How Many Infections Are Required for Herd Immunity?

As countries around the world struggle to control COVID-19 outbreaks, the concept of herd immunity is gaining momentum. A new study from the University of Nottingham and University of Stockholm has explored the concept further and found the threshold for herd immunity could be lower than originally thought.

Age and social activity affect herd immunity threshold

Using a simple model, mathematicians at the universities categorised people into groups according to their age and level of social activity. When factoring in these variables, the researchers say the threshold for herd immunity drops from 60% to 43%. This suggests that to develop indirect protection against COVID-19, communities or countries need to infect just over 40% of their population.

The findings were published in the journal Science, with Professor Frank Ball from the University of Nottingham explaining how the team used age and social activity variables to derive the 43% figure. “By taking this new mathematical approach to estimating the level for herd immunity to be achieved we found it could potentially be reduced to 43% and that this reduction is mainly due to activity level rather than age structure,” says Ball.

Breaking the transmission chain

Herd immunity occurs when an infectious disease spreads to the point that a significant percentage of the population develop natural immunity. This mass immunity then indirectly breaks the transmission chain and stops the disease from spreading. While herd immunity percentages for COVID-19 were initially estimated at 60% the new research suggests this figure could be much lower.

“We estimate that if R0 = 2.5 in an age-structured community with mixing rates fitted to social activity then the disease-induced herd immunity level can be around 43%, which is substantially less than the classical herd immunity level of 60% obtained through homogeneous immunization of the population,” reads the abstract.

Emerging from COVID-19

Ball says the findings could play important role in managing the COVID-19 pandemic and planning when and how to lift lockdown restrictions. “The more socially active individuals are then 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,” says Ball. “Consequently, the herd immunity level is lower when immunity is caused by disease spreading than when immunity comes from vaccination.”

Want to know more about how scientists are working to fight the SARS-CoV-2 virus? ‘Getting a grip on Covid-19 test samples’ explores the issues faced by laboratories around the world, with expert insight from Neil Benn MSc and Stephen Knight MA on behalf of Ziath Ltd.