Drugs giant AstraZeneca on Thursday said a Covid-19 vaccine could still be available by as early as the end of the year, despite a randomised clinical trial being paused.

"We could still have a vaccine by the end of this year, early next year," the UK-based company's chief executive Pascal Soriot said in comments at a media event.

AstraZeneca announced Wednesday it had "voluntarily paused" its trial of a drug developed alongside Oxford University after a UK volunteer developed an unexplained illness.

An independent committee was drafted in to review safety but the company said it was a "routine action" designed to maintain the integrity of the trials.

"We will be guided by this committee as to when the trials could restart, so that we can continue our work at the earliest opportunity," Soriot said in a statement.

AstraZeneca's vaccine candidate is one of nine around the world currently in late-stage Phase 3 human trials. In the US, the company began enrolling 30,000 volunteers across dozens of sites on August 31, and smaller groups are being tested in Brazil and elsewhere in South America.

The vaccine, called AZD1222, uses a weakened version of a common cold-causing adenovirus engineered to code for the spike protein that the novel coronavirus uses to invade cells.

After vaccination, this protein is produced inside the human body, which primes the immune system to attack the coronavirus if the person is later infected.

The director of UK scientific research charity the Wellcome Trust, Jeremy Farrar, said there were often pauses in vaccine trials.

He told BBC radio in an interview that it demonstrated the importance of conducting vaccine trials properly, with independent oversight and the involvement of the regulator.

"In the end the public must have absolute trust that these vaccines are safe and of course effective, and in the end will hopefully bring the pandemic to a close," he added.

Britain's chief scientific adviser, Patrick Vallance, on Wednesday said similar events should be expected but progress on vaccines and therapeutics was positive overall.

"Some will read out this year in terms of efficacy and safety," he told a Downing Street news conference. "And I think there's a reasonable chance that... we can think about the possibility of vaccination next year sometime at larger levels."

UK Health Secretary Matt Hancock said the pause was "not necessarily" a set-back, and said a similar pause occurred recently but was "resolved without a problem".