Vaccine effectiveness modest for 2011-2012 flu season

The influenza vaccine administered during the 2011-2012 season had low effectiveness against influenza A(H3N2), the predominant strain seen that season.

The overall vaccine effectiveness was 47%, researchers from the University of Michigan School of Public Health reported in Clinical Infectious Diseases.

“Although there was drift in the A(H3N2) viruses, we also demonstrated an apparent negative effect of repeated annual vaccination on effectiveness,” the researchers wrote. “Clearly, this phenomenon needs to be examined in other years when different strains are part of the vaccine, and, if present, a mechanism needs to be identified.”

For this study, 4,771 patients with medically attended acute respiratory illness beginning in early January 2012 were included. Among these, 681 (14.3%) had confirmed influenza. The most common influenza type was A(H3N2), with 440 patients. The remainder were A (H1N1)pdm09 (n=110) and influenza B (n=131).

When stratified by age category, the highest vaccine effectiveness was 58% (95% CI, 27-76) among children aged 9 to 17 years and the lowest was 43% (95% CI, –18 to 72) among adults aged at least 65 years. Among fully immunized children aged younger than 9 years, the vaccine effectiveness was 51% (95% CI, 27-68) compared with 18% (95% CI, –48 to 55) for partially immunized children.

Against A (H1N1)pdm09, the vaccine effectiveness was 65% (95% CI, 44-79), but the effectiveness against A(H3N2) was 39% (95% CI, 23-52). For influenza B, the vaccine effectiveness was 58% (95% CI, 35-73). For influenza B, the estimate was lower against the Victoria lineage included in the vaccine than against the Yamagata lineage not included in the vaccine.

The researchers also identified an interaction between current season (2011-2012) vaccination status and prior season (2010-2011) vaccination. Among those who were vaccinated in the prior season, the vaccine effectiveness of the current season was 33% (95% CI, –1 to 56). But among those who were not vaccinated in the prior season, the effectiveness was 56% (95% CI, 37-69).

Kathleen Neuzil

In an accompanying editorial, Kathleen Neuzil, MD, MPH, and John C. Victor, PhD, of PATH, Seattle, said that vaccine effectiveness studies such as this one help guide decisions about vaccine use and help focus research for improved vaccines. These studies are necessary to guide policy decisions and should be expanded.

“Larger multiyear prospective studies that include not only measures of the full spectrum of clinical illness caused by influenza but also immunologic measures may be required to allow us to understand the dynamic of the immune system in response to influenza vaccine and natural infection,” they wrote. “It is only through careful investigation that we can properly assess current vaccines and guide the development of better vaccines and delivery strategies.”

For more information:

Neuzil K. Clin Infect Dis. 2013;doi:10.1093/cid/cit739.

Ohmit S. Clin Infect Dis. 2013;doi:10.1093/cid/cit736.

Disclosure: Neuzil and Victor report no relevant financial disclosures. The study researchers report relationships with BioCryst, Cubist, GlaxoSmithKline, Inviragen, Medimmune, Merck, Novartis, Optimer, Sanofi and Sunovian.