Lockdowns come with hefty socio-economic costs. “Focused protection” arose as an alternative to lockdowns, and refers to a simple idea: instead of asking everyone to lock down and mask, let’s ask only the vulnerable to do that.
This idea requires that the vulnerable never spend time indoors with someone at risk of being infected. The problem is that it’s very hard to fulfill this requirement if the virus is allowed to spread in the non-vulnerable population.
Take the example of nursing homes. In theory, it’s possible to keep the vulnerable protected inside them, isolated from the outer world. In practice, there will be people from the outer world coming in – nurses, primarily, but also parents visiting. The issue is that, the more the virus is allowed to spread in the outer world, the more likely that a nurse or visitor are infected, and thus bring the virus inside.
Moreover, imagine that an older person, who, in theory, lives an isolated life, suddenly needs to visit a hospital. The higher the viral spread in the outer world, the more likely that she will come in contact with infected nurses, doctors, patients, or visitors inside the hospital.
Hence the counterintuitive principle that, to protect the vulnerable, the non-vulnerable must remain protected too.