The Merck MMR vaccine lawsuit is actually two linked federal cases: a 2010 whistleblower complaint accusing Merck of falsifying efficacy data for the mumps component of its MMR-II vaccine, and a 2012 antitrust class action by vaccine purchasers who said the alleged fraud kept competitors out of the U.S. market. Both cases reached the U.S. Court of Appeals for the Third Circuit in 2024, and Merck won both. The antitrust plaintiffs have since asked the U.S. Supreme Court to take up the case, and as of late 2025 the Court has not yet acted on the petition.
What the Whistleblowers Alleged
On August 27, 2010, two former Merck virologists, Stephen Krahling and Joan Wlochowski, filed a qui tam lawsuit under the federal False Claims Act in the U.S. District Court for the Eastern District of Pennsylvania.1CaseMine. United States ex rel. Krahling v. Merck & Co., Civil Action No. 10-4374 Both had worked in Merck’s West Point, Pennsylvania laboratory, and they said they saw the company manipulate testing to prop up its mumps efficacy numbers.2Syracuse.com. Whistleblowers Say Merck Vaccine Ineffective
Their core claim: Merck had for over a decade reported a mumps vaccine efficacy rate of 95 percent that did not reflect reality. According to the complaint, Merck knew the vaccine had grown less effective since its 1967 introduction and pressured employees to falsify data to hide the decline. The whistleblowers also alleged senior management threatened them with jail if they went to the FDA.2Syracuse.com. Whistleblowers Say Merck Vaccine Ineffective After a two-year investigation, the federal government declined to intervene but reserved the right to join later.3Berger Montague. Federal Judge Permits False Claims Act Case to Continue Against Vaccine Manufacturer Merck
The Testing at the Center of the Case
The technical dispute involves a Merck clinical trial called Protocol 007. By the late 1990s, the FDA had raised concerns that Merck’s vaccine lost potency toward the end of its 24-month shelf life. Merck tried to fix the problem by overfilling doses. The whistleblowers and later the antitrust plaintiffs said it did not work.4Justia Law. In re Merck Mumps Vaccine Antitrust Litigation
According to court filings, rather than testing children’s blood against the wild-type mumps virus that actually circulates, Merck tested it against the same weakened Jeryl Lynn strain used in the vaccine. Plaintiffs said that approach overstated real-world effectiveness.5Courthouse News Service. Class Says Merck Lied About Mumps Vaccine Merck also added rabbit antibodies to blood samples in what was known as the Anti-IgG Enhanced Neutralization Test, or AIGENT. Because the human immune system would never produce those animal antibodies, plaintiffs argued, the results said nothing about how the vaccine performed in vaccinated people. Initial testing against wild-type virus had produced seroconversion rates well below 95 percent.6A&O Shearman Antitrust Litigation Blog. In re Merck Mumps Vaccine Litigation Summary
The whistleblowers alleged that Dr. David Krah, who supervised the AIGENT lab, directed staff to selectively recount samples and to change pre-positive results to pre-negative to reach more favorable numbers. The FDA conducted an unannounced inspection and issued a Form 483 citing four deficiencies related to AIGENT testing, including an observation that “raw data [was] being changed with no justifications.”6A&O Shearman Antitrust Litigation Blog. In re Merck Mumps Vaccine Litigation Summary
The FDA’s concerns went beyond AIGENT. In October 2000 the agency issued a Form 483 for Merck’s failure to report mumps potency failures.7Protocol7.movie. Kessler Expert Report, Exhibit 3A In February 2001 it sent Merck a Warning Letter noting that multiple vaccine lots had failed the minimum potency specification and warning that continued deviations could result in license suspension or revocation. The FDA closed the letter in April 2001 after Merck said overfilling would resolve the problem, and no lots were pulled from the market.6A&O Shearman Antitrust Litigation Blog. In re Merck Mumps Vaccine Litigation Summary
How the Antitrust Case Grew Out of It
In 2012, Chatom Primary Care and other physicians and physician groups filed a class action alleging that Merck violated Section 2 of the Sherman Act by maintaining an unlawful monopoly on the mumps vaccine market. Merck had been the sole licensed U.S. manufacturer of mumps vaccine since 1967. By keeping inflated efficacy claims on its FDA-approved label, plaintiffs said, Merck raised the regulatory bar any competitor had to clear to prove its own vaccine was “not inferior.”4Justia Law. In re Merck Mumps Vaccine Antitrust Litigation
The competitor allegedly kept out was GlaxoSmithKline, which had been developing its own MMR vaccine, Priorix. Because GSK could not replicate the seroconversion rates on Merck’s label, GSK concluded the FDA would view Priorix as inferior. Plaintiffs said that delayed GSK’s U.S. market entry by more than a decade, and that purchasers overpaid for MMR vaccine as a result.8U.S. Supreme Court. Chatom Primary Care v. Merck, Cert Petition Appendix
GSK ultimately did receive FDA approval for Priorix on June 6, 2022. The FDA found Priorix safe, immunogenic, and noninferior to MMR-II based on four randomized clinical trials, ending more than four decades of Merck as the sole U.S. supplier.9CDC. Advisory Committee on Immunization Practices Recommendation for Use of PRIORIX For the antitrust plaintiffs, the 2022 approval confirmed their theory: GSK could have done a decade earlier what it finally did in 2022 if not for Merck’s label.
How the Courts Ruled in 2024
Both cases reached the Third Circuit in 2024. Both went Merck’s way.
