It had been suggested people who survive the virus could exit the lockdown first as they may have developed natural immunity – known as antibodies.
But Professor Danny Altmann admitted scientists “haven’t got the foggiest” about whether Covid survivors are actually protected from reinfection.
The immunology expert said antibodies might not actually kill the bug off for good.
It could cling on secretly in a person’s gut, according to evidence seen by ministers.
And he warned he would be “terribly worried” about lifting the lockdown without more information about the nation’s immunity levels.
Prof Altmann, from Imperial College, told MPs “What an antibody test tells you is that you have met the virus – you have had exposure to the virus and it triggered an immune response.
“It offers you no conclusion whatsoever about whether you will be immune to it next time you meet it.”
He said scientists “haven’t got the foggiest notion of” whether someone who has had Covid becomes immune to it.
Britain is in a global race to try to find an antibody test which can work on individuals and reliably say if they have had the virus.
Boris Johnson has said the tests could be a “game changer”.
But their usefulness will be limited if it turns out people can get reinfected by Covid anyway.
Prof Altmann warned that people produce wildly different amounts of antibodies, adding that the data “looks like somebody has fired a scattergun on the page”.
If he had to guess he thinks immunity is “variable” and probably “doesn’t last long – not more than a few years” he told the science and technology committee.
Professor Susan Michie, Professor of Health Psychology at University College London, also warned that the presence of antibodies does not mean a person is immune.