Credit NIAID
# 7873
Influenza viruses are constantly changing, which is why seasonal flu vaccines must be adjusted nearly every year in an attempt to keep up with their evolution. NIAID, part of the NIH, has a short (3 minute) video on Youtube that nicely illustrates how flu viruses change antigenically over time, and eventually mutate so that the current flu vaccine is no longer effective.
As you would expect, in the wild, avian and swine influenza viruses undergo similar changes.
Small, incremental changes are called antigenic drift, and they occur more or less all of the time. Most of these changes don’t convey any evolutionary advantage, and quickly disappear. Occasionally, a `more biologically fit’ virus emerges, and blows away the competition.
More drastic changes can come by way of antigenic shift, also called reassortment. Shift occurs when one virus swap out chunks of their genetic code with gene segments from another virus. While less common than drift, antigenic shift can quickly create a brand new `hybrid virus’. One that can sometimes have pandemic potential.
For shift to happen, a host (human, swine, bird) must be infected by two influenza different viruses at the same time.
While successful reassortment is relatively rare, as any virologist will tell you . . . Shift happens.
H5N1 (aka `bird flu’) is not a single strain of influenza, but rather, a group of similar viruses sharing the same HA (hemagglutinin) and NA (neuraminidase) genes segments, that has continually evolved and mutated since its emergence in 1996.
You can see the evolution of the virus through 2011 in the chart below, starting with Clade 0, first detected in 1996.
Over the years more than 20 genetically separate clades of the virus have emerged (not all continue to circulate), with many minor variants of each clade thrown in the mix.
Different areas of the world have seen different clades set up residence, with clade 2.3.2 (and now 2.3.2.1) very common in South East Asia, clades 2.2.1 and 2.2 endemic in Egypt and clades 2.1.1, 2.1.2. and 2.1.3 circulating in Indonesia.
The problem is, vaccines based on older clades may not provide significant protection against infection with one of the newer clades. So the World Health Organization, NIH, CDC, the Vaccine Manufacturing community, and others are pitted in an ongoing `arms race’ against a constantly changing set of viruses.
And we now have more than just H5N1 to concern ourselves with. There are multiple clades or variants of zoonotic H7N9, H9N2, H7N7, H3N2v, et. al. circulating around the world.
The list is long, and it grows longer each year.
Which brings us to a long report today, that appears in the World Health Organization’s Weekly Epidemiological Record (h/t @Ironorehopper) One that looks at the strains of these zoonotic viruses that are currently circulating, and the development of candidate vaccine viruses against them.
Although I’ve posted some small excerpts below, I would recommend that you follow the link to read the report and its accompanying charts in their entirety:
The development of representative candidate influenza vaccine viruses, coordinated by WHO, remains an essential component of the overall global strategy for pandemic preparedness. Comparisons of the candidate vaccine viruses with respect to antigenicity and their relationship to newly emerging viruses are ongoing and will be reported periodically by WHO.
(1) Influenza A(H5N1)
Since their re-emergence in 2003, highly pathogenic avian influenza A(H5N1) viruses have become enzootic in some countries and continue to cause outbreaks in poultry as well as sporadic human infections. The A(H5N1) viruses have diversified both genetically and antigenically leading to the need for multiple candidate vaccine viruses for pandemic preparedness purposes. This summary provides updates on the characterization of A(H5N1) viruses isolated from birds and humans, and the current status of the development of influenza A(H5N1) candidate vaccine viruses.
Given its duration, and continued evolution, not surprisingly the WHO is proposing that 4 new H5N1 candidate vaccine viruses be developed.
Based on the available antigenic, genetic and epidemiologic data, A/duck/Bangladesh/19097/2013-like (clade 2.3.2.1), A/duck/Viet Nam/NCVD-1584/2012-like (clade 2.3.2.1) and A/Cambodia/W0526301/2012-like (clade 1.1) candidate vaccine viruses are proposed.
The WHO has previously selected candidate vaccine viruses for H3N2v, H9N2, H7N7, H7N3, and H7N9, and have not proposed work on any new vaccine strains for these flu types in this latest report.
