The vaccine trial where the control group was protected too

Question

Researchers randomly vaccinate some people in a small town and leave others unvaccinated as controls, then compare infection rates. The gap between vaccinated and unvaccinated turns out smaller than the vaccine's known biological effect, and someone concludes the vaccine is weak.

Explain how spillover produces this, and how the design should change to measure the effect properly.

Accepted Answer

Protecting some people protects others. When a chunk of the town is vaccinated, the disease spreads less, so even the unvaccinated controls catch it less often. This is herd immunity, a positive spillover: the treatment given to some units improves the outcomes of untreated units nearby. The control group was quietly helped by the very treatment it was supposed to be compared against. Why it shrinks the measured effect. A clean trial needs the control group to show what happens with no protection at all. But here the controls enjoyed indirect protection from their vaccinated neighbors, so the…

The vaccine trial where the control group was protected too

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