The first Covid-19 vaccines are "likely to be imperfect" and "might not work for everyone", the chairman of the UK Vaccine Taskforce has said.
Kate Bingham said no vaccine in the history of medicine "has been as eagerly anticipated" and that vaccination is "widely regarded as the only true exit strategy from the pandemic".
However, she warned against over-optimism and highlighted that a vaccine might not work for everyone, or for very long.
"We do not know that we will ever have a vaccine at all," she wrote in The Lancet. "It is important to guard against complacency and over-optimism.
"The first generation of vaccines is likely to be imperfect, and we should be prepared that they might not prevent infection but rather reduce symptoms, and, even then, might not work for everyone or for long."
The vaccine taskforce was created by Sir Patrick Vallance, the UK Government's chief scientific adviser. It was set up under the Department for Business, Energy and Industrial Strategy in May, and Ms Bingham reports directly to the Prime Minister.
In her Lancet article, she said the "strategy has been to build a diverse portfolio across different formats to give the UK the greatest chance of providing a safe and effective vaccine, recognising that many, and possibly all, of these vaccines could fail".
George Eustice, the Environment Secretary, told Sky News Ms Bingham's analysis was "probably right", adding: "A vaccine will be the answer at some point, but it is too early to say precisely when that vaccine will come.
"I know there's some hope there might be something as quickly as Christmas, but that's not by any means a certainty. And it's also the case, it always is with a vaccine, that you don't know precisely what level of protection it will deliver. Some of them deliver full protection, some don't."
There are more than 200 vaccine candidates in development around the world, with 44 in clinical trials. 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.