On November 8, stocks of vaccine makers in Asia fell in the wake of U.S. pharmaceutical giant Pfizer’s announcement that its new antiviral pill called Paxlovid is 89% effective in reducing risk of hospitalization or death from COVID-19.

The share price for Chinese vaccine maker Cansino Biologics dropped 17% in Hong Kong by closing bell on November 8. Cansino's one-shot viral vector vaccine, based on similar technology as U.S. vaccine maker Johnson & Johnson, is being distributed in nine countries, largely in Asia and South America.

Chinese pharmaceutical firm WuXi Biologics, which makes ingredients for British vaccine giant AstraZeneca, fell by nearly 9% in Hong Kong.

Investors appear to believe that the introduction of highly effective treatment options may reduce some global demand for COVID-19 vaccines, which are currently the main proven tools on the market to prevent hospitalizations and deaths related to the virus.

Doctors say that while antiviral pills provide promising treatment options, they should be considered as supplements to vaccines rather than alternatives. “Getting vaccinated still must be at the heart of [the U.S.’s COVID-19] strategy, as a therapeutic pill is not a substitute for getting vaccinated,” Dr. Vivek Murthy, the U.S. surgeon general, told ABC News on November 7.

In its November 5 press release, Pfizer said its study demonstrated such “overwhelming efficacy” against COVID-19 that it would cease enrolling new patients in its trial and submit data soon to U.S. regulators.

“Today’s news is a real game-changer in the global efforts to halt the devastation of this pandemic,” Pfizer CEO Albert Bourla said in the announcement. “If approved or authorized by regulatory authorities, [Pfizer’s COVID-19 pill] has the potential to save patients’ lives, reduce the severity of COVID-19 infections, and eliminate up to nine out of 10 hospitalizations.”

Amid the positive antiviral news, investors appeared particularly wary of one company whose vaccine has not been approved yet.

Clover Biopharmaceuticals, a Chinese firm that plans to roll out a protein-based jab in coming months and debuted on November 5 on the Hong Kong stock exchange, saw its share price fall by 29% on November 8. In Tokyo, shares of pharmaceutical firm Shionogi & Co., which is developing a protein-based COVID-19 vaccine, dropped 6% on November 8, which represents the firm's largest single-day decline since the beginning of the pandemic.

But other vaccine makers fared slightly better.

The share price for Fosun Pharma, the distributor of Pfizer's and BioNTech's COVID-19 vaccine in the Chinese market, dropped by 9% shortly after trading began in Hong Kong on November 8 before nearly recovering to its opening price by closing bell. On the Indian stock exchange, shares for COVID-19 vaccine maker Bharat Biotech dipped 8% on November 8 morning before clawing back the losses.

It is unclear whether Pfizer's announcement will lead to similar selloffs in European and U.S. markets, but investors are salivating over Pfizer's new antiviral pill. Pfizer's stock price surged 11% in New York on November 5 immediately after its announcement.

Pfizer’s news comes just one week after the U.K. became the first country in the world to approve an antiviral pill for use against COVID-19. The U.K. approved molnupiravir, a pill jointly developed by U.S. pharmaceutical firms Merck and Ridgeback Biotherapeutics. The companies reported in October that the pill cut the risk of hospitalization or death by 50% for COVID-19 patients. The Pfizer and Merck pills are given to patients as soon as possible after a COVID-19 diagnosis to prevent an infection from getting worse.

“The success of these antivirals potentially marks a new era in our ability to prevent the severe consequences of SARS-CoV2 infection,” Dr. Stephen Griffin, associate professor in the School of Medicine at the University of Leeds, told the Science Media Centre.