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Swine flu's first
genetic analysis reveals hopefully lower
rate of transmission London:
While swine flu virus H1N1 continues to spread around the world, the first genetic
analysis of the efficacy of its transmission from person to person revealed that
it spreads barely well enough to keep itself going. The analysis also suggested
that the virus might have started circulating as long ago as January. However,
because of the scarcity of cases to analyse, the calculation is still uncertain,
as many believe that the circulation could have started more recently, or as far
back as September. Nicholas Grassly of Imperial College London and Andrew Rambaut
of the University of Edinburgh, UK, have analysed the rate of spread. Their analysis
is based on the small mutations that have accumulated in almost two dozen genetic
sequences produced so far, from viruses collected from patients in Mexico and
the US. Unlike H5N1 bird flu, all the genetic sequences of this H1N1 are being
posted on bulletin boards like GISAID, so that scientists can access them and
compare preliminary analyses. Scientists who protested that H5N1 sequences were
not being made freely available set up the GISAID system in 2006. "The limited
sampling so far gives rise to considerable uncertainty in the estimate," New Scientist
quoted Rambaut as saying. However, if the rate at which genes mutate is similar
for this virus as for other H1N1 viruses, the number of mutations that have accumulated
so far have indicated that it has been circulating since January - or even September
2008. If the new virus spreads from one infected person to the next at about the
same speed as ordinary flu, it could give an idea of how many cases there may
have been in that time. A mathematical model permits the calculation of an important
variable called R0 - the number of additional people infected, on average, by
each case. If R0 is less than one, an infection dies out. Also, Grassly cautioned
that the estimate is very preliminary. However, with newly available data, he
gets an R0 of 1.16 - enough for the virus to keep going, but only just. This comes
as good news, as epidemiological theory suggests that the lower the R0, the easier
it may be to snuff the virus out by further hindering its spread. And now the
onus lies on how quickly the new H1N1 virus from swine adapts to people. -May
2, 2009
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