An outbreak of equine influenza that affected vaccinated horses led the British racing authorities to cancel races for 6 days. Three vaccinated horses that participated in races tested positive for the disease, meaning that a large number of other horses were potentially exposed to the highly contagious virus. Racing was suspended on 7 February, resuming on 13 February after more stringent vaccine requirements were put in place.
More details on the outbreak and its affect on British horse racing can be found at the BBC here and here.
More information on equine influenza is available in the Merck Veterinary Manual.