The year we were born in and the strain of flu we were exposed to as children predicts immunity to flu in adulthood, new research suggests.
Until now, scientists thought that the virus you were exposed to as a child was irrelevant in predicting immunity to viruses that are transmitted from animals to humans. However, the latest research carried out by the University of Arizona, in collaboration with the University of California, suggests that the kind of flu virus we are exposed to as children predicts what avian-origin flu virus we will be protected from later in life.
The 1918 flu pandemic killed over 20 million people, more than twice the number of people killed in the First World War. More recently, the Asian H5N1 and H7N9 influenza have caused the highest number of flu-related illness and death worldwide. In May 2013, the World Health Organisation (WHO) reported 131 human cases of H7N9, of which 24 resulted in death. Both of these viruses cause severe respiratory problems, such as pneumonia and respiratory failure.
The scientists involved in the new research examined two types of avian flu strains, H5N1 and H7N9, both of which have the potential of spreading very quickly from animals to humans and among human hosts.
When someone gets a flu virus for the first time, the body produces antibodies that target haemagglutinin. This is a protein that sticks out from the surface of the virus and it causes red blood cells to clump together, or agglutinate.
Viruses differ according to the type of haemagglutinin that they have. There are only two main types of haemagglutinin and each of the 18 known types of influenza A virus falls into one of these two types.
One type of influenza virus includes human H1 and H2 viruses, as well as the avian H5 virus, and the other includes human H3 and avian H7.
Michael Worobey, the head of the Department of Ecology and Evolutionary Biology at the University of Arizona and senior author of the study, suggests that the results of the study are best understood by making the two types of haemagglutinin analogous to two different flavours of lollipop.
He explains further: "In this analogy, let's say you were first exposed to a human 'orange lollipop' flu as a kid. If later in life you encounter another subtype of flu virus, one from a bird and one that your immune system has never seen before but whose proteins also are of a similar 'orange' flavour, your chances of dying are quite low because of cross-protection. But, if you were first infected with a virus from the 'blue lollipop' group as a kid, that won't protect you against this novel, 'orange' strain."
This so-called immunological imprinting seems to depend exclusively on the very first exposure to a flu virus and is quite difficult to reverse.This study helps to explain why some age groups have been more prone to severe or fatal complications from an infection with new flu viruses. For instance, the Asian H5N1 virus has caused the higher mortality rates in people aged 10-19 years old and young adults.
"All sorts of possibilities have been put forth, and here my colleagues from UCLA and I present a strong result showing that whatever other minor factors are at play, there is one really major one, and that is, surprise, surprise, we're not a completely blank slate when it comes to how susceptible we are to these emerging flu viruses," says Michael Woberey.
Knowing this information might help us predict which of the 18 subtypes of influenza will cause the next pandemic, as well as which age groups it will hit the hardest. It also sheds light on why some groups of people were affected harder than others during past pandemics.
Following the lollipop analogy, people born before the late 1960s were exposed to the "blue" virus as kids. Researchers found that these older individuals are resistant to H5N1, which shares a "blue" haemagglutinin, but instead often die from "orange" H7N9. Conversely, those born after the late 1960s are resistant to the "orange" virus but are severely affected by "blue" H5N1.
The results of the study show that there is a 75 per cent chance of protection against severe illness from a "matched" virus that an individual was exposed to as a child, and an 80 per cent protection rate against death.
The study is published in the journal Science.