When someone falls ill, people they might have infected are vaccinated. Depending on how easily the disease spreads, contacts who could have been infected might include family, neighbours, and friends. Several layers of contacts may be vaccinated (the contacts, the contacts' contacts, the contact's contacts' contacts, etc.).
Ring vaccination thus requires contact tracing, figuring out who a person is likely to infect or have infected. This can be difficult. In some cases, it is preferable to vaccinate everyone within the area in which people have fallen ill (geographically-targeted reactive vaccination). If there is a tight geographic community boundary, it may be preferable to vaccinate the entire community in which the illness has appeared, rather than explicitly tracing contacts.
Many vaccines take several weeks to induce immunity, and thus do not provide immediate protection. However, even if some of the ill person's contacts are already infected, ring vaccination can prevent the virus from being transmitted again, to the ill contacts' contacts.[medical citation needed] A few vaccines can protect even if they are given just after infection; ring vaccination is somewhat more effective for vaccines providing this post-exposure prophylaxis.
When responding to a possible outbreak, health officials should consider which is best, ring vaccination or mass vaccination. In some outbreaks, it might be better to only vaccinate those directly exposed; variable factors (such as demographics and the vaccine that is available) can make one method or the other safer, with fewer people experiencing side-effects when the same number are protected from the disease.
In 2018, health authorities used a ring vaccination strategy to try to suppress the 2018 Équateur province Ebola outbreak. This involved vaccinating only those most likely to be infected; direct contacts of infected individuals, and contacts of those contacts. The vaccine used was rVSV-ZEBOV.
Ring vaccination has been used extensively in the 2018 Kivu Ebola outbreak, with over 90,000 people vaccinated. In April 2019, the WHO published the preliminary results of the research by its research, in association with the DRC's Institut National pour la Recherche Biomedicale, into the effectiveness of the ring vaccination program, stating that the rVSV-ZEBOV-GP vaccine had been 97.5% effective at stopping Ebola transmission, relative to no vaccination.
- Cocooning (immunization)
- Pulse vaccination strategy
- Herd immunity
- Targeted immunization strategies
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