"Because until we are absolutely sure about the relationship between the positive antibody tests and immunity, I think we as scientists would say we need to tread cautiously".
'This is an important milestone, and it represents further progress in our national testing programme, ' he added.
The problem is twofold; no one knows if having antibodies grants you superhero-like invulnerability against additional infection (and if it does for how long), plus a positive test doesn't mean you're over it and aren't still able pass it on to anyone else, as you may well be spewing virus out all over the place.
Earlier, the prime minister's spokesman said the tests will be "free for people who need them, as you would expect". The trial will involve around 4,000 people and will run for up to six weeks.
They will first be offered to health and social care staff as well as patients and care home residents.
But health officials caution that serology tests still do not confirm whether the antibodies found in a person who was previously infected with COVID-19 can protect them from getting the virus again.
If you are experiencing cold or flu symptoms, arrange a test by calling your doctor or contact the Coronavirus Health Information Hotline on 1800 020 080.
Announcing the antibody test rollout at the Downing Street press conference yesterday, Hancock said: "It's not just about the clinical advances that these tests can bring".
"Antibody surveillance study tells us 17 percent of people in London and 5 percent or higher in the rest of country have tested positive for coronavirus antibodies. We can not clap our carers one day and then charge them to use our NHS the next".
"These tell you if you have had the virus, and have developed antibodies in response that might help you to fight the virus in future", he said.
However, admitting that it is not certain that people who have antibodies can not get reinfected with Covid-19, he said that the availability of testing would help towards "developing this critical science, to know the impact of a negative antibody test".
Also speaking at the No 10 briefing, Prof John Newton, who is in charge of the government's testing efforts, said quick test results were a key element.
He said "All-cause mortality has come down at the same time as the COVID deaths have come down, and it is now at roughly the rate it is at in an average winter".
Hampshire Hospital NHS Trust will work closely with local authorities to identify priority care homes to visit and test.
A smartphone app is now being tested, while the government has also promised to recruit 25,000 tracing staff.
He said these would be rolled out as fast as possible if successful.