Pfizer vaccine and implication for herd immunity
The encouraging results of Pfizer’s vaccine against Covid-19 has renewed the idea of herd immunity. Central to the idea is the question “how many people in the general community need to be vaccinated to create herd immunity?” This note will answer that question.
By now, most people probably know what is herd immunity (also referred to as herd protection). Briefly, the idea of herd immunity is based on the conjecture that when a large proportion of people is immune to an infectious disease, these people will provide indirect protection to those not immune to the disease (see illustration). So, if 70% of a population is immune to, say, SARS-Cov-2, then 7 out of every 10 people who contact someone with the disease will not be infected. Consequently, the disease will not spread further and the epidemic is controlled. Nice idea.
There are basically two ways to create a herd immunity in the general population. One way is to let the epidemic take its course, which literally means the virus is allowed to spread as much as possible. This approach was once considered by the UK government, and needless to say, it is very controversial.
Another way to create herd immunity is by vaccination. The beauty is that vaccines can create immunity without causing illness or adverse effects. Moreover, vaccines can protect the whole…