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.
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