Explainer: A look at Hong Kong's worst COVID-19 wave, and mainland-aided fight

What makes the fifth wave more challenging?

The fifth wave of coronavirus infections in Hong Kong is led by the highly transmissible Omicron variant of COVID-19.

David Lam, a member of HKSAR's Legislative Council, said the strain has outrun Hong Kong's inefficient case tracking system. Omicron can fuel a surge in infections in a matter of days, and without the help of big data tools, Hong Kong's virus sleuths are overwhelmed by sprawling transmission chains that are hardly traceable.

Hong Kong has a high population density and many residents live in overcrowded rooms. Revamped sewage and ventilation pipes and poor air circulation in some houses have also sped up the transmission, he said in an interview with Xinhua.

A low vaccination rate among the elderly is also to blame. As of early February this year, the vaccination rate in elderly people over 80 years old in Hong Kong is only about 20 percent.

According to official data, up to 91 percent of the recent COVID-19 fatalities had not completed their vaccination, and the mortality rate for the elderly aged 80 or above without completed vaccination was 16 times higher than that for those who had done so.