There are critical gaps in the evidence for the effectiveness of licensed influenza vaccines in the USA, according to a meta-analysis published Online First in the Lancet Infectious Diseases.
"Evidence for consistent high-level protection is elusive for the present generation of vaccines, especially in individuals at risk of medical complications or those aged 65 years or older. The ongoing health burden caused by seasonal influenza and the potential global effect of a severe pandemic suggests an urgent need for a new generation of more highly effective and cross-protective vaccines that can be manufactured rapidly", explains Michael Osterholm from the University of Minnesota, USA, lead author of the study.
However, he cautions: "In the meantime, we should maintain public support for present vaccines that are the best intervention available for seasonal influenza."
In 2010, the USA expanded annual vaccination recommendations to include all individuals aged 6 months or older with the trivalent inactivated vaccine (TIV), or live attenuated influenza vaccine (LAIV) for healthy non-pregnant people aged 20 years.
Numerous studies have evaluated influenza vaccines against non-specific outcomes such as reduction in all cause deaths, or influenza-like illness. But, these less specific benefits provided by vaccination have potentially overestimated the effect of influenza vaccines.
To establish a more reliable estimate of the magnitude of benefit, the researchers analysed the pooled results of 31 studies published over a 40-year period with influenza-specific outcomes. The researchers included only studies using laboratory-confirmed influenza infections with highly specific diagnostic tests (RT-PCR or viral culture).
Overall, they found that the TIV (which accounts for 90% of influenza vaccination in the USA) had only 59% efficacy against influenza in healthy adults.
By contrast, LAIV showed significant protection against infection in young children, preventing influenza in 83% of children aged 7 years or younger. However, the Advisory Committee on Immunization Practices (ACIP) does not currently recommend LAIV over TIV in these children.
The median effectiveness of the monovalent pandemic vaccine was 69% against medically attended pH1N1 influenza in people aged less than 65 years.
The authors note: "This amount of protection is not adequate for a pandemic setting where the antigen match is ideal and antigen drift has not occurred. The difference between 69% effectiveness and 90% effectiveness (or greater) will have a major public health effect in any pandemic that causes serious morbidity or increased mortality."
In a Comment, Heath Kelly from Victorian Infectious Diseases Reference Laboratory, Melbourne, Australia, and Marta Valenciano from EpiConcept, Paris, France say: "Now might also be an appropriate time to use revised estimates of the most probable effectiveness of influenza vaccines to re-examine the effectiveness and cost-effectiveness of some policy options. This re-examination would need to be done in conjunction with studies that, similar to the new meta-analysis of the effect of influenza vaccines, use highly specific laboratory-confirmed outcomes to assess influenza burden."
Professor Michael Osterholm, University of Minnesota, Minnesota, USA. T) 1-612-626-6770 E) mto@umn.edu
Dr Heath Kelly, Victorian Infectious Diseases Reference Laboratory, Melbourne, Australia. Contact via e-mail. E) heath.kelly@mh.org.au
Journal
The Lancet Infectious Diseases