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while the contact does infect the other person, at work, in the thea-
ter, across the dining room table, or in some other place.
Jack is infectious to others for six days (during days four to nine),
during which he makes 96 contacts in total, and infects 2.4 other
individuals (the reproductive number, or the number of secondary
infections generated from one infected person, of the novel corona-
virus -- is thought to be between two and 2.6, according to models
fromImperial College). In this scenario, that comes out to about
40 contacts for every one infection, again a rough number broadly
consistent with the evidence about the rate of disease transmission.
There are two main ways to stop the pandemic. The first way --
the approach now widely used around the country -- is through an
economic lockdown and “shelter in place.” By deliberately closing
down most businesses, the daily contacts per person fall. Suppose
that falls by more than half, say to six per day. Over six days, Jack
now only makes 36 contacts rather than 96, and as a result now
infects on average less than one other person.
This is why the lockdown works, if it is enforced. Over time,
the number of infected people declines as those currently infected
recover, followed by the smaller number of people they have in-
fected. The lockdown policy, however, comes at a high short-term
cost. Daily life and the economy are put on hold. Incomes, jobs, tax
revenues, and businesses could collapse during the lockdown.
There is a much better way -- when it is feasible. Suppose that on
the evening of day five, after two days of symptoms, Jack is visited
by a public health worker who has conducted vigorous contact tra-
cing. The health worker explains, “We are here because your collea-
gue at work developed a mild Covid-fever three days ago. Do you
also have symptoms?”
“Well, yes,” comes Jack’s reply. “I woke up today feeling not so
great, but thought I could shake it.” The health worker is able to
perform a diagnostic test and Jack remains isolated at home until
the test confirms the Covid-19 infection. After a positive diagnosis,

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