The firm is developing the vaccine with Oxford University.
"The vaccine has to work and that's one question, and the other question is, even if it works we have to be able to demonstrate it".
Vaccine developed by Oxford University was also in the clinical trial stage, but latest reports claim that the vaccine has failed to prevent infection in animals.
Nevertheless he admitted that AstraZeneca and Oxford University are in a race against time as the coronavirus seems to be fading rapidly in the UK.
Government approval is now allowing researchers to move ahead with the second and third phases of testing, which will see more people across a wider age range - including older adults and children - tested.
The professor leading Oxford University's highly-anticipated coronavirus vaccine trial has poured cold water on much of the hype surrounding the project by warning that it has only a 50 percent chance of being successful.
Professor Adrian Hill, director of the Jenner Institute, told the paper that if the virus's spread is too low not enough of the volunteers will catch it and the trial will be unable to definitively say if the vaccine works.
Hill told The Telegraph newspaper that the number of people in the United Kingdom with the virus was falling at a rate that meant there might not be enough people to test the experimental vaccine known as ChAdOx1 nCoV-19.
Yesterday, this was followed up by a figure dwarfing the UK's commitment, as the US Biomedical Advanced Research and Development Authority (BARDA) offered up to $1.2bn (€1.1bn) to support to expansion of manufacturing and development activities for the vaccine.
On its chances of working, Hill said, "We still think they're fairly high but not guaranteed".
Leading virologist Shahid Jameel said India's vaccine manufacturing capacity is quite remarkable and at least three Indian companies - Serum Institute, Bharat Biotech and Biologicals E are at the forefont, working with global partners to manufacture a vaccine for COVID-19.
However, Pollard told BBC radio that "it is not possible to predict" when the potential vaccine could be ready.
The effectiveness of the vaccine is still to be determined.
Dr Rajeka Lazarus, a consultant in infectious diseases and microbiology at UHBW and the principle investigator for the study in Bristol, said: "Currently there are no licensed vaccines or specific treatments for COVID-19".
"The researchers will evaluate the immune response to the vaccine in people of different ages, to find out if there is a variation in the response of the immune system in older people or children", he said in a statement.
World Health Organization had recently announced that eight companies were front runners of developing the vaccine for coronavirus.