Live vs Inactivated Influenza Vaccine

The next issue of The Medical Letter begins with an article on influenza vaccine for the 2015-2016 season. Only one paragraph touches on the choice between the standard inactivated vaccine given by injection and the live attenuated vaccine, which is sprayed intranasally. Last year the CDC Advisory Committee on Immunization Practices recommended use of the live-attenuated vaccine over the inactivated vaccine in healthy children 2-8 years old, based on clinical trials conducted in 2002-2003 and 2005-2006, but this year the ACIP is not recommending either vaccine over the other for use in children. Our explanation for the ACIP’s change of heart was that observational data from recent seasons have not found the live-attenuated vaccine to be consistently more effective in children. That is demonstrably true, but there is a good deal more we could have said about it.

The live-attenuated vaccine (LAIV) was moderately to highly effective in preventing laboratory-confirmed influenza caused by A/H3N2 and B strains in 2010-2011, 2011-2012, 2012-2013, and 2013-2014. In 2014-2015, however, a year in which 95% of influenza cases were due to H3N2 viruses, more than 80% of the isolates were substantially drifted from the H3N2 vaccine virus, and LAIV was not effective (the inactivated vaccine was poorly effective, with confidence limits that included zero effectiveness).

In addition, LAIV showed low effectiveness in 2010-2011 and 2013-2014 in preventing influenza in the US caused by the 2009 pandemic strain of H1N1, which is still circulating. But in Canada in 2013-2014, LAIV did protect against the H1N1 2009 pandemic strain. How can that be? The manufacturer believes that the poor performance of LAIV in US studies in 2013-2014 could be explained by the vulnerability of the vaccine virus to heat degradation and the multiple exposures to temperatures higher than 70 degrees F that occur during the normal US distribution process. In 2015-2016, the H1N1 strain in LAIV has been replaced by a more robust strain that is less susceptible to heat degradation.

All things considered, nevertheless, the ACIP decided not to recommend LAIV over inactivated vaccine for use in young children, who would undoubtedly prefer an intranasal spray to an injection. Hopefully this additional information about the background for that decision will help practitioners make their own decisions about which vaccine to choose.

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