COVID-19: Achieving herd immunity may occur sooner than previously thought

Mathematicians at the University of Nottingham in the United Kingdom and Stockholm University in Sweden realised that different groups of people within a population spread infections at different rates. Incorporating different ages and social activity, the herd immunity level reduced from 60% to 43%. The figure of 43% should be interpreted as an illustration rather than an exact value or even a best estimate. The research has been published in Science.

Herd immunity occurs when many people in a community become immune to an infectious disease. This happens when people contract the disease and build up natural immunity or when people receive a vaccine. When a large percentage of the population becomes immune to a disease, the spread of that disease slows down, stops and breaks the chain of transmission.

This research takes a new mathematical approach to estimate the herd immunity figure for a population to an infectious disease, such as the current COVID-19 pandemic. The herd immunity level is the fraction of the population that must become immune for spreading the disease to decline and stop when all preventive measures are lifted. For COVID-19, it is often stated that this is around 60%, a figure derived from the fraction of the population that must be vaccinated (in advance of an epidemic) to prevent a large outbreak.

The 60% assumes that each individual in the population is equally likely to be vaccinated, and hence immune. However, it won’t be true if immunity arises from spreading the disease in a population comprising people with different behaviours.

The more socially active the individuals are, the more likely they are to get infected than less socially active ones, and they are also more likely to infect people if they become infected.

“[T]he herd immunity level is lower when immunity is caused by disease spreading than when immunity comes from vaccination,” says Prof. Frank Ball from the University of Nottingham, one of the authors of the study.

The model suggested that the population would achieve herd immunity once 43% of the people had contracted the virus. At that point, the infection would stop spreading, and the outbreak would come to an end.

        GraphHerdImmunity2

Fig. 1

Plot of the overall fraction infected over time for the age and activity structured community with R0 = 2.5, for four different preventive levels inserted March 15 (day 30) and lifted June 30 (day 135). The blue, red, yellow and purple curves correspond to no, light, moderate, and severe preventive measures, respectively.                        

Fig. 2
Plot of the cumulative fraction infected over time for the age and activity structured community and R0 = 2.5, for a four different preventive levels inserted March 15 and lifted June 30. The blue curve corresponds to no preventive measures, the red with light preventive measure, the yellow to moderate preventive measures and the purple corresponding to severe preventive measures.

Image source: sciencemag.org 

 


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