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Did you know that you protect yourself and those around you by getting vaccinated? Diseases can’t spread easily when enough people in a population get vaccinated. This effect is called herd immunity. Cholera is a big threat in countries that don’t have safe water and toilets for everyone. These countries sometimes vaccinate large numbers of people (mass vaccination). This creates herd immunity and prevents disease outbreaks. But it is hard to know how long herd immunity will last. 

In one camp in South Sudan, people who had fled their homes during a war received mass vaccination. However, there was a cholera outbreak the following year. We developed a mathematical model to find out what affects how long herd immunity lasts. We found that (a) it lasts for a shorter time when a lot of people move into and out of an area, and (b) the vaccine gets less effective for each person over time. Our results suggest that herd immunity lasts longer if authorities do two things: 1. vaccinate everyone and 2. always give vaccines to new arrivals to the camp and to those who were vaccinated a long time ago.

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