Ab&B Bio-Tech soared 158 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 closed at HK$33.28 ($4.24), 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.

“The strong demand, especially for sub-$1 billion deals, suggests growing comfort with risk in the biotech space,” said Gary Tan, portfolio manager at Allspring Global Investments. But it may also “indicate short-term overheating", Tan said.

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 under the so-called clawback mechanism. Starting this August, they will only be able to get up to a 35 percent allocation in oversubscribed deals that trigger the mechanism.

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.

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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.