(Bloomberg) -- Novavax Inc. shares fell after the company released data showing its current Covid-19 vaccine produced a better immune response than an experimental bivalent shot that targets omicron variants.

Novavax compared its bivalent vaccine to its shot that’s already been cleared along with an experimental shot that targets the original omicron strain and found no benefit in terms of immune response, according to a statement. The study also found that the company’s authorized vaccine induced broad immune responses against omicron strains BA.1 and BA.5. The shares fell 2% at the New York market open. 

The newly released findings “may spell trouble,” Bloomberg Intelligence analyst Sam Fazeli said in a note. “Novavax says the data show its prototype holds up, but uptake and confidence in future strain changes may remain low.” 

The data could further add to conflicting results regarding the advantages bivalent shots may hold against newer variants. Recent studies of bivalent shots from Pfizer Inc. and Moderna Inc. have produced mixed results. Pfizer’s own trial, as well as research from Emory University, found that bivalent boosters produced enhanced levels of protective antibodies versus current omicron strains. By contrast, studies from Harvard University and Columbia University suggested that bivalent mRNA shots didn’t generate much incremental benefit compared to a fourth dose of the original shot.

‘Compelling Case’

In June, US regulators advised vaccine manufacturers to develop Covid-19 boosters tailored to the currently circulating omicron variants for new shots. Earlier findings together with the new data released from Novavax raises the question of whether the switch to a new version of the vaccine is necessary.

The original Novavax shot is currently authorized for use as a single booster, and the findings may persuade regulators to authorize them for more than one use, Chief Medical Officer Filip Dubovsky said Monday on a call with reporters. The company has a “compelling case” for sticking with the current vaccine over the bivalent version, he said.

“It appears to be working now, and it’s the way to go into the future,” Dubovsky said. 

Novavax’s two-dose primary series, cleared in July, is the first protein-based Covid vaccine available for use in the US. It works by a different mechanism than messenger RNA vaccines from Moderna Inc. and Pfizer Inc., which induce cells to make vaccine-like proteins, and the viral vector vaccine from Johnson & Johnson.

Novavax’s booster for people 18 and older was authorized last month by US regulators, offering another option to those seeking protection ahead of a potential wave of cases this winter. The company is scheduled to release quarterly results later Tuesday, with a conference call at 4:30 p.m. New York time. 

--With assistance from Robert Langreth.

(Updates with analyst’s comment starting in third paragraph)

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