Wednesday, Aug. 12We often hear the term herd immunity. What does it mean?In epidemiological terms, infectious disease spreads from person to person. Each person who gets infected will infect, on average, Rt (rate of transmission at that point in time) other people in each infection cycle. In a community that does not attend to the virus, initial spread will have Rt equal to R0.That is, the rate of transmission will equal the transmission constant. For COVID-19, estimates are that R0 is between 2 and 3. Each person infected with COVID-19 will infect, on average, two to three others. For sake of example, let’s assume the R0 for COVID-19 is 2.5.Initially, each person with COVID-19 will spread the virus to 2.5 others. In herd immunity, 1.5 of those 2.5 people will have already been infected and will have resistance to developing new infection, so each person with COVID-19 will only infect one other person, which means the infection will not grow. If 1.5 out of 2.5 people (on average) are infected, then the percent with resistance is 60 percent.There is a formula for determining your herd immunity threshold, and it is:Percent population not susceptible = 100 * (1 – 1/R0)Similarly, people have been looking for signs of partial immunity, because at any point in time, the Rt is scaled by 1- the fraction of people already infected, or Pi:Rt= R0* (1-Pi)In New York City, for example, where 25 percent of the population is estimated to have been infected, the formula would yield a 25-percent reduction in Rt. If Rt would be 2.5 without immunity, in NYC it would be only 1.875. All this is well and good, but are there signs of partial or herd immunity looming?The following plot is taken from the Italian COVID-19 dashboard, and it shows the Bergamo province (Lombardia on the plot, third row from the top), where Italian scientists have estimated 50 percent of the population has already been infected, still has a looming outbreak compared to neighboring regions. For whatever reason, herd immunity, or partial immunity, does not seem to be slowing the virus down:The short translation from this is that it is a long road ahead of us, and suppressing virus transmission is the only proven way to restore normalcy in our communities.Some links for further reading:On herd immunityhttps://www.cebm.net/covid-19/when-will-it-be-over-an-introduction-to-viral-reproduction-numbers-r0-and-re/Italian dashboard (in Italian). Click on “Dati cumulative”https://www.epicentro.iss.it/coronavirus/sars-cov-2-dashboard Contact Dave Blake.