How does ring vaccination work, and what did the Guinea Ebola trial show?
In 2015, during the Ebola epidemic, people living next to the sick were vaccinated at once or three weeks later, to find out whether the vaccine worked.
Ring vaccination works by vaccinating the people most likely to be infected, the contacts of a confirmed patient and the contacts of those contacts, so that a ring of protection forms around each case instead of trying to reach a whole country. In Guinea in 2015 a trial of the rVSV-ZEBOV vaccine used exactly this idea, and its results were encouraging but later debated.
The logic is simple. Everyone who has been, or could have been, exposed to a patient receives the vaccine, creating a ring of protection that can limit the spread of a pathogen. It requires thorough and rapid and case investigation, and the smallpox eradication programme used it with great success. Many vaccines take weeks to work, but even if some of a patient's contacts are already infected, ring vaccination can prevent the virus from being transmitted again, to the contacts' contacts.
Step 1: A confirmed case
The centre of the ring
Step 2: Contacts
Family, neighbours, colleagues
Step 3: Contacts of contacts
The outer ring
Step 4: Vaccinate the ring
Protection around the case
Step 5: Chain limited
The virus has fewer people to reach
The Guinea study was a cluster trial. In all, 3,537 contacts and contacts of contacts of laboratory-confirmed cases received either "immediate" or 21-day "delayed" vaccination. Nobody was given a dummy injection; a arm was omitted for ethical reasons. In the immediate group of 2,108 people, no case began more than ten days after vaccination; in the delayed group of 1,429 people, there were ten. Because immediate vaccination showed such protection, all recipients were then immediately.
Guinea ring trial: cases with onset more than 10 days after vaccination
cases
| Cases | |
|---|---|
| Immediate (2,108 people) | 0 cases |
| 21-day delayed (1,429 people) | 10 cases |
Experts later added a caution. In 2017 a committee of the US National Academy of Medicine agreed the vaccine probably had some , but found statements that it had substantial or 100 percent efficacy to be unsupportable. In the Democratic Republic of the Congo, a peer-reviewed study of the 2018 to 2020 outbreak found 84 percent protection for people vaccinated at least 10 days before exposure. The vaccine, known as Ervebo, was approved in the European Union and the United States in 2019.
Try it
Protect enough people and the chain breaks
It keeps growing
- Each case now infects
- 2
- Cases after 6 rounds
- 127
- Vaccinated to stop it
- 50%
Challenge · Start with R at 2. Raise the vaccinated share until each case infects fewer than one. How many people had to be protected?
Quiz me
0/3
Recap
Vaccinate the ring around each case, and compare immediate with delayed vaccination to see whether it works.
💡 A trick to remember it · Draw a circle of protection round every spark before the wind can carry it.
Surprising fact · The Guinea trial had no placebo group: everyone got the vaccine, some immediately and some after 21 days.
Connects to
- 🧪 How do Ebola treatments like Inmazeb and Ebanga work, and what did the PALM trial show?
- 📉 How do R and vaccination decide whether an Ebola epidemic ends?
- 🔍 How does contact tracing stop an Ebola outbreak?
- 💉 How did the world wipe out smallpox, and why only smallpox?
- 💉 How does a vaccine train your body to fight a germ it has never met?
Sources (4)
No source, no claim. Every fact in this lesson (15 claims) cites at least one of these.