December 5, 2017

Information from CDC


There has been a bit of discussion about the New England Journal of Medicine (NEJM) commentary on the pursuit for a universal influenza vaccine. Summit partners have requested talking points on vaccine effectiveness (VE) specific to the final part of the commentary which reflects, “it is possible that we will experience low VE against influenza A (H3N2) viruses and a relatively severe influenza season if they predominate.” Additionally, there is some interesting discussion in the commentary regarding the role of egg-adaptation in vaccine effectiveness.


CDC Talking Points on Influenza Vaccine Effectiveness
Currently, the CDC does not have talking points specific to the paper (see above) that is generating the recent press about Australia’s low VE and what that might mean for the US this season. However, there is a lot of information on the CDC website that Summit partners might find helpful, including influenza VE and what some of the issues are with VE for H3N2. Should more specific materials be developed, Summit partners will be informed.

These two questions are of relevance:

Does flu vaccine effectiveness vary by type or subtype?
Yes. The amount of protection provided by flu vaccines may vary by influenza virus type or subtype even when recommended flu vaccine viruses and circulating influenza viruses are alike (well matched). Since 2009, VE studies looking at how well the flu vaccine protects against medically attended illness have suggested that when vaccine viruses and circulating flu viruses are well-matched, flu vaccines provide better protection against influenza B or influenza A (H1N1) viruses than against influenza A (H3N2) viruses. A study [505 KB, 10 pages] that looked at a number of VE estimates from 2004–2015 found average VE of 33% (CI = 26%–39%) against H3N2 viruses, compared with 61% (CI = 57%–65%) against H1N1 and 54% (CI = 46%–61%) against influenza B viruses. VE estimates were lower when vaccine viruses and circulating viruses were different (not well-matched). The same study found pooled VE of 23% (95% CI: 2% to 40%) against H3N2 viruses when circulating influenza viruses were significantly different from (not well-matched to) the recommended influenza A(H3N2) vaccine component.

Why is flu vaccine typically less effective against influenza A(H3N2) viruses?
There are a number of reasons why flu vaccine effectiveness against influenza A(H3N2) viruses may be lower.

    1. While all influenza viruses undergo frequent genetic changes, the changes that have occurred in influenza A(H3N2) viruses have more frequently resulted in differences between the virus components of the flu vaccine and circulating influenza viruses (i.e., antigenic change) compared with influenza A(H1N1) and influenza B viruses. That means that between the time when the composition of the flu vaccine is recommended and the flu vaccine is delivered, H3N2 viruses are more likely than H1N1 or influenza B viruses to have changed in ways that could impact how well the flu vaccine works.


  1. Growth in eggs is part of the production process for most seasonal flu vaccines. While all influenza viruses undergo changes when they are grown in eggs, changes in influenza A(H3N2) viruses tend to be more likely to result in antigenic changes compared with changes in other influenza viruses. These so-called “egg-adapted changes” are present in vaccine viruses recommended for use in vaccine production and may reduce their potential effectiveness against circulating influenza viruses. Other vaccine production technologies, e.g., cell-based vaccine production, or recombinant flu vaccines, could circumvent this shortcoming associated with the use of egg-based candidate vaccine viruses in egg-based production technology, but CDC also is using advanced molecular techniques to try to get around this short-coming.

Seasonal Influenza Surveillance

CDC has released Seasonal Flu Key Points for December 1, 2017. In addition, the FluView report for Week 47 (ending November 25) has been published.

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