Strategies to Maximize Participant Retention in the Sierra Leone Trial to Introduce a Vaccine Against Ebola (STRIVE)

Background: STRIVE, a phase 2/3 trial of investigational rVSV∆G-ZEBOV (Merck) vaccine was implemented in 5 districts in Sierra Leone during the 2014-2015 Ebola epidemic. Eligible health care workers and front line Ebola response workers were individually randomized to immediate (within 7 days) or deferred (within 18-24 weeks) vaccination and followed for 6 months after enrollment or vaccination for medical events, including Ebola virus disease (EVD). There were concerns that high rates of follow-up would be difficult to achieve in a low resource setting with poor communications infrastructure.

Methods: The study design incorporated several features to maximize participant retention, especially for the deferred vaccination group. To facilitate safety follow-up, STRIVE provided cell phones to participants with free calls to study staff and a dedicated hotline for participants to report health concerns and be referred for clinical care. Trial staff conducted monthly calls to participants using their study phone or personal/other phone number if provided, making up to six attempts. Home visits were made if participants could not be contacted by phone. Preliminary follow-up data are presented.

Results: Of the approximately 8650 participants enrolled and randomized, half were assigned to the immediate vaccine group and half to the deferred group. Approximately 1% withdrew or were lost to follow-up within 6 weeks of vaccination (immediate group) or enrollment (deferred group). About 95% of the immediate vaccination group completed 6 months of safety follow-up, and 88% of the deferred vaccination group completed pre-vaccination follow-up and were vaccinated. Post-vaccination follow-up in the deferred group is ongoing. The most common reasons for early termination were declining deferred vaccination followed by losses to follow-up. To date, no vaccine-related serious adverse events or EVD cases have been reported.

Conclusion: High retention of study participants in a vaccine clinical trial in a low resource setting was achievable using multiple follow-up methods. Use of a deferred intervention control group in the design of a randomized clinical vaccine trial in an outbreak in such a setting is feasible.