There are currently hundreds in development around the world.
In a review published in The Lancet Infectious Diseases journal, researchers from the University of Oxford said a meaningful comparison of different candidates is required to ensure only the most effective vaccines are deployed.
Dr Susanne Hodgson, of the University of Oxford, who is the lead author of the review, said: “It is unlikely that we will see a single vaccine winner in the race against Covid-19.
“Different technologies will bring distinct advantages that are relevant in different situations, and additionally, there will probably be challenges with manufacturing and supplying a single vaccine at the scale required, at least initially.
“Taking a standardised approach to measuring the success of vaccines in clinical trials will be important for making meaningful comparisons, so that the most effective candidates can be taken forward for wider use.”
There are more than 200 vaccine candidates in development around the world, with 44 in clinical trials - including the one by Oxford Uni.
Of the 44, nine are in the phase three stage of clinical evaluation and are being given to thousands of people to confirm safety and effectiveness.
An effective vaccine is one that can act against infection, disease, or transmission – potentially keeping the Covid-19 pandemic under control.
The review authors said that to help compare clinical efficacy between the current and future vaccine candidates, “standardised, quantifiable endpoints” need to be applied to clinical trials and the “limitations and potential for bias” also need to be understood.
They also said long-term follow-up of the Covid-19 vaccine recipients will be needed to evaluate efficacy against severe disease and mortality as well as to ensure the ongoing evaluation of vaccine safety.
Review co-author Dr Kate Emary, also of the University of Oxford, said: “To determine whether a vaccine protects against severe Covid-19 disease, a clinical trial needs to show that there are significantly fewer cases of severe disease in individuals vaccinated with a Covid-19 vaccine, compared with individuals who are not.
“However, only a small proportion of individuals infected with Sars-CoV-2 develop severe disease, which means an extremely large number of volunteers is needed in a clinical trial for there to be enough cases to get a reliable measure of vaccine efficacy.
“This means that it is likely that we will only know if a vaccine protects against severe disease once it has been deployed and given to a large population.”
The authors also discussed controlled infection studies in humans, known as challenge trials, as a way to measure vaccine efficacy as coronavirus transmission declines in the community.
But they argued that while human challenge trials could allow rapid assessment of vaccine efficacy, “it is unclear if results from these studies – which are likely to only include young volunteers – will predict vaccine efficacy in older adults”.
The review comes after the Government announced last week that it is putting £33.6m towards human challenge studies, with trials set to begin in early January 2021.