We followed the bovine potency test protocol described by the OIE to test this subunit vaccine potency. Three groups of five cattle per group and a control group of two non-vaccinated animals were vaccinated. The vaccinated groups were administered different doses (1 dose, 1/3 dose, 1/9 dose)of the subunit vaccine prepared from diluted expression products and all animals were challenged 3 weeks after vaccination with 10,000 BID 50 of Asia I/HNK/CHA/05 to the vaccine under test by intradermal inoculation into two sites on the upper surface of the tongue. The animals were observed daily for 10 days after challenge for clinical signs of FMD. Control animals developed lesions on at least three feet. Unprotected animals showed lesions at sites other than the tongue. From each animal protected in each group, the PD 50 (50% bovine protective dose) content of the vaccine was calculated based on the Reed-Muench method. The effect of network mixing patterns on epidemic dynamics and the efficacy of disease contact tracing In networks, nodes may preferentially contact other nodes with similar (assortatively mixed) or dissimilar (disassortatively mixed) numbers of contacts. Different patterns of contact support different epidemic dynamics, potentially affecting the efficacy of control measures such as contact tracing, which aims to identify and isolate nodes with infectious contacts. We used stochastic simulations to investigate the effects of mixing patterns on epidemic dynamics and contact-tracing efficacy. For uncontrolled epidemics, outbreaks occur at lower infection rates for more assortatively mixed networks, with faster initial epidemic growth rate and shorter epidemic duration than for disassortatively mixed networks. Contact tracing performs better for assortative mixing where epidemic size is large and tracing rate low, but it performs better for disassortative mixing at higher contact rates. For assortatively mixed networks, disease spreads first to highly connected nodes, but this is balanced by contact tracing quickly identifying these same nodes. The converse is true for disassortative mixing, where both disease and tracing are less likely to target highly connected nodes. For small epidemics, contact tracing is more effective on disassortative networks due to the greater resilience of assortative networks to link removal. Multi-step contact tracing is more effective than single-step tracing for assortative mixing, but this effect is smaller for disassortatively mixed networks. For a wide range of epidemic and epizootic diseases, individuals can be usefully modelled as nodes in a network, where the network links represent potentially infectious contacts between individuals. This network representation applies to many complex systems such as the Internet, the World Wide Web (Albert & Barabási 2002) , social and transportation networks (Liljeros et al. 2001; Jones & Handcock 2003; Hufnagel et al. 2004) and livestock movement networks Robinson et al. 2007) . Models that use explicit contact structures between individuals, households, cities, regions, countries or farms have been used to analyse the spread of human diseases, such as SARS (Hufnagel et al. 2004; Meyers et al. 2005) and pandemic influenza (Eubank et al. 2004; Ferguson et al. 2005) , and animal diseases, such as foot-and-mouth disease (FMD; Green et al. 2006a; Kao et al. 2006; Kiss et al. 2006a ) and avian influenza (Le Menach et al. 2006) . Much attention is now focused on analysing network properties and the dynamic processes they support (for reviews, see Newman (2003b) and Keeling & Eames (2005) ), as well as how to effectively control these processes.