Thread by @kevinpurcell

Vaccine Efficacy Needed for a COVID-19 Coronavirus Vaccine to Prevent or Stop an Epidemic as the Sole Intervention

Or “How good does a vaccine have to be to get us ‘Back to Normal’?” ajpmonline.org/article/S0749-…

4 pairs of plots of reduction in cases vs vaccine efficiency.

Each pair of plots for two different values of R0 (2.5 and 3.5) for a fraction of the population already immune to the disease (0%, 5%, 15% and 30%).

The lines on a plot are the fractions of the population vaccinated

Simulation experiments revealed that to prevent an epidemic (reduce the peak by >99%), the vaccine efficacy has to be at least 60% when vaccination coverage is 100% (reproduction number [R0]=2.5–3.5).

This vaccine efficacy threshold rises to 70% when coverage drops to 75% and up to 80% when coverage drops to 60% when R0 is 2.5, rising to 80% when coverage drops to 75% when R0 is 3.5.

To extinguish an ongoing epidemic the vaccine efficacy has to be at least 60% when coverage is 100% and at least 80% when coverage drops to 75% to reduce the peak by 85%–86%, 61%–62%, and 32% when vaccination occurs after 5%, 15%, and 30% are immune to the disease.

A vaccine with an efficacy between 60% and 80% could still obviate the need for other measures (NPIs) under certain circumstances such as much higher, and in some cases, potentially unachievable, vaccination coverages.

This should set expectations for vaccine efficiency.

This study found that the vaccine has to have an efficacy of at least 70% to prevent an epidemic and of at least 80% to largely extinguish an epidemic without any other measures (e.g., social distancing).