While we tend to think of seasonal flu as a monolithic, slowly evolving strain, in truth the H1N1 and H3N2 viruses that circulate around the world are made up of numerous clades, and within each of these clades we can find a large number of variants.
Due to antigenic drift (see NIAID Video), new strains are continually emerging and auditioning for a place on the world stage. Fortunately most are less `biologically fit' than their competition, and quickly fade away.
The most recent ECDC Influenza Virus Characterization report describes pH1N1's recent evolution:
Since 2009, the HA genes have evolved, and nine clades have been designated. For well over a year, viruses in clade 6, represented by A/St Petersburg/27/2011 and carrying amino acid substitutions of D97N, S185T and S203T in HA1 and E47K and S124N in HA2 compared with A/California/7/2009, have predominated worldwide with a number of subclades emerging.
Within each these clades and sub-clades, small amino acid substitutions - like D225G (see EID Journal: Emergence of D225G Variant A/H1N1, 2013–14 Flu Season, Florida) which is linked to severe pneumonia - and H275Y, which conveys antiviral resistance - may also emerge.
So, with this degree of global influenza diversity, it is not at all surprising that this past winter - when North America saw one of its mildest flu seasons in recent years - parts of Eastern Europe were being hammered (see ECDC Risk Assessment : Reports Of Severe A(H1N1)pdm09 In Europe).
Small pockets of unusually severe, antigenically drifted, or antiviral resistant flu are often short-lived - and most are destined to go undetected - since the genetic changes responsible for those traits aren't necessarily conducive to better spread.
But minor events, such as the rise in antiviral resistance in seasonal H1N1 virus in 2007, and the emergence of an antigenically drifted H3N2 virus in 2014, show that all big flu changes started small somewhere.
In a story I recounted last in 2014 (see When Influenza Goes Rogue), in December of 1950 a new, particularly virulent influenza virus appeared in Liverpool, England during the midst of what had been a very mild flu season.
The EID Journal article by Viboud C, Tam T, Fleming D, Miller MA, Simonsen L. 1951 influenza epidemic, England and Wales, Canada, and the United States, described the impact:
The 1951 influenza epidemic (A/H1N1) caused an unusually high death toll in England; in particular, weekly deaths in Liverpool even surpassed those of the 1918 pandemic. . . . . Why this epidemic was so severe in some areas but not others remains unknown and highlights major gaps in our understanding of interpandemic influenza.
Mysteriously, while this virus spread to large areas of England, Wales, and parts of eastern Canada and the United States over the next few months, it burned itself out by the end of spring, and did not return the following fall.
While game-changing influenza events are pretty rare, they can happen, and so we pay attention when we see unusual reports anywhere in the world.
Which brings us to a report overnight from Fiji, in the South Pacific. Several weeks ago 3 pregnant women died from severe influenza, and their local labs had difficulty determining the subtype.
We've long known that Pregnancy & Flu Are A Bad Combination, and seasonal influenza can - on occasion - kill even healthy adults. But three deaths in such a short period of time raised alarms.
The Health Ministry sent samples to a WHO collaborating lab in Melbourne, Australia for identification, and Fiji has received thousands of doses of flu vaccine over the past couple of weeks (see Seqirus donates vaccines to Fiji to alleviate increase in influenza).
Overnight the Fiji Times reports that the virus was characterized as `similar to pH1N1' but was a `variation in its genotype and it was more virulent than usual."
Intriguing, but not terribly specific. Hopefully we'll get a more detailed analysis at some point from WHO.
The article goes on to say the local health ministry was `concerned', but that `all efforts were being made to properly deal with the situation.'
You can read the full article HERE.
Given that these deaths occurred in May, and there are no newer reports of severe flu and fatalities, it seems likely that this outbreak has already been brought under control.
But it does serve to remind us that flu is ever changing, and even seasonal flu can sometimes have tragic consequences.
Something to think about the next time you have to decide whether you are going to get your annual flu shot.