An empty Covent Garden in locked-down London (Image: Vuk Valcic/SOPA Images/Sipa USA)

A highly infectious strain of COVID-19 is spreading through the UK causing officials to delay the country's reopening. But how infectious is the strain, really?

The World Health Organization puts variants of COVID-19 into two categories: variants of interest, which show the virus has mutations that have caused COVID clusters around the world, and variants of concern, which are more transmissible, more severe, or vaccines or other medications are less effective.

While most variants have been detected in hotel quarantine, only two of the four variants of concern have led to community transmission in Australia: Alpha and Delta.