Why Herd Immunity Against COVID-19 Is 60%, Not 100%

Herd immunity is a critical concept in public health, particularly in the context of infectious diseases like COVID-19. This phenomenon occurs when a large portion of a community becomes immune to a disease, thereby providing a protective effect to those who are not immune.

The Threshold for Herd Immunity

The threshold for herd immunity against COVID-19 has been a topic of discussion among experts. Traditionally, it is believed that herd immunity begins to occur when around 60-75% of the population is immune. However, the exact percentage can vary based on the transmissibility of the virus.

For example, the transmissibility of the virus can be measured by its reproductive number, or R0 (reproductive rate). When R0 is high, meaning that a large proportion of the population will get infected, it becomes harder to achieve herd immunity. Conversely, a low R0 value implies that only a few people exposed to the virus will get infected, making it easier to reach herd immunity.

Understanding 60% Herd Immunity

At 60% herd immunity, the system is just beginning to show signs of protection. This means that the number of people who will become infected is so small that it is not a significant number. However, it is important to note that this does not mean that children or other vulnerable populations will not catch the infection. What it does mean is that the disease spread will be minimal, and these populations will face a lower risk of infection due to the larger proportion of immune individuals.

The concept of herd immunity is based on the idea that if a sufficient number of people get infected and recover, their immune systems will prevent the virus from spreading further. This chain of infection is broken, thereby protecting those who are not immune. In simple terms, it is like a natural barrier that protects the non-immune individuals in a population.

Factors Influencing Herd Immunity

Several factors can influence the exact percentage required for herd immunity, including the degree of infectivity of the virus. The infectivity, or the ease with which a virus can spread from one person to another, plays a crucial role. High infectivity means a higher R0, which makes it harder to achieve herd immunity. Conversely, a lower R0 means that only a few people will get infected, making it easier to reach the threshold for herd immunity.

Historical examples, such as the eradication of smallpox, provide insights into this concept. In the 1970s, smallpox was almost eradicated with a vaccination rate very close to 100%. However, with the measles, it has been challenging to reach the same level of herd immunity, with recent outbreaks demonstrating that effective herd immunity requires a higher percentage of the population to be immune.

Sweden's Approach

Sweden has been one of the countries experimenting with a more relaxed approach to managing the spread of COVID-19. The country is aiming to reach around 50% herd immunity by mid-May. This approach is based on the assumption that a significant portion of the population will naturally acquire immunity through infection, balancing the need for societal and economic activities with public health measures.

Conclusion

In conclusion, while the exact threshold for herd immunity against COVID-19 might be subject to some debate, the 60% figure represents a minimal level of herd immunity where the disease spread is significantly slowed down. It is a complex interplay of factors such as the virus's transmissibility and the proportion of the population that has been infected or vaccinated. Achieving higher levels of herd immunity, like 90% or above, remains challenging due to various factors, including the mutation of the virus and ongoing transmission dynamics.