Evidence shows most people who recover from Covid-19 do produce antibodies, one of the government's most senior medics said today.
Meanwhile the health secretary said he would "not yet" feel comfortable being in a crowded room despite already having had the virus.
Matt Hancock tested positive for coronavirus in late March, spending two weeks in self-isolation before recovering and returning to work.
At today's Downing Street press conference, he and deputy chief medical officer Professor Jonathan Van-Tam were asked about the evidence on whether people could catch the disease twice.
Professor Jonathan Van-Tam said currently available scientific evidence showed the “overwhelming majority” of people who had recovered from the disease were found to have antibodies in their bloodstream but it was not yet known for sure whether they would be immune from a second case of the disease or for how long the antibodies would last.
“We just haven’t had this disease around on the planet in humans for long enough to know whether those who had recovered were immune," he said.
He said that antibodies produced in response to other human coronaviruses “don’t persist necessarily for years and years and years” although it was unknown what would happen in the case of Covid-19.
A "massive piece of work" looking at antibodies was underway in the UK, he said.
However, he said: “We can’t make the science go any faster than how quickly our bodies go in terms of maintaining the antibodies.
“Like everybody else in the world we just have to be patient and cautious until we get those answers."
Mr Hancock, who said he is one of those taking part in a trial to analyse patients’ antibodies, said he would “not yet” feel happy being in a crowded room.
“I very much hope that the science shows that the people with antibodies, who have tested positive for having antibodies, have a low risk of transmitting the disease and a low risk of catching it – both are important, low risk of catching it to protect me, low risk of transmitting it to protect others.
“But until we know that’s the case we can’t be sure of it, we can’t base a policy decision on it."
He added he could not be “comfortable” being in a crowd because “that might trigger a rise in the number of infections if the science turns out to be wrong."
Mr Hancock did however reveal there was “very positive” indications about a new antibody test which will be able to determine if people have already had the virus.
“Today Roche, the Swiss global diagnostics company, made a very positive announcement about progress with their antibody test and we are in discussions with them about a very large-scale roll-out of antibody testing, as well as with some others," he said.
Roche has said its new serology test has a specificity greater than 99.8% and sensitivity of 100%, meaning there is a low chance of an inaccurate test.
Mr Hancock acknowledged that there had been problems with antibody testing before, having previously said the UK would order 17.5 million home testing kits only to find they did not work.
“There has been false hope before in antibody testing,” Mr Hancock said, “so we will make announcements when we are absolutely ready.”
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