There is no doubt that herd immunity was the government’s initial Covid strategy
4 min read
Herd immunity was never a realistic approach but probably contributed to the delay in government taking action back in March 2020.
For an infectious organism to spread in a population, it needs contact with people susceptible to the disease. If there are not enough people susceptible, then the organism cannot continue to spread.
Individuals may not be vulnerable to infection because they have a degree of immunity conferred by a previous infection or by immunisation. As the proportion of the population who are susceptible to being infected falls, the organism finds it more and more challenging to move from person to person, and the chain of infection may come to an end.
A population where the organism can’t spread in the community because of the level of immunity that people possess, is said to have herd immunity.
There is no doubt that the facilitation of herd immunity was the favoured approach amongst senior government officials in early March 2020. Some used the term ‘herd immunity’ openly; others spoke of how the numbers of cases would fall when the virus “effectively runs out of people in the population to infect”.
Successful isolation on a mass scale was always impossible
It was never a realistic approach and only came to brief prominence because of the dearth of high-level public health medicine skills amongst senior officials. It probably did, however, contribute to the delay in taking action.
Not surprisingly, the term herd immunity has its origins in the veterinary world. The theoretical basis for the concept was developed in the 1920s and 1930s in laboratory experiments often using homogeneous populations of laboratory mice. Mice infected with an infectious disease were placed amongst healthy populations of mice, and the spread of the disease studied. The basic concepts developed in that period still underlie discussions of herd immunity.
The difficulty with the practical application of a strategy of developing herd immunity in the Covid-19 era is that we are not dealing with a homogeneous laboratory setting, plus we are dealing with a previously unknown organism and the disease it causes.
There are four main reasons why a strategy of leaving the population open to a dangerous virus like Covid-19 to let herd immunity develop is an unacceptable and impracticable approach.
Firstly, in the absence of vaccination, the only way to build immunity is to experience the infection. Although we can identify many of the people at high risk of a fatal outcome from Covid-19, we can’t identify all of them. Some of those with only moderate or even low risk would suffer fatal disease if the infection were allowed to spread freely.
Secondly, even if we could identify all those at most risk, to protect them, they would have to be in complete isolation until the infection had spread far and wide in the population. Given how many of those at high risk require routine care, successful isolation on a mass scale was always impossible.
Thirdly, even if a level of herd immunity were achieved, once those who had been isolated for their protection were back in the community, the proportion of susceptible people would immediately increase. If someone with the infection were to arrive in the community, an outbreak would be more than possible; it would be inevitable.
Fourthly, we still do not fully understand the natural history of this virus. The nature of the long-term disability caused by the virus is far from being understood, and there may even be other long-term consequences to health that emerge in the coming years.
In Covid-19, the virus is highly infectious, with new variants increasing that infectivity. The chances of reaching a position where herd immunity will keep us safe, even with vaccination, are slim. Our population is very far from homogeneous, as the substantial variation in the uptake of immunisation confirms.
Our future protection from Covid-19 requires building a robust public health system at a local level capable of dealing with flareups in communities as they happen. It also depends on the success of the global effort to suppress the virus plus the availability of effective vaccination for everyone.
Gabriel Scally is a visiting professor of public health at the University of Bristol and member of Independent SAGE.
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