
CDC. Interim adjusted estimates of seasonal influenza vaccine effectiveness—United States, February 2013. MMWR 2013 Feb 22;62(7):119-23 [Full text]
Jan 11 CIDRAP News story "CDC's early-season look finds flu vaccine 62% effective"
Feb 14 CIDRAP News story "Euro studies show flu vaccine working best against type B"
Oct 15, 2012, CIDRAP News story "Report: Complacency, misperception stymie quest for better flu vaccines"