Wearing a surgical mask can lessen the risks of droplet transmissions by up to 40 percent, the world's first animal experiment on the effectiveness of masks has found.

Led by top microbiologist Yuen Kwok-yung from the University of Hong Kong, the study also suggests that even when mask wearers are infected, their viral load is up to 10 times lower than those who don't wear a mask.

"This explains why the mortality rate in Hong Kong is only 0.4 percent when it can be up to 10 percent in other countries," Yuen said.

"It's because during the peak of the outbreak, 97 percent of Hongkongers wore masks when outside. Therefore, only 40 percent of our cases are local infections and many only suffered from mild symptoms."

For the study, hamsters were infected with Covid-19 and put in an enclosed cage where porous partitions separated them from healthy hamsters.

A ventilation system facilitated droplet transmission by circulating air from the infected hamsters to the healthy ones. The partitions were then covered with surgical masks to evaluate the masks' effectiveness in preventing the spread of the virus.

In the first set of experiments where no masks were used, eight out of 12 healthy hamsters contracted the virus within seven days, an infection rate of 67 percent.

In the second set, the cage of the infected hamsters was masked and only two of 12 healthy hamsters, or 17 percent, caught the virus, while in the third set the healthy hamsters' cage was masked and three of 12, or 25 percent, fell sick.

Unmasked healthy hamsters also caught the virus much earlier and suffered more damage in their lungs and tracheae.

"The experiment suggests that masking infected people is more effective. But a major problem with Covid-19 is that there are asymptomatic carriers, so it's inevitable that everyone needs to wear a mask to protect themselves," Yuen said.

He pointed out that some overseas experts were skeptical about the effectiveness of masks as there was no scientific evidence on the subject.

"This experiment gives a clear answer that what we believed in is right," he said.

However, Yuen said his team has noticed that the ratio of Hongkongers wearing masks has fallen below 90 percent due to the hot weather.

"I hope citizens can continue wearing masks and hold onto the practice longer as we believe the pandemic will be with us for some time, probably years," he said.

"You can take off the mask when in an outdoor area with not many people, but you must wear a mask on public transport and in indoor areas, especially where there is air-conditioning."

The results will be published in the scientific journal Clinical Infectious Diseases.

Yuen Kwok-yung gives the inside story on tests into effectiveness of masks. Inset: one of the hamsters used.

HKU tests show masks work