Reading an FDA AdCom: Voting Outcomes, Base Rates, and the Trade After the Vote

FDA Advisory Committee meetings are among the most overreacted-to catalysts in biotech. The tape moves in 90 seconds on a number — 14-2, 9-4, 6-8 — and positions get marked before anyone has read what the committee actually voted on.

The number is almost never the whole story. This note walks through what an AdCom vote signals, how predictive it is of the subsequent FDA decision, what to watch in the meeting itself, and how the stock typically behaves in the days and weeks that follow.

What AdComs actually vote on

The first thing to internalize: AdComs do not vote to approve drugs. FDA approves drugs. Advisory Committees vote on specific questions that FDA poses to them, and the answers are non-binding.

The questions almost always take one of these forms:

• Benefit-risk — "Do the data support a favorable benefit-risk profile for [drug] in [indication]?" This is the closest to a proxy for approval, but it is still not an approval vote.
• Efficacy — "Has the applicant demonstrated substantial evidence of effectiveness?"
• Safety — "Is the safety profile acceptable for the proposed indication?"
• Subgroup questions — "Should approval be limited to [subpopulation]?" or "Should the label include a boxed warning for [event]?"
• Post-marketing — "Are the proposed post-marketing studies adequate?"

The practical implication: a 10-4 "yes" on benefit-risk is not the same as a 10-4 "yes" on efficacy paired with a split safety vote. When the tape prints, pull the actual voting questions from the FDA briefing document before trading the move.

The base rates: how often FDA follows

Across the FDA as a whole, the historical concordance between AdCom recommendations and final FDA decisions sits around 75% overall, based on multiple published analyses covering the 2008-2022 window. That headline number is the most misleading figure in biotech investing because it varies enormously by division.

• Oncology (ODAC): follow rates run meaningfully lower than the overall average. FDA's Oncology Center of Excellence has shown increased willingness to diverge from ODAC recommendations in both directions — approving after negative votes where there is strong unmet need and signal of activity, and requesting confirmatory data after favorable votes. For oncology AdComs, discount the vote substantially.
• Psychiatry and CNS: follow rates trend higher than the average, and FDA rarely approves over a strongly negative psychiatry AdCom.
• Cardiorenal and metabolic: near the overall average, with divergences clustering in cases where post-marketing commitments can address open questions.
• Vaccines and infectious disease (VRBPAC, AMDAC): historically high follow rates, though recent cycles have seen more public daylight between the committee and FDA.

Uncertainty flag: the aggregate numbers mask important selection effects. FDA does not convene an AdCom for every NDA. Meetings are increasingly reserved for applications with meaningful open questions, which means the average AdCom sits on a more contested decision than the average NDA. Follow rates on AdCom'd drugs understate FDA's agreement rate with the center of scientific opinion generally.

The voting questions that actually matter

When reviewing the briefing document the night before the meeting, rank the voting questions by their weight in the final decision:

1. The benefit-risk question carries the most signal. If it passes decisively (12-2 or better), the regulatory path is usually intact even if subordinate questions split.
2. The efficacy question matters more than safety in most divisions, because unresolved efficacy kills applications outright while unresolved safety often produces labeling constraints rather than Complete Response Letters.
3. Safety questions produce post-marketing requirements, REMS programs, and boxed warnings. They rarely kill approvable drugs on their own unless the signal is severe.
4. Subgroup questions are where label construction happens. A "yes" on benefit-risk with a subgroup "restrict to prior-therapy patients" vote is an approval with a narrower label — often worth more in the stock than the headline vote suggests, because a narrow label still clears the marketing path.

Pay particular attention to the order in which FDA stacks the questions. FDA sets the agenda, and it front-loads the questions it considers foundational. If benefit-risk is question one, FDA is telling you it wants that answer to frame the rest.

Reading the committee

Three things about committee composition worth tracking in advance of the meeting:

• Voting members vs. non-voting members. Only voting members count to the tally. Industry representatives on the committee almost always hold non-voting seats. Patient representatives are sometimes voting, sometimes not — check the roster.
• Clinical specialty mix. A cardiology drug being reviewed by a committee heavy on general internists will tend to weight safety more heavily than efficacy magnitude. A rare disease drug reviewed by a committee with one specialist in the disease will often see that specialist's view carry outsized weight in the discussion, even if the vote is binary.
• Industry-funded dissent. Committee members disclose industry relationships. Members with relevant consulting arrangements are typically excluded from voting under 21 U.S.C. §355 conflict-of-interest rules, but residual relationships — grant support, unrelated consulting — are disclosed and should be read for directional bias, in either direction.

Listen to the morning discussion, not just the vote. A 9-5 "yes" where the five dissenters cite a specific safety signal is a different outcome than a 9-5 "yes" where the dissent is scattered. FDA reads the transcript. So should you.

Post-AdCom trading windows

The intra-day move on an AdCom vote is usually wrong in magnitude, often wrong in direction, and almost always faded partially over the following five trading days. This is the "AdCom fade" pattern, and it has held reasonably well across the last decade.

The typical rhythm, in my observation:

• T+0 (vote day): binary move, often double-digit percent. Options vol collapses post-print. Liquidity is poor in the first 15 minutes post-vote as dealers reprice.
• T+1 to T+5: fade of roughly 20-40% of the initial move. This is where algorithmic flow and generalist profit-taking meet specialist repositioning. Shorts often re-initiate into strength on positive votes; longs often cover into weakness on negative ones.
• T+5 to PDUFA: the fade typically reverses as the PDUFA date approaches, if — and only if — the vote was aligned with the scientific center of gravity. A 14-2 yes that faded 30% into T+10 typically retraces back toward the initial close and beyond as PDUFA approaches. A 6-8 no where the stock rallied on "closer than expected" will often roll over into PDUFA as the market remembers the vote was still a no.

Two patterns that break the fade:

1. Post-vote FDA signaling. If FDA staff make public comments within the first week that clearly align with one side of the vote, the fade compresses or inverts. Watch division director statements and any CDER press activity.
2. Label-limiting subgroup votes. When the benefit-risk vote passes but a subgroup vote narrows the likely label, the initial move often underreacts on the downside. The stock tends to work lower over the subsequent two weeks as sell-side models rebuild on the narrower addressable market.

A working checklist for the day of

1. Pull the voting questions from the FDA briefing document before the meeting starts. Do not trade the number without knowing the question.
2. Identify the benefit-risk question and weight it most heavily.
3. Track dissent reasoning, not just dissent count.
4. Discount oncology votes against the ~75% baseline. Weight psychiatry and vaccine votes closer to that baseline or above.
5. Expect a 20-40% fade in the first week. Size entry and exit accordingly rather than chasing the T+0 close.
6. Map the PDUFA date. The vote is a waypoint. The decision is the catalyst.

AdComs reward investors who read the agenda and punish investors who read the tape.

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