Why Not Get It Over With?: A Selection from Nicholas A. Christakis’s Apollo’s Arrow (2020)

“Should we just let the epidemic hit us like a big wave? Why not get it over with? This line of reasoning feels sound in some ways; we cannot really stop the germ from infecting many people unless we have a vaccine, and efforts to stop it require us to devastate our economy and might possibly create many more deaths because of our response. After all, poverty is deadly too.

Some countries did consider this approach in 2020. The British toyed with, but ultimately rejected, the idea of ‘taking it on the chin.’ Sobering estimates of a rapid increase in deaths prompted a belated national lockdown in the UK. Prime Minister Boris Johnson himself contracted the disease after affecting an air of nonchalance for months, and he ended up in intensive care for several days in April 2020. Sweden, alone among its Scandinavian neighbors, adopted a tactic like this, aiming to isolate the vulnerable and aged while allowing the young and healthy to go about their business as sensibly as possible in order to achieve sufficient levels of population immunity. By May 2020, Sweden had at least four times the rate of COVID-19 deaths as its Nordic neighbors with similar demographics and economies, and it began to gingerly correct its course. Ultimately, the architect behind the strategy admitted that the policy had not worked out as planned. Crucially, the Swedish economy was just as hard-hit as the economies of its neighbors who had pursued full lockdowns. Physical and economic health are inextricably linked.

But early on, some politicians and pundits in the United States made noise about how the country should follow the Swedish model or otherwise charge its way toward herd immunity. But it’s ludicrous to compare Sweden to the United States. The former is a country of 10.2 million people with a cradle-to-grave social-welfare system, relatively few health problems, a culture of outdoor living, and a rule-abiding, collectively oriented citizenry. None of this resembles the United States, whose citizens have vastly higher rates of poverty, poor health, and other risk factors. . . .

Herd immunity is the idea that a group of people can be collectively immune to an infectious disease even if not everyone in the population is individually immune. The term has veterinary origins, but it applies equally well to human beings. The concept is that, if a sufficiently large number of people have acquired immunity to a disease (either by getting it and surviving or by being vaccinated), then any individual in this population who somehow contracts the illness is unlikely to encounter another person to whom he or she can transmit it. Hence, even if the chain of transmission somehow got started, it would die out. . . .

The proportion of people who need to be immune for a society to have herd immunity depends on how infectious the disease is; the less infectious the disease, the fewer individuals need to be immune. And the higher the R0, the higher the fraction of people who must be immune to stop an epidemic. This is the reason that measles, which is among the most contagious diseases known, requires such high levels of vaccination in a population to avoid outbreaks. About 6 percent of unvaccinated people in an area is enough to give measles an opportunity to create an outbreak, as we have seen in recent years in regions where a large number of people have refused to get vaccinated. That is, 94 percent or more of the population must be immune, either naturally or via vaccination, to stop measles epidemics.

However, for pathogens with a lower R0, a smaller percentage of immune people will get the job done. The formula (R0-1)/R0 gives this percentage. Using an R0 for SARS-2 of 3.0 means that the calculated percentage of the population that must be immune is 67 percent. But this percentage is an overestimate to some degree. The epidemiological basis for calculations of the reproduction number of pathogens makes an assumption that every person in a population has an equal chance of interacting with every other person, but we know this is not true in the real world. Some people have few social connections and few social interactions and others have many . . . . If popular people become immune, either naturally or via vaccination, this means that fewer people must be immune to reach herd immunity than the foregoing calculation suggests.

It’s difficult to estimate the precise size of the effect since it depends on many factors. But the implication for reaching the herd-immunity threshold is that if the more popular people are overrepresented early on during the COVID-19 pandemic, a lower percentage of the whole population must become immune—perhaps only 40 to 50 percent.”—Nicholas A. Christakis, Apollo’s Arrow: The Profound and Enduring Impact of the Coronavirus on the Way We Live (2020)