Recruiting Against a Closing Window: Seasonal Study Planning

Flu vaccine trials, RSV studies, seasonal allergy protocols, and norovirus challenge studies share one brutal property: the enrollment window closes whether or not you filled it. There is no "catching up in Q2." If you miss the window, you miss the season, and the sponsor remembers.

The planning logic for these studies is different from a standard rolling-enrollment protocol. Here's how to run them.

Work Backward From the Last Possible Randomization Date

Start with the hard stop — the date the sponsor will no longer accept randomizations — and subtract everything that has to happen first:

• Screening-to-randomization interval (often 7–28 days)
• Lab turnaround for central eligibility labs
• Time to schedule the screening visit after a candidate says yes
• Time between first contact and the candidate returning your call

In most site datasets, the gap between first contact and screening visit is the single largest consumer of calendar time, and it's the one sites consistently underestimate. If your typical contact-to-screen interval is two weeks, your real recruitment deadline is a month earlier than the protocol's.

Put that date on a whiteboard. Everyone on the team should be able to recite it.

Front-Load Everything You Can Do Before Activation

Seasonal studies punish sites that start recruiting at site initiation. Before the SIV:

1. Get ads IRB-approved early. Submit recruitment materials with the initial package, not as an amendment three weeks later.
2. Pre-screen your registry against draft criteria. You can build and segment the candidate list before you can consent anyone.
3. Confirm staffing for the peak weeks. A six-week enrollment window with a coordinator on vacation in week three is a planning failure, not bad luck.
4. Pre-book block visit slots. Hold mornings on your schedule now; release them later if you don't need them.

Build a Pre-Consent Waitlist

The highest-leverage move for a time-sensitive study is a warm list of people who have already said "yes, call me when it opens." Reaching out to a known-interested candidate converts far faster than cold outreach, and the difference matters enormously when you only have weeks.

Maintain the list with the information you'd need to prescreen fast: age, key comorbidities, medication flags, distance from site, and availability pattern. Ask explicitly for permission to contact about future studies — that consent language, approved once, saves you repeatedly.

Overbuild the Top of the Funnel

For rolling studies you can tune outreach gradually. For seasonal studies, assume you'll need more candidates than your historical ratio suggests, because speed forces you to accept a less-qualified pool. Plan to run two or three channels simultaneously from day one rather than sequencing them.

Also plan for scheduling density. If you can only screen eight people a week, a thousand responses doesn't help. Match outreach volume to visit capacity, and expand capacity before expanding outreach — Saturday screening clinics, a float coordinator, or a second exam room during peak weeks.

Decide Your Stop Rule in Advance

Set a checkpoint roughly a third of the way into the window. If you're below pace, escalate then: add a channel, request a second site PI for consent capacity, or tell the sponsor early so they can reallocate. Sites that flag shortfalls at week two keep their reputation. Sites that flag at week six don't.

The seasonal study you ran this year is next year's playbook. Write down the actual intervals, conversion rates, and bottlenecks while they're fresh — that document is worth more than any forecast you could build from scratch.