Early findings on development of Mexican swine flu reported in new
13 May 2009
Researchers from the Medical Research Council Centre for Outbreak
Analysis and Modelling at Imperial College London, working in
collaboration with the World Health Organisation and public health
agencies in Mexico, have assessed the A(H1N1) flu epidemic using data to
the end of April. Their key findings are as follows:
- the data so far is very consistent with what researchers would
expect to find in the early stages of a pandemic.
- the researchers' best estimate is that in Mexico, influenza A
(H1N1) is fatal in around 4 in 1,000 cases, which would make this
strain of influenza as lethal as the one found in the 1957 pandemic.
The researchers stress that healthcare has greatly improved in most
countries since 1957 and the world is now better prepared.
- the epidemic of influenza A (H1N1) is thought to have started in
Mexico on 15 February 2009. The data suggests that by the end of
April, around 23,000 people were infected with the virus in Mexico
and 91 of these died as a result of infection. However, the figures
are uncertain — for example, some mild cases may have gone
unreported. The numbers infected could be as low as 6,000 people or
as high as 32,000 people.
- the uncertainty around the numbers of people who have been
infected with influenza A (H1N1) in Mexico means that the case
fatality ratio (CFR) of 0.4% (4 deaths per 1000) cannot be
definitely established. The CFR is in the range of 0.3% to 1.5%, but
at this stage the researchers believe that 0.4% is the most likely.
- For every person infected, it is likely that there will be
between 1.2 and 1.6 secondary cases. This is high compared to normal
seasonal influenza, where around 10-15 percent of the population are
likely to become infected. However, it is lower than would be
expected for pandemic influenza, where 20-30 percent of the
population are likely to become infected.
- In an outbreak in an isolated village called La Gloria,
Mexico, children were twice as likely to become infected as adults,
with 61% of those aged under 15 becoming infected, compared with 29%
of those over 15. This may suggest that adults have some degree of
immunity against infection, because of having been previously
infected with a related strain of influenza, or it may mean that
children are more susceptible to infection because they interact
much more closely together, for example in school, than adults.
Professor Neil Ferguson, the corresponding author of today's research
from the MRC Centre for Outbreak Analysis and Modelling at Imperial
College London, said: "Our study shows that this virus is spreading just
as we would expect for the early stages of a flu pandemic. So far, it
has been following a very similar pattern to the flu pandemic in 1957,
in terms of the proportion of people who are becoming infected and the
percentage of potentially fatal cases that we are seeing.
"What we're seeing is not the same as seasonal flu and there is still
cause for concern — we would expect this pandemic to at least double the
burden on our healthcare systems. However, this initial modelling
suggests that the H1N1 virus is not as easily transmitted or as lethal
as that found in the flu pandemic in 1918," added Professor Ferguson.
Christophe Fraser et al.
Pandemic Potential of a Novel Strain of Influenza A (H1N1): Early
Findings. Science, 11 May 2009.
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