Due to a decline in infection rate, the team at Oxford University developing a Covid-19 vaccine believe that the chances of the trial yielding "no result" is now 50 per cent, The Telegraph reported.
The University of Oxford last week announced that the advance human trial of the vaccine will involve up to 10,260 volunteers across the UK.
While explaining when the results of the trial will be available, the university said that to assess whether the vaccine works to protect from Covid-19, the statisticians in the team team will compare the number of infections in the control group with the number of infections in the vaccinated group.
For this purpose, it is necessary for a small number of study participants to develop Covid-19.
"How quickly we reach the numbers required will depend on the levels of virus transmission in the community. If transmission remains high, we may get enough data in a couple of months to see if the vaccine works, but if transmission levels drop, this could take up to 6 months," the university said.
This is the reason why recruitment of those who have a higher chance of being exposed to the SARS-CoV-2 virus is being prioritised, such as frontline healthcare workers, frontline support staff and public-facing key workers, in an effort to capture the efficacy data as quickly as possible.
"It's a race against the virus disappearing, and against time," Professor Adrian Hill, director of the university's Jenner Institute, told the Telegraph.
"We said earlier in the year that there was an 80 per cent chance of developing an effective vaccine by September. But at the moment, there's a 50 per cent chance that we get no result at all."
The professor told the newspaper that if fewer than 20 of the 10,000 volunteers in the trial test positive, the results may be useless.
However, Professor Andrew Pollard, head of the Oxford Vaccine Group, last week assured that "the clinical studies are progressing very well".
Earlier, drugmaker AstraZeneca finalised its licence agreement with Oxford University for the recombinant adenovirus vaccine.
The licensing of the vaccine, formerly ChAdOx1 nCoV-19 and now known as AZD1222, follows the recent global development and distribution agreement with the University's Jenner Institute and the Oxford Vaccine Group.