U.S. vaccine developers tumbled as competition from the U.K. and a focus on COVID-19 plasma treatments curbed investor enthusiasm for the stocks.

Novavax Inc. and Inovio Pharmaceuticals Inc. each sank more than 15 per cent on Monday. They’re among approximately 30 companies that have begun testing their vaccines in patients. More than 130 others are undergoing animal and laboratory testing, according to the World Health Organization.

Shares of vaccine makers have been volatile since the outbreak of the pandemic as Wall Street tries to forecast the timing and revenue potential of an inoculation for the virus as the Trump administration targets one of the fastest development time-lines ever.

The Food and Drug Administration confirmed on Sunday that it had granted emergency use authorization for convalescent plasma treatments, developed from the blood of people who have gotten ill and recovered, clouding the outlook for the scope and timing of a vaccine.

Some were quick to criticize the agency’s move, with the presidential election happening in a little more than two months.

“Such action seriously damages the FDA’s credibility during the greatest public health crisis in 100 years,” Michael Carome, health research group director at the consumer advocacy group Public Citizen said in a statement. “This does not bode well for future FDA decisions regarding COVID-19 vaccines and therapeutics between now and election day.”

A plasma treatment probably won’t do much ultimately to alter the demand for a preventative shot, according to Cowen biotechnoloty analyst Yaron Werber.

“The data has been mixed,” Werber said by phone. “On top of that, there’s limited supply for convalescent plasma. All of the vaccines are able to neutralize antibodies from the virus, but not all them can mobilize a T-cell response, which is needed.”

Vaccine trials are “still important,” Cantor Fitzgerald analyst Charles Duncan wrote in a note defending his buy-equivalent rating on Novavax. The phase 2 study the Gaithersburg, Maryland-based biotech started Monday in older patients “is key,” he said.

FDA Commissioner Stephen Hahn pointed to early promising data for plasma treatments. “The data from studies conducted this year shows that plasma from patients who’ve recovered from COVID-19 has the potential to help treat those who are suffering from the effects of getting this terrible virus,” he said in a statement.

Russia has already given the go-ahead for a vaccine, before large-scale efficacy trials were completed. That’s drawn skepticism and concerns that politics may be driving decisions more than science.

Other vaccine makers with human trials underway also slumped in U.S. trading on Monday. Moderna Inc. dropped as much as 5.4 per cent, while American depository receipts of Pfizer Inc.’s German partner BioNTech fell 4.5 per cent.

The slide was driven in part by a Financial Times report saying the Trump administration is contemplating fast-tracking a vaccine being developed by the University of Oxford and AstraZeneca Plc, though a spokesman for the U.S. Health and Human Services department denied that an emergency use authorization would come before the election.

AstraZeneca’s benefit comes in part from initial results from a larger study, as well as a more competitive price on its vaccine, Werber said. “They have a tremendous amount of capacity at a very cheap cost. The price to the U.S. government is US$4 a dose which is by far the cheapest,” Werber said.