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A top epidemiologist in Hong Kong says it is safer for people to wear surgical masks if they are available, but health workers must have first call when supplies are short.

Public health professor Ben Cowling of Hong Kong University said there was no need to wear a mask when out in the fresh air, as Covid-19 did not survive well outside.

New Zealand's Ministry of Health says it is keeping close watch on a review by US authorities into whether wide use of masks could reduce the spread of Covid-19.

The World Health Organisation is also now looking at this, though its advice remains that healthy people only need to wear masks if they are caring for others suspected of being infected or if they themselves are coughing or sneezing.

"In the past we've talked mainly about the use of face masks in people who are symptomatic to reduce the chance that they spread infection further," Cowling said.

"But now there's an emerging debate about whether it might be useful to even suggest people who don't have symptoms to wear masks."

There was mixed evidence, but most indications were that surgical masks would have some benefits, and even an improvised cloth mask was likely to be better than nothing, though the US Centers for Disease Control was looking into this, he told RNZ.

"There's a good chance that wearing a mask will have some effect but I don't think it will stop transmission," Cowling said.

"Now the times have changed with coronavirus, we're looking for every edge we can get.

"The public health advice would still be to stay at home as much as possible," he said, adding that his two sons only left the house once a week. "But if you do need to go out, and you have a mask, if it's a surgical mask, wear it - if it's a cloth mask, wear it - it's probably doing some good and better to wear it than to wear no mask."

Full-filter N95 masks should be reserved for health workers only, said Cowling, who co-authored a Lancet report 'Rational use of face masks in the Covid-19 pandemic'.

In the United States, Johns Hopkins Center for Health Security director Thomas Inglesby said authorities should urge people to use non-medical masks or face coverings as a "prudent step".

However, the Washington Post reported there was no consensus on mask use.



Surgical mask production soared from 10 million a day in China in early February, to 115 million a day a month later. However, N95 production had barely increased by early March and was at just 1.66 million per day, because they require special fabric.



China bought up masks and other personal protection equipment supplies in other countries when it was in the midst of the virus crisis.

Deliveries to New Zealand of 41 million masks by privately chartered plane, and to public Ministry of Health stocks, will begin to arrive from tomorrow, and the ministry is trying to secure 20 million more masks per month.

The ministry said some of these could go to the general public if that was "where they are most valued".

Cowling said surveys showed masks in very wide use in Hong Kong - 99 percent now during Covid-19 - as well as Taiwan, South Korea, and parts of China, but in these places the supplies were good.

These countries are often held up as role models for New Zealand to combat the virus.

Taiwan stockpiled masks for a pandemic and had local production and a distribution network so everyone could get three masks a week at a controlled price, similar to the set-up in South Korea, he said.

A New Zealand expatriate in Shanghai, Brendon Ritchie, received 10 surgical masks a week for his family from the Chinese government.

"When they did find out the seriousness, they did their absolute most to protect their people," Ritchie said.

All these countries were doing well against the virus, and all had widespread mask use, but they all also had many other measures such as physical distancing, types of lockdown or quarantining, and very good contact tracing.

There was short supply of masks worldwide and health workers should get them first in every instance, Cowling said.

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