Studies show that people with Covid-19 are most infectious when they first show symptoms, according to a scientist with the World Health Organization.

Maria van Kerkhove, an epidemiologist, is the WHO's technical lead on the pandemic.

In a briefing today, she said that a sub-set of people do not develop symptoms, but can still infect others, and as many as 40% of transmissions maybe by asymptomatic cases.

Yesterday Ms van Kerkhove said that on the basis of studies carried out in several countries, transmission of the virus by an asymptomatic person seemed "very rare".

Her remarks, which were widely relayed on social media networks, sparked a reaction from part of the scientific community.

During a discussion today, Ms Van Kerkhove said she wanted to clarify a misunderstanding.

"I was referring to very few studies, some two or three" and answering a question, she said.

"I was not stating a policy of WHO.

"I used the phrase 'very rare', and I think that is a misunderstanding to state that asymptomatic transmission globally is very rare. What I was referring to was the subset of studies," she added.

Meanwhile, Spain's health minister has said that wearing face masks will be compulsory there until the coronavirus epidemic is defeated once and for all.

Salvador Illa said the measure would remain in force after the state of emergency ends on 21 June and will "remain in place until we permanently defeat the virus, which is when we have an effective treatment or vaccine against it". 

Since 21 May, it has been compulsory for everyone aged six and over to wear a mask in public where it is not possible to maintain a two-metre security distance from other people.

But when the lockdown formally ends, the measure will remain in place with the government introducing a fine of up to €100 for non-compliance.

The new regulation will, however, slightly reduce the security distance to 1.5 metres.

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Mask-wearing was initially imposed as a requirement for those using public transport in early May but was later expanded in a country where the virus has killed more than 27,000 people. 

With the epidemic now well under control, Spain has been easing out of its mid-March lockdown in a staged process due to finish by 21 June, with the country to open its international borders from 1 July. 

Mask wearing is not compulsory for very young children although it is "recommended" for those between three and five.