People with coronavirus but no symptoms infecting others is "very rare", a World Health Organization scientist has said.

Although a proportion of people test positive with no symptoms, it is believed these infections are mostly not passed on.

But people can pass on the disease just before symptoms develop.

The evidence comes from countries that carry out "detailed contact tracing", Dr Maria Van Kerkhove said.

Dr Van Kerkhove, the WHO's head of emerging diseases, made the distinction between three categories:

• People who never develop symptoms (asymptomatic)
• People who test positive when they don't yet have symptoms - but go on to develop them (pre-symptomatic)
• People with very mild or atypical symptoms who do not realise they have coronavirus

Some reports distinguish between these categories while others do not and she said this, along with the relatively small groups of people studied, make it difficult to draw firm conclusions.

But Dr Van Kerkhove said the weight of evidence suggested people who never develop symptoms do not play a significant role in passing on the virus, the WHO said.

And WHO guidance on wearing masks published at the weekend, says. "The available evidence from contact tracing reported by member states suggests that asymptomatically-infected individuals are much less likely to transmit the virus than those who develop symptoms,"

In England, the Office for National Statistics (ONS) has been regularly testing a sample of the population.

It has found that, of those who have so far tested positive for Covid-19, only 29% reported "any evidence of symptoms" at the time they were tested, or at the previous or following visits.

People with symptoms 'highest risk'

A key question has been whether asymptomatic people pass on their infections on to others.

Contact-tracing studies from a number of countries suggest that while "true" asymptomatic cases "rarely transmit", infection transmission can occur before or on the day symptoms first appear when they may be very mild, according to Prof Babak Javid, an infectious diseases consultant at the University of Cambridge.

People can have detectable amounts of the virus in their system roughly three days before developing symptoms and appear to be capable to passing it on during this period, especially the day before or on the day symptoms begin.

And since people who haven't yet developed symptoms are unlikely to know that they are contagious, pre-symptomatic transmission has "important implications" for track, trace and isolation measures, Prof Javid said.

This emphasises the importance of lockdown measures in "massively reduc[ing] the numbers of people infected," said Prof Liam Smeeth, an epidemiologist at the London School of Hygiene and Tropical Medicine.

While people without symptoms do seem to be capable of infecting others, current evidence still suggests people with symptoms are the highest risk.

A positive result alone doesn't tell you how much of the virus someone has in their system.

And this - what is known as the viral load - along with whether an infected person is sneezing and coughing and what kind of contact they are having with other people, influences how likely they are to pass the illness on.

Dr Van Kerkhove pointed out since coronavirus "passes through infectious droplets", it is when people are coughing or sneezing that they are most able to transmit the disease.