Scrambled Egg Virus: mutations explain ineffectiveness of 2012 flu vaccine


A report published in PLoS ONE last week has shed light on why the effectiveness of the 2012 flu vaccine proved to be well below expectations.

Despite assurances by the US Centres for Disease Control and Prevention (CDC) in November 2012 that “most (90%) of influenza viruses…are well matched to the 2012-2013 influenza vaccine,” post season analysis found that the 2012 vaccine was just 46% effective in 18-49 year olds against the dominant strain that year (H3N2), 50% in 50-64 year olds, and a mere 9% in those over the age of 65.

So why was the vaccine (and perhaps even subsequent vaccines) so poorly matched to the virus strain?

The authors, led by Danuta M. Skowronski, report that three mutations, resulting from the process through which viruses are prepared for mass production, were to blame for the vaccines lack of effectiveness.

In order to prepare a flu vaccine, first the World Health Organisation isolate the particular flue strains, before sending them off to lab to be modified to improve their ability to multiply and grow during the manufacturing process. This involves a process known as ‘reassorting’ where by the wild flu strain, in this case H3N2, is combined with a flu strain known for its high growth yield – strands of H1N1 are often used for this purpose.

The process seeks to replicate the efficiency of the H1N1 virus, whilst keeping the surface antigens (the part that is recognised by our immune system) identical to the wild H3N2 strain. The process through which this is achieved involves the co-infection of an egg with the wild flu virus and the high yield strand, and then a series of incubation and re-infection cycles. Once ready, the new high yield strains of H3N2 are sent to the CDC for testing against the original wild strain, before being sent out to manufacturers.

What the report in PLoS ONE claims is that, somewhere along the line, the surface antigens of the H3N2 strain suffered a number of mutations as a result of the reassorting process.

This of course resulted in the ineffectiveness of the mass produced flu vaccines in 2012/13 because our bodies were being trained to fight a strain of H2N3 that was not much of a threat.

These revelations have prompted concerns that the egg incubation method that has been used to produce vaccines since the 1930s may be simply outdated. Nature reports that Michael Osterholm, head of the University of Minnesota’s Centre for Infectious Disease research and Policy in Minneapolis believes that “this has happened before,” and that “it’s another reason why we need to accelerate efforts to come up with really game-changing flu vaccines.

Find out more about how viruses are modified to produce higher yields here.

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