LONDON—British enthusiasm for the AstraZeneca COVID-19 vaccine has faded in the past month, reflecting rising unease about its possible links to rare adverse side effects, though overall UK confidence in vaccines is high, an updated survey has found. The survey of almost 5,000 people showed a significant increase in the proportion who said they want to be vaccinated against COVID-19 as soon as possible, but also found that almost a quarter of those asked now believe the AstraZeneca vaccine causes blood clots—up from 13 percent last month. Reports of possible links to very rare blood clots have dented confidence in the AstraZeneca shot, which was developed with Oxford University scientists and shown in trials to be 76 percent effective at preventing symptomatic COVID-19. More than a dozen European countries temporarily suspended its use after reports of blood clots combined with low platelets in a very small number of people who …