during each of these encounters, assuming the other person is
susceptible.
R therefore depends on four factors: the duration of time a person
is infectious; the average number of opportunities they have to
spread the infection each day they’re infectious; the probability an
opportunity results in transmission; and the average susceptibility of
the population. I like to call these the ‘DOTS’ for short. Joining them
together gives us the value of the reproduction number:
R = Duration × Opportunities × Transmission probability ×
Susceptibility
Breaking the reproduction number down into these DOTS
components, we can see how different aspects of transmission trade
off against each other. This can help us work out the best way to
control an epidemic, because some aspects of the reproduction
number will be easier to change than others. For example,
widespread sexual abstinence would reduce the number of
opportunities for transmission, but it’s not an appealing or
practical option for most people. Health agencies have therefore
focused on getting people to use condoms, which reduce the
probability of transmission during sex. In recent years, there has also
been a lot of success with so-called pre-exposure prophylaxis (PrEP),
whereby -negative people take anti- drugs to reduce their
susceptibility to the infection.[47]
The type of transmission opportunities we’re interested in will
depend on the infection. For influenza or smallpox, transmission can
occur during face-to-face conversations, while infections like and
gonorrhea are spread mostly through sexual encounters. The trade-
off in the DOTS means that if someone is infectious for twice as long,
in transmission terms it’s equivalent to them making twice as many
contacts. In the past, smallpox and have at times both had an R
of around 5.[48] However, people are generally infectious with
smallpox for a shorter period, which means that there must be more
opportunities to spread infection per day, or a higher transmission
probability during each opportunity, to compensate.