Ab&B Bio-Tech soared as much as 164% in its trading debut in Hong Kong, after retail investors flocked to the Chinese vaccine maker’s initial public offering.

The stock climbed to as high as HK$34.02 per share on Monday morning, more than double its HK$12.90 IPO price, which was set at the low end of the marketed range.

The share sale’s success builds on the rally of health-care stocks, which have been the best performers on the Hang Seng Composite Index this year. Ab&B, which sells an influenza vaccine, was so sought after among retail investors that they applied for more than $20 billion worth of margin loans to bid for the stock. That’s thousands of times the number of shares available to them.

Ab&B is also the last company to go public in Hong Kong under listing rules that allowed retail investors to grab as much as half of all allocated shares in an oversubscribed new listing. Starting this August, mom-and-pop investors will only be able to get up to 35% of such deals.

Aside from flu shots, Ab&B is also developing vaccines for rabies, herpes and other respiratory diseases, with plans to commercialize them in China and beyond.

Though much smaller in scale, Ab&B follows other health-care companies in starting out with a bang in Hong Kong this year. Cancer drugmakers Duality Biotherapeutics and Nanjing Leads Biolabs saw their stocks more than double on their trading debuts.