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Is vaccination our only hope for attaining herd immunity?

When a sufficiently large percentage of a population is immunised against a pathogen, this latter can no longer cause large-scale epidemics. Immunity may be acquired through vaccination, or natural infection. What are the practical issues involved in attaining it?

In the absence of vaccination, and if immunity is long-lasting and has a sterilising effect (absence of infection and of transmission after recovery), the epidemic curve is typically composed of an ascending phase, a peak (herd immunity threshold, HIT), then a phase of decline in infections helped by the population’s immunity. In this simplified scenario, future infections will only occur in unvaccinated new-born infants. The critical vaccination threshold (CVT) is the number of vaccinated people required to stop the circulation of a pathogen within a population.

The reality is much more complex. Despite numerous effective vaccines, only smallpox has been eradicated. Vaccination and natural immunity do not always sterilise. This is true in particular for any population that does not operate as a closed system. From one country to another, public hygiene policies are variable and vaccines are not distributed in a consistent way. Contact between people evolves, as does the nature of the pathogens themselves. Non-pharmaceutical measures such as masks, lockdowns and social distancing also have a great deal of influence. It is therefore difficult to estimate the HIT/CVT in real situations. In addition, immunity does not progress in a linear way: a minority infects a majority of persons, generating clusters of various sizes. If the chain of transmission continues to be very active when HIT has been attained, then a large number of infections will occur after this point, a phenomenon known as “overshoot”.

Before the deployment of anti-COVID-19 vaccines, the prevailing notion was to wait for herd immunity to be attained naturally through the spread of infections. Although it is an empirical fact and a direct consequence of transmission dynamics, the notion of “herd immunity” is, however, poorly understood. This term saw the light of day in the last century, at the time of bacterial epidemics in American cattle farms. Only immunised animals were to be reared, and the introduction of new animals was  to be prevented. The term then became applicable to viruses.

Recent examples of controlling an epidemic by this approach are rare (chickenpox, and flu before the arrival of vaccinations). It is not a responsible choice, in view of the risks for the population and for health structures. When hospital provision is overwhelmed by COVID-19 cases, other illnesses take second place.

However, since the evolution of epidemics depends on the availability of naïve hosts, vaccinating a large part of the population during an epidemic brings many benefits: it limits clusters and overshoots, and restricts mortality and morbidity which occur before HIT is reached. Herd immunity is therefore linked nowadays with vaccination, since this is undoubtedly the best way of attaining it.

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