What to do next
Show the working, step by step
Where I used this
Coverage is a trust problem before it is a supply problem. On the Lifebuoy hotline we set up at Praava, doctors took about 3,000 calls a day because help arrived through a channel people already trusted. A catch-up campaign needs that kind of channel before it needs more doses.
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Averages hide the pockets
- This is the simple model. It assumes people mix evenly. They do not: unvaccinated children cluster in the same schools, neighborhoods and faith communities, so an area at 95% can hold a school at 70%.
- Run it for the pocket, not the county. Use school or neighborhood coverage where you can get it. That is where the next chain starts.
- Immunity from past infection is not counted here, so the result is cautious for older adults and accurate for young children.
- When the number needed is above 100%, the vaccine alone cannot stop spread. Pertussis is the classic case: protect babies by vaccinating in pregnancy and the people around them.
Supply gaps and trust gaps need different fixes
The same gap can have opposite causes. In the US in 2026, measles coverage slipped because some parents chose not to vaccinate; in Bangladesh, a vaccine shortage left children unprotected, and a national campaign closed much of the gap within weeks once supply returned. The calculator tells you how many doses you need. Whether the problem is supply or trust tells you how to deliver them. More in Losing measles elimination is a trust bill coming due.
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Questions
What is the herd immunity threshold for measles?
Measles is one of the most contagious diseases, with an R0 of about 12 to 18. With two doses of MMR at about 97% effectiveness, around 95% of people need to be immune to stop sustained spread, which is why 95% is the usual coverage target.
How do you calculate the herd immunity threshold?
The share of people who must be immune is 1 minus 1 divided by R0. Divide that by the vaccine’s effectiveness to get the vaccination coverage needed. For R0 15 and 97% effectiveness: (1 − 1/15) ÷ 0.97 = about 96%.
Why can measles spread when coverage is above 90%?
At 92% coverage with a 97% effective vaccine, each case still infects about 1.5 people on average, so outbreaks can grow. And averages hide pockets: a district at 92% can include schools far below it, where transmission takes off.