Flu viruses spread differently around the world

15 Jun

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 world.

man with flu
There are four types of virus that cause seasonal flu in humans.

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.

In summing up the findings Dr. Barr says:

“[…] while the spread of influenza in Asia is a good indication of which H3N2 virus might spread

worldwide, it’s only part of the solution.”

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


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 strains.

Written by Catharine Paddock PhD