http://www.nature.com/nsu/021223/021223-1.html

Foot-and-Mouth would have jumped fence
Vaccine model suggests a shot in the rump is worth two in the head.
23 December 2002
TOM CLARKE

Six million farm animals died in Britain's 2001 foot-and-mouth outbreak.

Ring-fence vaccination would not have halted Britain's devastating 2001
epidemic of foot-and-mouth disease (FMD), a new analysis suggests1.

Blanket prophylactic vaccination or targeting high-risk farms first are the
best ways to avert or minimize future outbreaks of FMD, say the report's
authors, Matthew Keeling of the University of Warwick, UK, and colleagues.

Similar tactics, they reckon, could work for other animal and even human
diseases, such as influenza and smallpox.

The heat from pyres of burning carcasses has died down, but argument about
the management of Britain's worst-ever FMD outbreak has not. More than 6
million animals, mainly cows, sheep and pigs, were slaughtered before the
outbreak ended.

Vaccination wasn't used because of pressure from farmers. Under current
European law, vaccinated animals cannot be bought or sold.

New tests for distinguishing vaccinated animals from infected ones, and
strategies such as those proposed in the new study, show that it's "time for
the rule book to catch up with the science", comments veterinary
immunologist Ian McConnell of the University of Cambridge, UK.

Action replay

Based on data from the 2001 outbreak, Keeling and his colleagues designed a
model that predicted the infectious state of every farm in Britain each day.
They then replayed each day of the seven-month outbreak with four different
vaccination strategies.

Nation-wide vaccination of all animals before an outbreak offered the
greatest protection. Localized culling on and around infected farms was
still required, as there are always gaps in vaccine coverage.

Mass vaccination in response to an outbreak was the next best strategy. The
FMD vaccine takes seven days to confer immunity, so extensive culling was
again required. In the model, this strategy knocked four months off the
outbreak.

Ring-fence vaccination - where farms surrounding infected hotspots are
treated to contain the disease - failed because of the
vaccination-to-immunity lag and the rapid movement of animals before, and
for a time after, the outbreak was detected. Without a complete movement ban
on animals and people, says Keeling, "sparks of infection jump out of the
rings" and propagate the epidemic.

The researchers also used their model to identify a 'predictive' vaccination
strategy to be used in response to an epidemic. This tactic targets the
farms at greatest risk. In the model it shortened the outbreak almost as
much as mass vaccination.

Assuming that the necessary information about farms and animal movements is
at hand before an outbreak, the predictive approach is preferable, says
McConnell. Mass vaccination requires large amounts of vaccine and systems
for delivering it. In most parts of Europe, which suffer only occasional
outbreaks, it doesn't make economic sense. Predictive vaccination is "a
precise strategy", he says.

The poor performance of ring-fence vaccination in Keeling's study suggests
that it might fail for other diseases too. Infectivity and immunization are
different for different diseases, but "all have this transmission pattern",
says Keeling. Indeed, recent research showed that ring-fence vaccination
probably wouldn't contain smallpox2.


References
Keeling, M. J., Woolhouse, M. E. J., May, R. M., Davies, G. & Grenfell, B.
T. Modelling vaccination strategies against foot-and-mouth disease. Nature,
Published online, doi:10.1038/nature01343 (2002). |Article|
Kaplan, E. H., Craft, D. L. & Wein, L. M. Emergency response to a smallpox
attack: The case for mass vaccination. Proceedings of the National Academy
of Sciences, 99, 10935 - 10940, (2002). |Article|