Oxford University, have been put on hold after a participant had a suspected adverse reaction in the UK.AstraZeneca described it as a "routine" pause in the case of "an unexplained illness".The outcome of vaccine trials is being closely watched around the world.

The AstraZeneca-Oxford University vaccine is seen as a strong contender among dozens being developed globally. Hopes have been high that the vaccine might be one of the first to come on the market, following successful phase 1 and 2 testing.Its move to Phase 3 testing in recent weeks has involved some 30,000 participants in the US as well as in the UK, Brazil and South Africa. Phase 3 trials in vaccines often involve thousands of participants and can last several years.At first glance this may seem alarming. A vaccine trial - and not just any vaccine, but one receiving massive global attention - is put on hold due to a suspected serious adverse reaction. But such events are not unheard of. Indeed the Oxford team describe it as "routine". Any time a volunteer is admitted to hospital and the cause of their illness is not immediately apparent it triggers a study to be put on hold.

This is actually the second time it has happened with the Oxford University/AstraZeneca coronavirus vaccine trial since the first volunteers were immunised in April. An Oxford University spokesperson said: "In large trials, illnesses will happen by chance but must be independently reviewed to check this carefully."A final decision on restarting the trial will be taken by the medical regulator the MHRA, which could take only days. But until then all international vaccination sites, in the UK, Brazil, South Africa and the USA are on hold. The companies, including AstraZeneca, Pfizer Inc and GlaxoSmithKline, issued what they called a "historic pledge" after a rise in concern that safety standards might slip in the face of political pressure to rush out a vaccine.

The vaccine, called AZD1222, uses a weakened version of a common cold causing adenovirus that has been 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.