Saturday, June 20, 2015

Flu viruses spread differently around the world

A new study published in Nature reveals how differences among seasonal flu viruses and the populations they infect
influence their different patterns of spread around the

There are four types of virus that cause seasonal flu in humans. Every year, drug developers try to predict which strains are likely to dominate in the next flu season so as to create an effective flu vaccine.

A good understanding of the rate and pattern of virus evolution
helps these predictions, as one of the authors, Dr. Ian Barr, of
the World Health Organization (WHO) Collaborating Centre for Reference and Research on Influenza in Melbourne, Australia, explains: "This work represents another piece in the complex puzzle of influenza virus circulation and human infections and provides insights that will help develop better influenza vaccines that match strains circulating in the community."

The four viruses that cause seasonal flu in humans are:
influenza A viruses H3N2 and H1N1, and influenza B viruses
Yamagata and Victoria.

The viruses cause similar symptoms - for instance sudden
fever, tiredness and weakness, dry cough, headache, chills,
muscle aches, sore throat - and they evolve in similar ways.

But what has not been well understood is their different
patterns of spread around the world and what influences them.

H1N1 and B viruses persist locally between epidemics

The authors note that while the global circulation patterns of
H3N2 viruses have been well researched, we know little about
the patterns of the other three types of flu virus: the H1N1 and
the B viruses.

For instance, we know from previous studies that H3N2
viruses circulate all year round in East and Southeast Asia, and
spread to the rest of the world to cause seasonal epidemics.

Before this new study, it was assumed the other types of flu
virus would follow a similar pattern, given they are
fundamentally similar.

But Dr. Barr and colleagues found some surprising differences. They discovered that while local strains of H3N2 viruses die out between epidemics, and new epidemics are seeded from new strains that emerge from East and Southeast Asia every year, strains of H1N1 and the B viruses persist locally between epidemics and are less strongly influenced by new strains emerging from East and Southeast Asia every year.

The researchers also found links between the less frequent global movement of H1N1 and B viruses and other factors, such as slower rates of evolution, a greater likelihood of infecting children than adults, and smaller, less frequent epidemics, compared to H3N2 viruses.

For the study, the researchers analyzed 9,604 hemagglutinin
sequences of human seasonal influenza viruses from 2000 to
2012. Hemagglutinin is a protein on the surface of the flu virus
that plays a key role in determining how infectious it is.

India's role could be as central as China's in global flu spread
The team also made an important discovery about the role of
India in the global spread of seasonal flu.

For some time we have known that China and Southeast Asia have played an important part in influencing the evolution and
spread of seasonal flu viruses.

But it appears - based on the analysis of many samples from
India - that India's role may be as central as China's in
influencing the evolution of new strains of seasonal flu viruses.

The researchers also hope their findings will help toward a
one-shot flu vaccine that provides immunity against all
strains, replacing the need for annual vaccination.

Earlier this year, MNT reported how a discovery about how human immune cells respond to infection by a strain of avian flu may also help with progress toward a one-shot flu vaccine.

The team behind that study - including members from the
University of Melbourne - said their discovery could lead to an
approach that changes cellular memory in the immune system
rather than the more traditional method of targeting specific flu