The False Claims Act Case
On August 6, 2024, the Third Circuit affirmed judgment for Merck in the whistleblower suit. The court held that Krahling and Wlochowski lacked evidence to create a triable issue on whether Merck’s alleged misrepresentations were “material” under the False Claims Act. It pointed to the government’s “actual knowledge of the facts concerning the alleged misrepresentations and fraudulent acts as to testing, potency, shelf-life and the like,” reasoning that misrepresentations the government already knew about could not be material.10U.S. Supreme Court. Chatom Primary Care v. Merck, Brief in Opposition
The Antitrust Case
On October 7, 2024, the Third Circuit ruled that Merck was protected by the Noerr-Pennington doctrine, which shields parties from antitrust liability for petitioning the government even when the petitioning is ethically questionable or produces anticompetitive results.4Justia Law. In re Merck Mumps Vaccine Antitrust Litigation
The court found that Merck’s communications with the FDA about Protocol 007 and its label constituted “genuine and successful petitioning.” The FDA had approved Merck’s supplemental biologics license application and maintained the label claims, so the petitioning was not objectively baseless and the “sham” exception did not apply. The court declined to recognize a separate exception to Noerr-Pennington immunity for fraudulent misrepresentations in regulatory proceedings, and it held that any antitrust injury flowed from the FDA’s own discretionary approval decision rather than from independent private conduct by Merck.4Justia Law. In re Merck Mumps Vaccine Antitrust Litigation
Where the Case Stands Now
The antitrust plaintiffs sought en banc rehearing before the full Third Circuit. On February 10, 2025, the court denied the petition, with three judges dissenting.11CCH Business. Chatom Primary Care v. Merck, Petition for Certiorari
On July 10, 2025, the plaintiffs filed a petition for certiorari with the U.S. Supreme Court, Docket No. 25-45. The question presented: “Is intentional deception of the government in an adjudicative proceeding completely immunized from antitrust liability so long as the deception succeeds?”11CCH Business. Chatom Primary Care v. Merck, Petition for Certiorari The petition argues that a circuit split exists, with the First, Ninth, Eleventh, D.C., and Federal Circuits all recognizing exceptions to Noerr-Pennington immunity for intentional misrepresentations to government agencies. The American Antitrust Institute filed an amicus brief supporting the petition in August 2025.12American Antitrust Institute. AAI Amicus Brief, Merck Mumps Vaccine Noerr-Pennington
Merck opposed the petition, calling the case an “exceptionally poor vehicle” for review, in part because the plaintiffs had expressly disclaimed reliance on a misrepresentation exception in their Third Circuit briefing and raised it for the first time in their en banc rehearing petition.10U.S. Supreme Court. Chatom Primary Care v. Merck, Brief in Opposition As of late 2025, the Court had not yet acted on the petition. The whistleblower case is over.
Real-World Mumps Data
Separately from the litigation, real-world data has continued to raise questions about how well the mumps component of MMR performs. Mumps outbreaks in the United States have increased since 2006 and frequently occur in highly vaccinated populations. From January 2016 through June 2017, there were 150 reported outbreaks accounting for 9,200 cases. Among patients whose vaccination status was known, 70 percent had received two doses of MMR before becoming infected.13CDC. Use of a Third Dose of Mumps Virus-Containing Vaccine in Persons at Increased Risk
Studies of college campus outbreaks in 2016 and 2017 estimated a median two-dose effectiveness of 60.5 percent against clinical mumps, well below the CDC’s published median of 88 percent.14National Center for Biotechnology Information. Mumps Outbreaks and Vaccine Effectiveness on College Campuses In October 2017, the Advisory Committee on Immunization Practices unanimously recommended a third mumps-containing vaccine dose during outbreaks for people who had already received two doses, citing “waning of vaccine-induced immunity with time.”13CDC. Use of a Third Dose of Mumps Virus-Containing Vaccine in Persons at Increased Risk Current CDC guidance for providers reports two-dose effectiveness at 88 percent but adds that “serologic and epidemiologic studies suggest this effectiveness decreases with time.”15CDC. MMR Vaccination Information for Healthcare Providers None of this resolves whether Merck committed fraud, but it is the backdrop against which the whistleblowers’ central claim was made.
Merck’s Position
Merck has consistently described the lawsuits as “completely without merit” and said its “labeling for M-M-R II accurately reflects the safety and efficacy of the vaccine as approved by the U.S. FDA.”16Fierce Pharma. Whistleblowers Accuse Merck of Withholding Info on Mumps Vaccine Its most effective argument in court has been the FDA’s own conduct: despite years of investigation and a detailed 2019 submission by former FDA Commissioner Dr. David Kessler urging label changes, the agency has not ordered a label change, recalled MMR-II lots, or brought enforcement action.10U.S. Supreme Court. Chatom Primary Care v. Merck, Brief in Opposition The Third Circuit treated that inaction as strong evidence that Merck’s representations to the FDA were not objectively baseless.
The current FDA-approved MMR-II prescribing information, most recently revised in November 2025, states that a single injection induced mumps neutralizing antibodies in 96 percent of susceptible individuals in clinical studies, with more recent open-label trial data showing seroresponse rates of roughly 97 to 98 percent.17FDA. M-M-R II Package Insert Whether the Supreme Court agrees to hear the antitrust plaintiffs’ challenge to Noerr-Pennington immunity is the next open question in the case.