The complexity of understating the spread of influenza was again highlighted in a paper in Eurosurveillance recently. In this paper the authors have looked at antibody levels and antibody quality to H3N2 and H1N1 influenza A viruses from pigs that have been known to spread to humans. H and N; or haemagglutinin and neuraminidase to give them their full names, are two molecules found on the outside of the influenza virus, and it is changes in these molecules which mean that it is necessary to revaccinate against influenza each year. They constantly change in a process called antigenic drift, and sometimes change suddenly in a process known as antigenic shift; the latter often occurring when an animal virus passes into and then spreads between humans.
This is a particular problem with influenza because unusually humans share influenza viruses with a variety of animals with which we have frequent contact, namely birds and pigs. A sudden antigenic shift, as occurred in 2009 with the introduction of a new H1N1 virus into the human population can spread because, being a new virus, there will be little immunity in humans. The important thing to remember here is that it is not the jump into humans that is the problem (unless you are that human of course); but the subsequent transmission between humans. That was the difference between avian influenza in 2005 and swine influenza in 2009, the former did not transmit widely between humans and the latter did. Incidentally the latest virus to worry the world, H7N9 has made the first step, and there are tentative signs that it might have made the second, being transmitted between humans, but so far only a very small number of very close contacts.
Understanding the make-up of the H and N of viruses that are likely to circulate in the human population is important to ensure that vaccine can be made and that preparations can be put in place for health services. However, it is a bit more complicated than just characterising the H and N numbers, since not all H1N1 for example are the same.
In this study the authors looked at antibody titres (levels) to different strains of influenza of pig origin, remembering that the last pandemic strain came from pigs. When they looked at a variety of different H1N1 and H3N2 viruses what they found was that there were marked differences in antibody levels in different age groups to different viruses. For example, those born between 1968 and 1999 had high levels to one type of H3N2, but very low levels to another more recent variety. These differences reflect exposure to viruses over time, and have been seen before, for example older people tended to be less at risk of catching the last H1N1 pandemic virus because of existing immunity from previous exposure to a similar virus.
What we learn from this is that influenza should probably not be considered to be a single virus, but a large and ever changing family of viruses that can affect different groups in different ways. The other thing that comes across is that immunity, like knowledge, is a life-long affair. Just as we learn and collect knowledge, so we learn and collect immunity. The key is to avoid damage while we are collecting it, which once again emphasises the importance of age and group-specific vaccination policies that reflect the ever-changing world of influenza. If you want to know more about the UK situation, Public Health England is the place to look. It is worth keeping an eye on this page, because the situation is constantly changing.