Using a Zaire vaccine against the Sudan virus isn’t an option, says Pontiano Kaleebu, director of the Uganda Virus Research Institute. “This has already been proven in the laboratory. The neutralizing antibodies do not respond,” he says. This means two things: that surveillance and physical control measures are currently the only tools available for limiting the virus’s spread, and that a working vaccine needs to be found as quickly as possible.

The candidate that’s farthest along is the single-dose ChAd3 Ebola Sudan vaccine, which is being developed by the Sabin Vaccine Institute, a nonprofit based in Washington, DC. By working with the World Health Organization (WHO), the Coalition for Epidemic Preparedness Innovations, and other organizations, the institute is planning to run a clinical trial in the current outbreak to see how well the vaccine works.

But there are only 100 doses available. With limited supply, health officials plan to give doses of the vaccine to immediate contacts of confirmed Ebola cases. Scientists then hope to use these contacts as potential candidates in the vaccine’s clinical trial—though the exact testing protocol they will use is still being worked out.

Kaleebu says they are hoping for accelerated production from the Sabin Vaccine Institute now that more doses are needed. But even if the number of vaccines used in the trial is small, they will still provide useful data, says Bruce Kirenga, a senior respiratory physician at Makerere University College of Health Sciences on the outskirts of Kampala.

“Trials use power calculation,” Kirenga says, referring to sums that allow researchers to work out the minimum number of people you need to involve to see whether a vaccine or medicine has an effect. A well-designed trial in an emergency situation in need of a drastic change of course—such as whether a vaccine stops someone from getting or dying from a severe disease like Ebola—doesn’t necessarily need to involve lots of people.

Doses of another candidate vaccine, designed to protect against both the Zaire and Sudan forms of the virus and developed by the University of Oxford, are also being sent to help in the outbreak. But the WHO has said that vaccine trials won’t start for another couple of weeks, meaning that for now, Ugandan authorities are relying on non-pharmaceutical interventions.

Contact tracing is being used to follow people who have been close to known cases, with more than 1,500 contacts having been traced as of October 16. Over a third of these are no longer being followed, having been traced for 21 days without developing symptoms.

New cases are also being picked up outside of the contacts being traced, says Daniel Kyabayinze, director of public health at Uganda’s Ministry of Health. “It’s a good sign of optimal surveillance,” he says. But it is also a sign that the limits of the outbreak are still uncertain.

On October 12, the Ministry of Health announced that a man had died of Ebola in a hospital in Kampala, having traveled to the capital from his local village. Three days later, two districts—Mubende and Kassanda—entered a three-week lockdown to try to stop the virus spreading. Bars, nightclubs, and places of worship have been closed, and only cargo trucks are allowed to enter or leave the districts.