Merck is paying $9.2 billion to acquire a drug company primarily for an experimental antiviral flu-prevention drug, marketed specifically as “not a vaccine.”
TrialSite News reported that Merck’s move to acquire Cidara Therapeutics marks a “strategic pivot” toward long-acting antivirals as demand for traditional flu vaccines drops amid failure to match the circulating flu strain.
Daniel O’Connor, founder and CEO of TrialSite News, told The Defender:
“Whether industry leaders admit it publicly or not, Merck’s pivot reflects a market transformed by vaccine fatigue. After years of polarized debate and fluctuating efficacy, long-acting antivirals could be emerging as a politically safer, commercially scalable alternative — and Merck clearly wants to lead that post-vaccine era.”
Merck said in a press release that it will acquire Cidara via a subsidiary for $221.50 per share in cash — more than double the company’s last closing price, CNBC reported. The deal is expected to close in early 2026.
“That’s a lot of money,” Dr. Meryl Nass told The Defender. TrialSite News said Merck’s move raises the question of whether “strategic desperation inflated the price.”
James Harlow, senior vice president at Novare Capital Management, told CNBC, “There definitely seems to be a sense of urgency at Merck to add growth to their pipeline via deals.”
Reuters reported that Merck appears to be diversifying its product portfolio ahead of losing the patent to its lucrative cancer drug Keytruda.
Merck highlights that drug is ‘not a vaccine’
Merck pursued the acquisition largely to secure Cidara’s CD388, an experimental drug designed to prevent seasonal influenza infections.
The company’s press release stressed that the drug is “not a vaccine.”
Harlow told CNBC that CD388’s “non-vaccine nature” is significant given the current “uncertainty” surrounding how the U.S. Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) view vaccines.
CD388 is a long-acting antiviral that uses a fragment of a human antibody to prevent seasonal and pandemic influenza. Unlike annual flu vaccines that often miss the circulating strains, the drug is “strain-agnostic,” meaning that it targets most influenza A and B viruses.
And since CD388 is not a vaccine, its efficacy doesn’t depend on generating an immune response — a feature Merck believes will make it effective regardless of a person’s immune status.
Jeffrey Stein, Ph.D., Cidara’s president and CEO, said in Merck’s press release that the drug offers “an additional option to vaccines and antivirals to help address unmet needs in influenza prevention.”
Merck’s chairman and CEO, Robert M. Davis, said that he’s confident CD388 “has the potential to be another important driver of growth through the next decade, creating real value for shareholders.”
A ‘breakthrough? Or ‘clearly not clinically significant’?
Merck’s flu antiviral is in Phase 3 clinical trials involving about 6,000 adolescents and adults in the U.S. and U.K. who face higher risks of flu complications, according to the press release.
The FDA granted the drug “fast track” status in June 2023 and, after reviewing Phase 2b results last month, designated the drug a “breakthrough therapy,” a move that could reduce regulatory hurdles.
Stein said the Phase 2b trial delivered “compelling results” of CD388’s potential effectiveness.
However, Ronald Brown, Ph.D., a public health researcher at the University of Waterloo in Canada, who reviewed the Phase 2b trial data, told The Defender the results are “clearly not clinically significant.” He said:
“The results for efficacy of the primary endpoint are reported as relative risk reductions: 450 mg [milligrams] (76.1%), 300 mg (61.3%), and 150 mg (57.7%).
“But it is important to also consider the unreported absolute risk reductions, which are the actual percentage point differences in outcomes between the two groups, offering a more direct measure of efficacy: 2.1%, 1.7%, and 1.6%.”
In other words, the drug reduced a person’s chance of getting the flu by only about 2%.
Merck did not respond to our comment request by the deadline. Cidara declined to comment.
Nass noted that even if the antiviral proves highly effective, the CDC has long relied on annual flu vaccine campaigns and may resist moving away from a shot-based model.
“We had a huge embedded culture where the flu vaccine program was CDC’s biggest program every year,” she said.
However, the CDC could shift its approach under U.S. Secretary of Health Robert F. Kennedy Jr., Nass suggested. “Would he be able to get the employees at the CDC to make the pivot to not doing it anymore? I think it would be a very good thing if that happens,” she said.
In February, the U.S. Department of Health and Human Services directed the CDC to shut down its long-running “Wild to Mild” flu vaccine ad campaign and shift the agency’s focus to informed consent. The campaign targeted pregnant women and children.
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