Ab&B Bio-Tech soared as much as 169 percent in its trading debut in the Hong Kong Special Administrative Region, after retail investors flocked to the Chinese mainland vaccine maker’s initial public offering.

The stock climbed to as high as HK$34.64 ($4.41) 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 healthcare 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 is thousands of times the number of shares available to them.

Ab&B is also the last company to go public in the HKSAR 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, they will only be able to get up to 35 percent 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 on the mainland and beyond.

READ MORE: New IPO may wave in HK with Chinese concept stocks return, analysts say

Ab&B follows other healthcare companies in starting out with a bang in the HKSAR this year. Cancer drugmakers Duality Biotherapeutics and Nanjing Leads Biolabs saw their stocks more than double on their trading debuts.