Media playback is unsupported on your device Media caption When asked about the scarcity of masks, Donald Trump suggested scarves as protection against Covid-19

US health authorities are debating whether to recommend face coverings for everyone when they go out in public.

One internal memo for the Centers for Disease Control and Prevention says that even simple cloth masks could help reduce the risk of virus transmission, the Washington Post reports.

President Donald Trump has suggested that individuals could "use a scarf".

Current guidelines say that masks only need to be worn by healthcare workers, the sick, and those caring for them.

There is currently a shortage of N95 and surgical masks in the US, with healthcare workers saying they have been told to reuse masks several times, or use bandanas when masks are not available.

There have been over 190,000 cases of coronavirus in the US, with about 4,000 deaths.

In other developments on Wednesday:

US Vice-President Mike Pence warned the US appears to be on a similar trajectory as Italy, which has had the most deaths of any country

In the UK, Wimbledon tennis tournament has been cancelled for the first time since World War Two

British government officials vowed to ramp up testing within weeks

Canadian Prime Minister Justin Trudeau has promised the largest economic programme in the country's history

The official death toll in Iran has passed 3,000

How could the US advice on face masks change?

The Washington Post obtained memos from the CDC, which said that cloth masks could help prevent the spread of the virus if people wore them in public, even during simple tasks like visiting the supermarket.

The advice is being shared with the White House coronavirus task force for their consideration, the Washington Post reports.

While there has been no change to the official guidance yet, there has been a clear change in tone from officials over the last few days.

Back in February, Surgeon General Jerome Adams had tweeted: "STOP BUYING MASKS! They are NOT effective in preventing general public from catching #Coronavirus, but if healthcare providers can't get them to care for sick patients, it puts them and our communities at risk!"

He also said in an interview that individuals could "increase [their] risk of getting" the virus by wearing a mask, because "folks who don't know how to wear them properly tend to touch their faces a lot".

However, on Wednesday, Dr Adams told Good Morning America that the CDC had been asked to review its guidelines on masks, because "we've learned there's a fair amount of asymptomatic spread" - although he stressed that medical masks should still be left to health care professionals who needed them most.

Media playback is unsupported on your device Media caption Coronavirus: Lack of medical supplies 'a national shame'

Similarly, on Tuesday, the US's top infectious diseases expert, Anthony Fauci, said the White House coronavirus task force was actively discussing whether to recommend a wider use of masks,

Dr Fauci told CNN he would "lean towards" recommending a "much more broad, community-wide use of masks outside of the health care setting" once there was a sufficient supply of masks for healthcare workers.

However, experts have stressed that it is vital to maintain social distancing measures and thorough hand washing, even when wearing a mask.

Do masks work - and why the confusion?

There have been a lot of confusing, or contradictory, messages on this.

Even Dr Fauci acknowledged on Tuesday that "this whole thing of masks has been confounded and conflated to create confusion".

Essentially, there are three different issues that have been linked together, causing that confusion:

Whether masks work at reducing transmissions Whether masks are effective enough at preventing community spread to justify them being used by the general public, and; How to address the shortage of supplies for healthcare workers

On the first point: Coronavirus is spread by droplets that can spray into the air when those infected talk, cough and sneeze.

As a result, healthcare workers who are at risk of encountering infected airborne droplets are told to wear N95 masks, which filter out at least 95% of airborne particles, or, if those are not available, surgical masks.

There is debate amongst health experts on the second point - whether masks are effective when used by the public.

Many have argued that hand-washing and social distancing are more effective, that masks could offer a false sense of security, and that people could infect themselves if they touch the masks incorrectly. Experts have also argued there is no clear evidence that face masks work when used by the wider community.

However, more and more health experts now say there are benefits.

They argue that the public use of masks can primarily help by preventing asymptomatic patients - people who have been infected with Covid-19 but are not aware, and not displaying any symptoms - from unknowingly spreading the virus to others.

Masks may help lower the risk of individuals catching the virus through the droplets from another person's sneeze or a cough - and people can be taught how put masks on and take them off correctly.

Experts have also argued that even small measures to reduce transmissions can be worth taking, especially when combined with other steps such as hand washing.

Media playback is unsupported on your device Media caption US death rates v UK, Italy and South Korea

"Something doesn't have to be 100% effective to be beneficial," Dr Fauci told CNN.

There is widespread agreement on point three - that while supplies are limited, medical face masks need to be left for those who need them the most, especially as healthcare workers play a vital role in protecting everyone.

Dr Fauci said: "If everyone wears masks then healthcare workers who need them may not get them."

But, he added, "If you have enough masks… we may need to modify the stance on whether people should be wearing masks."

In recent days, health experts have started suggesting a new approach - that the public should try to wear non-medical, cloth masks - which they could even make themselves - to help flatten the curve without using up medical supplies.

What do other countries do?

In some parts of Asia everyone now wears a mask by default - it is seen as safer and more considerate.

In mainland China, Hong Kong, Japan, Thailand and Taiwan, the broad assumption is that anyone could be a carrier of the virus, even healthy people. So in the spirit of solidarity, you need to protect others from yourself.

Some of these governments are urging everyone to wear a mask, and in some parts of China you could even be arrested and punished for not wearing one.