A Belgian biotech firm has started producing tests to rapidly detect antibodies against coronavirus infections, part of a push several countries are making to determine who might be immune.

The Liege-based company, ZenTech, told AFP Tuesday it has started making tens of thousands of the government-certified tests and plans to ramp up output to eventually make up to three million per month.

Founder and CEO Jean-Claude Havaux said diagnosis takes just 10-15 minutes and "sensitivity is 100 percent -- meaning all patients who have COVID-19 antibodies, we see them with our test". He emphasised that the test kits were only for medical professionals, first in Belgium and then later in other countries in the EU and beyond. They are not for the public to use at home.

"We don't want, and don't intend for, these tests to be used by just anybody. It's not a pregnancy test," said Havaux. "It's really pretty complicated to carry out and to interpret the results."

Antibody, or serology, tests are seen as a crucial tool for determining who has had COVID-19 -- especially non-symptomatic carriers -- and could therefore be immune to it, at least for a time. Such tests could possibly pave the way to allowing people to return to work as countries mull easing widespread lockdown measures.

That could be especially important for healthcare workers on the front line of the novel coronavirus pandemic. As a result, labs in several countries are racing to roll out huge numbers of reliable testing kits.

But the World Health Organization has warned that while reliable testing is welcome, the presence of COVID-19 antibodies is not proof that an individual is immune. Even if there was immunity, it says, it is unknown how long it might last.

Dr Pascale Huynen, clinic head of the microbiology unit in Liege's university hospital -- which confirmed ZenTech's test was reliable to 97 percent -- also underscored the point. "Nobody knows if the (COVID-19) antibodies are protective," she said.

She added that scientists also do not know how long immunity could last, or whether the new coronavirus might mutate around any initial immunity, as happens with the flu. "This is a virus that we don't know very well," she said.

ZenTech's test, she said, simply indicates whether a patient has "come into contact" with COVID-19 with a positive or negative result. It does not show the level of antibody response. But that in itself is useful for determining the spread of the virus in a population, and for patients who have not received a nasal swab tests that detect whether a person is in the infectious phase, thought to last around two weeks.