Writing in the British medical journal The Lancet, Kate Bingham, the head of the UK Vaccine Task Force, cautioned that the first generation of vaccines for coronavirus "is likely to be imperfect" and "might not work for everyone."
She also noted that no vaccine in the history of medicine "has been as eagerly anticipated" and that it is "widely regarded as the only true exit strategy from the pandemic.
"We do not know that we will ever have a vaccine at all," she added. "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."
Bingham also pointed out that the global manufacturing capacity for vaccines was far from being capable of providing the billions of doses that governments would be seeking to purchase.