The eyes are a key route by which the novel coronavirus infects humans, a study by the University of Hong Kong has found, in addition to the upper respiratory tract, which includes the nose, mouth and throat.

The study made public on Friday also found that Sars-Cov-2 - the strain of coronavirus that causes the Covid-19 disease - was up to 100 times more infectious than the severe acute respiratory syndrome (Sars) virus, or Sars-Cov, and bird flu viruses such as H5N1, reported newswire agency Xinhua.

The research team led by Dr Michael Chan Chi Wai, an associate professor at the university's School of Public Health, found that the coronavirus is much more efficient in infecting the human conjunctiva - the tissue lining the surface of the eye - and the upper respiratory airways, and its level of infection is comparable to that observed for H1N1, the swine flu virus that caused a pandemic in 2009.

"This explains the higher transmissibility of Covid-19 than that of Sars. This study also highlights the fact that eyes may be an important route of Sars-Cov-2 human infection," Dr Chan said, according to the South China Morning Post.

The research team had previously discovered that the coronavirus remains alive for up to seven days on smooth surfaces such as stainless steel, glass and plastic. It is therefore possible for people to get infected when they touch these surfaces with their hands and then touch their face.

The study was published in the latest issue of The Lancet Respiratory Medicine. According to SCMP, it was assumed in the early stages of the coronavirus crisis that medical personnel would be sufficiently protected by wearing N95 face masks and protective clothing, and did not need specialist glasses. These latest findings by Dr Chan's team, it says, challenge that assumption.

The Covid-19 outbreak first came to light in late December after cases were detected in the Chinese city of Wuhan.