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Share-based payments - Estimated the fair value of the share options granted (Details)
12 Months Ended
Dec. 30, 2021
Options
Y
$ / shares
Nov. 15, 2021
Options
Y
$ / shares
Jun. 17, 2021
Y
Options
$ / shares
May 21, 2021
Options
Y
$ / shares
Mar. 11, 2021
Y
Options
$ / shares
Dec. 22, 2020
Options
Y
$ / shares
Dec. 15, 2020
Y
Options
$ / shares
Nov. 16, 2020
Options
Y
$ / shares
Aug. 17, 2020
Y
Options
$ / shares
Jun. 08, 2020
Options
Y
$ / shares
May 20, 2020
Options
Y
$ / shares
Mar. 12, 2020
Y
Options
$ / shares
Dec. 31, 2021
Options
Dec. 31, 2020
Options
Share-based payments                            
Exercise price | $ / shares $ 14.90 $ 14.96 $ 23.14 $ 22.08 $ 28.16 $ 25.01 $ 23.02 $ 24.31 $ 20.39 $ 16.87 $ 17.44 $ 15.15    
Expected volatility 84.00% 84.00% 85.00% 85.00% 84.00% 85.00% 85.00% 85.00% 83.00% 84.00% 84.00% 82.00%    
Expected life of share options (in years) | Y 6 6 6 6 6 6 6 6 6 6 6 6    
Risk-free interest rate 1.69% 1.54% 1.25% 1.34% 1.09% 0.48% 0.45% 0.46% 0.48% 0.58% 0.46% 0.60%    
Number of share options issued | Options 105,071 24,500 1,000 555,464 12,000 11,500 10,000 84,900 2,900 80,000 481,405 16,550 698,035 687,255