Tuesday, 14 July 2009

Confirmed: Swine flu more virulent than Common Flu

At least on animals. Swine flu effects on humans are very clear for
everyone to see for those honest enough to study based on facts,
instead on hunches or common cold, which is completely different from
flu, common or swine.

Swine flu: just how virulent is it?

Swine flu As two more UK deaths reported today --a 6-year-old girl and
a GP -- brought Britain's total toll to 17, it is becoming clearer and
clearer that it's wrong to see swine flu as a mild disease. Yes, it's
nowhere near as lethal as the Spanish Flu of1918-19, but that doesn't
mean it's OK to rush out and organise a swine flu party (not that
anyone necessarily is -- Martin Robbins has taken down the evidence
for that rather nicely). The H1N1 virus can be extremely nasty, and we
shouldn't be complacent about it.

Swine flu's pathogenic potential was illustrated two weeks ago in two
papers published by Science, which Ed Yong has summarised rather
nicely. Their verdict -- that animal models suggest swine flu is more
virulent than seasonal H1N1 strains -- has today been endorsed by a
more extensive piece of research by Yoshihiro Kawaoka and colleagues,
published by Nature and freely available online.

Kawaoka's study shows that the CA04 swine flu strain, isolated from a
patient in California, replicates more easily than H1N1 in the lungs
than seasonal H1N1 in ferrets, mice and macaque monkeys. It also
inflicts greater damage on the lungs, creating larger lesions and
penetrating as far as the alveoli. The scientists speculate that this:

"might contribute to a viral pneumonia characterised by diffuse
alveolar damage that contributes to hospitalisations and fatal cases
where no underlying health issues exist."

This is animal work, but the models, particularly ferrets and
macaques, are those that best mimic flu infections in people. The
message should be pretty obvious. It's misleading to think of this as
a mild pandemic -- it's mild only in comparison to 1918, and
complacency is dangerous.

That said, however, and without questioning the rigour of the Nature
study, it's important to be aware of a couple of caveats. Speaking
this afternoon to Wendy Barclay, Professor of Virology at Imperial
College, London, she pointed out two factors that mean swine flu might
not be quite as bad in the field as it looks in the animal-testing

First, there are the doses involved. Kawaoka et al infected their
animals with between 104 and 106 particles of virus. Professor Barclay
said that most people who become infected with swine flu are exposed
to the order of just 10 viral particles -- several orders of magnitude
fewer -- and this may explain why symptoms are not generally serious.
Healthy people's immune systems can fight the virus off, and while
they do fall ill, they don't require hospital treatment and they don't
die. These experiments may be more representative of what happens when
immunocompromised people, or those with other underlying health
problems, get sick with swine flu.

The second issue is the comparison that's involved. In Kawaoka's
paper, and in the two papers published in Science last month, swine
flu's effects were compared to those of a seasonal H1N1 virus. But,
Professor Barclay said, it's been acknowledged for a while that H1N1
seasonal flu doesn't generally cause severe infections. H3N2 flu is
the type that causes more serious seasonal cases, and no comparison
has yet been made between it and swine flu.

None of this means that swine flu isn't serious, and that it isn't
capable of causing serious illness and death. It is. But the fact that
it's had these worrying effects in animal studies is not necessarily a
cause for especially grave concern. It's not a mild illness, but
neither does it seem especially severe. Perhaps "moderate" is the best

No comments: