Federal Circuit Affirms Gamboa: A Fact-Specific Ruling on Althen Prong One and Special Master Divergence in PCV-GBS Claims

The Federal Circuit recently issued its decision in Gamboa Avila v. Secretary of Health and Human Services, affirming a denial of compensation in a pneumococcal conjugate vaccine (PCV)–Guillain-Barré Syndrome (GBS) case. [Read the full decision here: https://www.cafc.uscourts.gov/opinions-orders/24-1765.OPINION.2-11-2026_2646330.pdf.] The ruling clarifies how reliability is evaluated under Althen Prong One in off-Table claims, but it does not foreclose future PCV-GBS cases. Rather, it is a fact-specific affirmance grounded in the particular record before a particular special master, and it expressly acknowledges ongoing divergence within the Office of Special Masters (OSM).

This post explores what the decision does — and does not — hold, and what practitioners should take from it when litigating PCV-GBS claims.

The Holding: Deference to a Special Master’s Reliability Determination

In Gamboa, the petitioner alleged that Prevnar 13 caused GBS, proceeding on an off-Table theory supported by expert testimony advancing a molecular mimicry mechanism.

The special master concluded that petitioner failed to satisfy Althen Prong One, finding that the proposed theory lacked sufficient indicia of reliability. The Court of Federal Claims affirmed, and the Federal Circuit upheld that decision.

On appeal, the petitioner argued that the special master effectively imposed a requirement of peer-reviewed literature directly linking PCV and GBS — contrary to Althen v. Secretary of Health & Human Services, which permits reliance on circumstantial and inferential evidence. The Federal Circuit rejected that characterization.

Citing Moberly v. Secretary of Health & Human Services and Boatmon v. Secretary of Health & Human Services, the court reaffirmed that while peer-reviewed literature is not a prerequisite to recovery, a petitioner must present a medical theory supported by “some indicia of reliability.” The absence of supporting studies may be considered as part of the overall reliability analysis.

Importantly, the Federal Circuit emphasized that the special master did not deny the claim merely because there was no published literature directly linking PCV to GBS. Rather, the decision rested on the special master’s holistic assessment of petitioner’s theory and relied on other “foundational deficiencies” unrelated to the absence of medical literature.

In short, the Federal Circuit did not announce a new legal standard. It affirmed the application of existing Althen principles to a particular evidentiary record.

OSM Divergence in PCV-GBS Cases: Acknowledged by the Federal Circuit

One of the most notable aspects of Gamboa is the Federal Circuit’s express recognition that different special masters have reached different conclusions on similar PCV-GBS theories.

The court observed that “different special masters appear to have reached inconsistent results on identical facts,” and remarked that if “the identity of the special master assigned to [the] case determines the outcome, [that is] a result that is both arbitrary and unfair.” It encouraged consideration of related-case procedures or other coordination mechanisms to avoid inconsistent rulings within the OSM.

That acknowledgment is significant.

In prior PCV-GBS cases, special masters found similar molecular mimicry theories sufficiently reliable to satisfy Prong One. See, e.g., Gross v. Sec’y of Health & Hum. Servs., No. 17-1075V, 2022 WL 9669651 (Fed. Cl. Spec. Mstr. Sept. 22, 2022); Pierson v. Sec’y of Health & Hum. Servs., No. 17-1136V, 2022 WL 322836 (Fed. Cl. Spec. Mstr. Jan. 19, 2022). Those outcomes stand in contrast to the result in Gamboa.

The Federal Circuit did not resolve that split. Nor did it disapprove the contrary PCV-GBS decisions. Instead, it affirmed that substantial deference is owed to a special master’s evaluation of scientific reliability — even where other special masters have evaluated similar evidence differently.

What Gamboa Does — and Does Not — Establish

What it does:

• Reaffirms that Althen Prong One requires a reliable medical theory supported by some indicia of scientific credibility.
• Confirms that special masters may weigh the presence or absence of supporting literature as part of that reliability analysis.
• Emphasizes appellate deference to a special master’s fact-specific evaluation of expert testimony.
• Highlights systemic fairness concerns stemming from inconsistent outcomes within OSM.

What it does not do:

• It does not hold that PCV cannot cause GBS.
• It does not require peer-reviewed literature directly linking PCV and GBS.
• It does not overrule prior PCV-GBS cases in which petitioners prevailed.
• It does not establish a categorical bar to recovery in future PCV-GBS claims.

The holding is limited to the record presented and to the special master’s evaluation of that record. Different evidentiary showings — or different credibility determinations — may produce different results.

Practical Implications for Practitioners

• The absence of direct literature linking PCV to GBS is not dispositive, but it remains a meaningful component of the broader reliability inquiry.
• Expert testimony should clearly bridge mechanistic theory to disease-specific evidence and address analytical gaps head-on.
• The assigned special master continues to play a role in PCV‑GBS case outcomes; however, given the Circuit’s scrutiny of inconsistencies in prior decisions, this influence may evolve.
• Practitioners should carefully analyze prior divergent OSM decisions to anticipate how reliability arguments may be received.

Concluding Remarks

Gamboa is best understood as a fact-specific reaffirmation of deference — not a doctrinal shift. It underscores that Althen prong one remains a flexible, evidence-driven inquiry, applied by individual special masters who retain substantial discretion in evaluating scientific reliability.

For PCV-GBS claims, the landscape remains evolving. The Gamboa decision sets no categorical limits but reinforces that special masters properly exercise their discretion in assessing the reliability of a petitioner’s medical theory.