In theory, just
The term herd immunity (literally – herd immunity) was coined by scientists from the University of Manchester almost a hundred years ago. They infected the mice with salmonella and watched how quickly the infection spreads. It turned out that in the enclosures, where there are many animals with developed protection, the epidemic was slowing down, and mortality was decreasing. The infected rodents did not infect themselves and did not transmit the infection to others.
In those years, it was important for the world to defend itself against ailments that killed a large number of children – diphtheria (caused by a bacterium), smallpox, and polio. The idea of herd immunity proved the need for mass vaccination. Vaccinations cause a lifelong immune response in the body, a person will not be able to get sick and transmit the infection. If you vaccinate everyone without exception, the disease will disappear.
Herd immunity is the only way to restrain the spread of such misfortunes as measles, rubella, and mumps. Observations showed: outbreaks are recorded in populations with a critical number of people immune to these infections – unvaccinated or not ill, for example, in the case of chickenpox.
Is the armor strong? Who has immunity against SARS-CoV-2
In the 1970s, mathematicians proposed a simple model for calculating population immunity. The main parameter is the reproductive index R0, which shows how many, on average, an infected person can infect. Suppose the indicator is two, then one infects two, each of them, in turn, two more and so on. But if one in a pair has immunity to infection, on it the chain breaks. How many of these should be in the population in order to avoid an outbreak, are calculated by the formula (R0 – 1) / R0. In our example, half.
The higher R0, the more contagious the disease, the greater the proportion of those immunized must be in order for collective protection to form. For measles, R0 = 18, which means that herd immunity begins to act when 95 percent of the population is vaccinated. And for the flu, this index is only 1.8. But since there is no mass vaccination against viruses of this group, most often the authorities choose a targeted vaccination campaign strategy – only in the layers that serve as a source of distribution. In the 1990s, compulsory flu shots were given in schools in Japan. This has reduced morbidity and mortality among the elderly.
How is collective immunity formed: 1 – one infected infects two, 2 – part of the people are vaccinated in the population, so one infected infects only one, 3 – in such a simplified model, it is enough to vaccinate half of the people so that collective protection appears
Smallpox was completely eradicated by the end of the 1970s. But poliomyelitis and measles, despite the mass vaccination of newborns for half a century, no. Why? As it turned out, herd immunity is a more complex phenomenon than theorists thought, and it does not always work, which has a number of explanations from British researchers.
Not every vaccine provokes an equally strong immune response; there will certainly be those who have poorly formed defenses or none at all. In such cases, in order for herd immunity to work, it is necessary to immunize more than the calculations show.
Another problem is the different behavior of people within and between groups, for example, age groups, which affects the rate of infection. Then R0 becomes a complex function taking approximate values.
In addition, the default models assume that people are vaccinated at random. If vaccination is targeted, only among risk groups, theoretically, population immunity can be obtained with less effort, as, for example, with seasonal flu. However, this may not work if a large number of infected individuals are concentrated in one place. And then flashes can not be avoided. This happens in the case of measles and poliomyelitis, which have spread in recent years in religious and anti-vaccination communities.
It is difficult to take into account the “freeloaders” – those who are not vaccinated for various reasons. Many factors influence their decision: there is no money, time, they are afraid of negative consequences. Knowing that collective immunity exists in society, which means that the probability of infection is close to zero, such people expect to ensure their own health at someone else’s expense.
Coronavirus for a long time
When the epidemic of the new coronavirus began, quarantine was introduced in most countries to contain the infection, smooth out the growth curve of the number of infected people and prevent the healthcare system from drowning from the influx of severe patients.
But Sweden, for example, was on the sidelines. Although not declaring it directly, the authorities chose a strategy to achieve collective immunity. Given that for SARS-CoV-2, the reproduction index is approximately 2.5, in order to achieve the goal, approximately 60 percent of the population should receive immunity (that is, get sick naturally because there is no vaccine yet). In a ten-millionth country, this is more than six million. However, the price of such a solution was too high. Chief epidemiologist Anders Tegnell in an interview with Sveriges Radio acknowledged his fallacy. On June 3, officially in Sweden, 41 thousand were sick, 4.5 thousand were killed.
According to published data, in France at the beginning of May, approximately 4.4 percent of the population was immunized. This is very far from achieving herd immunity.
In Moscow, the figure is higher – 14 percent, but it can change when the results of testing for antibodies across the country appear.
In a recent article in Science, scientists calculated that collective immunity to coronavirus in the United States would be achieved by mid-2022. Perhaps earlier if a vaccine is developed. While it is not there, it is necessary to maintain measures of social distance at least another two years – in case the virus consolidates in the population and becomes seasonal.