Understanding Herd Immunity

The principle of herd immunity is straightforward: when a significant portion of the population is immune to an infectious disease, the spread of that disease is hindered, thereby offering protection to those who are not immune. This concept is effective with diseases that have vaccines available. For instance, measles once caused widespread outbreaks, but now, due to vaccination, it rarely spreads beyond a few individuals in most regions.

The challenge with COVID-19 lies in the absence of a vaccine. Immunity can only be acquired through infection and recovery, and we know that the virus can be fatal for a significant number of those infected.

Despite understanding this for months, there are still claims suggesting that herd immunity is imminent.

Questionable Science

Two main arguments have emerged suggesting that herd immunity is close. The first, and clearly flawed, claim is that approximately 50% of the population is already immune due to pre-existing T-cell responses from previous coronavirus infections (like those causing the common cold). Proponents argue this means only a small percentage of the population needs to contract COVID-19 for herd immunity to be achieved, implying we are nearly there. However, evidence from superspreading events indicates that most individuals can contract COVID-19 upon exposure; pre-existing T-cells may reduce severity but do not confer immunity.

The second argument revolves around population heterogeneity—essentially, the differences in behaviors among individuals within a community. Traditionally, the herd immunity threshold is viewed as a fixed figure, influenced primarily by how contagious a disease is. For highly infectious diseases like measles, a near-total immunity is necessary, while less contagious diseases require fewer immune individuals.

However, this idea assumes uniformity in how people can spread the virus, which is not realistic. For instance, a solitary individual living away from others poses less risk than a service worker interacting with the public.

Mathematical models have attempted to account for this variability, suggesting that fewer infections may be needed to curb the virus's spread. Yet, these models depend heavily on the assumption that the behavior of individuals remains constant over time. If individuals frequently change jobs or social circles, the herd immunity threshold may rise again.

Some countries, like Sweden, have been cited as examples of achieving herd immunity, with reports of reduced COVID-19 cases despite a relatively low percentage of the population having antibodies. However, Sweden's restrictions are still in place, and the country continues to report hundreds of new cases daily. If herd immunity were truly established, one would expect a significant decline in new infections.

The Concept of Herd Resistance

This brings us to a fundamental issue with the notion of herd immunity concerning COVID-19. True herd immunity implies a lasting cessation of disease transmission, whereas the scenarios being described are more akin to endemicity, where a disease persists within a population but does not completely disappear. In regions where 20-30% of individuals have been infected, we would still anticipate ongoing transmission if restrictions were lifted.

Thus, we have not reached herd immunity. It is unlikely we will see the end of this virus without widespread vaccination. While some models suggest a possibility, the uncertainty remains high, and the likelihood is quite low.

The reality is that pursuing herd immunity without a vaccine poses serious risks. A significant percentage of COVID-19 cases result in death, and even individuals in their 40s and 50s can face severe consequences. High infection rates inevitably lead to high mortality rates, and the long-term effects of the virus on recovered individuals are still not fully understood.

It is disconcerting that a strategy relying on mass infection with a dangerous virus might not be the optimal solution.

Ultimately, the most prudent course of action is to continue with our preventive measures—maintaining social distancing, wearing masks, and avoiding large gatherings. The notion of herd immunity may seem appealing, but as previously discussed, it demands considerable sacrifices, particularly from vulnerable populations, all for the sake of economic stability. This reality remains unchanged.

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