Phase 3
Final clinical trial stage; tests efficacy and safety at scale; required before regulatory authorisation.
WHO's 20,000-dose Phase 3 ring trial of Ervebo against Bundibugyo Ebola had still not begun on 10 September 2026; WHO expects it in Ituri in October or November, where falling case numbers may leave too few patients.
Last refreshed: 4 October 2026 · Appears in 1 active topic
Can pre-pandemic Phase 3 data actually unlock 100-day vaccine authorisation?
Timeline for Phase 3
Mentioned in: WHO: 2,000 Congolese workers get Ervebo
Pandemics and BiosecurityMentioned in: 70,000 Ervebo doses, 20,000 as a trial
Pandemics and BiosecurityMentioned in: $4.17m to test Zaire vaccine antibodies
Pandemics and BiosecurityMentioned in: Only Ebola treatment still cannot dose
Pandemics and BiosecurityMentioned in: CEPI and GAVI commit $112m, no treatment yet
Pandemics and BiosecurityBackground
Phase 3 is the third and usually final stage of clinical testing before a regulator decides whether to approve a vaccine or medicine. After Phase 1 checks safety in a small group and Phase 2 refines the dose, a Phase 3 trial enrols thousands or tens of thousands of people to measure whether the product prevents or treats disease, and to catch rarer side effects. Its results form the core of an application to regulators such as the EMA, the MHRA or the US FDA.
In an outbreak, a Phase 3 vaccine trial often uses ring vaccination: contacts of each confirmed case, and their contacts, are vaccinated, so the trial follows the virus. That makes timing decisive, because a trial that starts as an epidemic wanes may not find enough cases. Outside outbreaks, Phase 3 can run before the disease spreads widely: Moderna dosed the first participants in a Phase 3 trial of its mRNA H5N1 bird flu vaccine on 21 April 2026, the furthest any mRNA H5N1 candidate has reached.
Phase 3 is the costliest and slowest stage, so funding decides which candidates reach it. When the US Department of Health and Human Services withdrew funding from the Moderna H5N1 trial, CEPI committed $54.3m in May 2026 to keep it going.
The Bundibugyo ring trial keeps slipping
On 20 August 2026 the International Coordinating Group released 70,000 Ervebo doses to DR Congo and set 20,000 of them aside for a Phase 3 trial against Bundibugyo ebolavirus. Ervebo is licensed only against the Zaire species, so the trial is the test of whether it protects anyone against this one.
By 10 September the trial had not started, and WHO expected it in October or November, beginning in Ituri. That timing is a design problem: a ring trial vaccinates the contacts of new cases, and Ituri's cases have been falling since a mid-August peak. A separate MSF study, BRAVO, began on 19 September and vaccinates about 20,000 frontline workers in Ituri and North Kivu; it does not replace the randomised trial.