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 (AZN.L) vaccine causes blood clots – up from 13% 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% 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 had received it. Many countries have resumed using the shot, but with some restrictions

"The blood clot scare has affected how some of the (UK)public view the AstraZeneca vaccine – but has not reduced confidence in vaccines overall," said Bobby Duffy, director of King's College London's Policy Institute which co-led the study.

"In fact, the trend has been towards increased commitment to get vaccinated – and quickly – as the rollout has progressed so well, with no sign of serious widespread problems."

Both the European Medicines Agency (EMA) and Britain's Medicines and Healthcare Regulatory Agency (MHRA) have reviewed the vaccine's safety in the wake of the blood clot reports. They say no causal link has been established, and that the vaccine's benefits in preventing COVID-19 far outweigh any risks.

The UK survey was conducted between April 1 and 16 and covered 4,896 UK adults aged between 18 and 75. It was designed as a follow-up to a study first conducted late last year to track how and why views of COVID-19 vaccines have changed.

It found that 17% of respondents now say that if they had a choice, they would prefer to have the AstraZeneca vaccine - down from 24% towards the end of March.

It also found that the majority of those asked whether the shot causes blood clots are most likely to say that claim is false, or that they don't know whether it is true.