Herd immunity

The figure below is adapted from an article published in 2011 by Fine and colleagues (). The article discusses the concept of “herd immunity”: individuals with immunity against a disease act as a “shield” for the community, slowing or stopping the spread of the disease.

At the top of the figure, the number of infected individuals grows exponentially, until a certain number of individuals with immunity is achieved. At the bottom of the figure, those with immunity “absorb and kill” the infectious agent, without passing it forward – significantly limiting the progression of the disease.

This illustrates the likely impact of vaccination in cases where immunity being acquired through full infection is problematic, such as with COVID-19. In these cases, vaccination would slow or stop the spread of the disease, even if only a proportion of the community is vaccinated.

That is, until the infectious agent mutates!

China’s relaxing of COVID-19 social distancing policy after containment appears to have worked

The graphs below summarize key results from a study published in early 2020 by Ainslie and colleagues (). Dr. Ainslie is in the Faculty of Medicine, School of Public Health, Imperial College London. The study looked at within-city movement, as a proxy for economic activity, and how that movement has influenced the numbers of new cases of COVID-19 in various areas, after initial containment.

As you can see, after initial containment is achieved, within-city movement (measured through a “Movement Index”) seems to be uncorrelated with new COVID-19 cases; or somewhat negatively correlated, as the authors note.

This surprising and counterintuitive outcome may be due to people becoming much more cautious about social interactions.