All countries must remain "on alert" over the possibility of further transmission of the coronavirus, the World Health Organisation (WHO) has warned.

Maria Van Kerkhove, WHO technical lead on Covid-19, said that many people remain susceptible to the virus, even where transmission has been suppressed.

Asked on the BBC's Andrew Marr programme if a second wave of infection could be likely in countries that have started to ease their lockdowns, Ms Van Kerkhove said: "It's certainly possible. What we're seeing in a number of countries that have been successful in suppressing transmission is that many more people remain susceptible."

She added: "All countries must remain on alert for the possibility of additional transmission even if they have been successful in suppressing transmission in the first round."

When asked about getting closer to finding a vaccine, she said hundreds of clinical trials are currently underway. 

"We need to be careful when we look at individual results as they need to have a large sample size etc to see which drugs are safe and effective."

She said a vaccine should not be developed in some countries and not made available to everyone everywhere.  

Crowds wearing face masks gathered yesterday in Busan, southeastern South Korea

On face masks being a useful part of measures to come out of lockdowns, Dr Van Kerkhove said while masks are useful, they alone will not solve the problem. 

"Masks are very helpful in a number of situations, firstly for healthcare workers. We also recommend for people in the community who are feeling unwell to wear a mask.

"Masks alone cannot solve the problem especially if you are thinking of lifting lockdowns. It must be done in a slow and staggered approach."

Asked about the longer-term health impact of the virus, Dr Van Kerkhove said: "The majority of people who are infected with Covid-19 will make a full recovery.

"But there will be some people that may have some longer term effects. It affects the lungs, it affects the body in different ways and so we may see some damage to the lungs.

"We need to follow individuals over time. We're in our fourth month of this pandemic so it's very, very early days."

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Dr Van Kerkhove also addressed claims by US President Donald Trump that he had seen evidence that the virus had originated in a laboratory in Wuhan, China.

She said: "Coronaviruses normally circulate in animals and many coronaviruses are circulating in bats.

"Most emerging pathogens, viruses, come from an animal reservoir. Everything that we have seen about the novel coronavirus, Covid-19 or Sars-CoV-2, of the tens of thousands of sequences that are available, full genome sequences and partial sequences, compared to other coronavirus sequences that are available, this is of a natural origin.

"And it originally comes from bats because the coronaviruses come from bats.

"What we need to do is really understand what we call the intermediate host - what is the animal that was infected from bats that potentially infected humans?

"It's important that we know this because from a public health point of view, it's very important that we find the animal host so that we prevent this, (what) we call spillover from transmission from an animal to a human, we prevent that spillover from happening again."