Hello, this is Dr. Russell Kohl, chief medical officer at TMF Health Quality Institute with another installation of our Medical Minute. I’m joined today by Dr. Clifford Moy, TMF’s behavioral health medical director. In today’s episode, we’re going to talk about herd immunity – and no, we’re not referring to livestock. Community immunity, which is probably a better phrase for it, recognizes that both a vaccine and disease is not about an individual. Certainly an individual can contract a disease, but you have to get it from somebody else and then you are likely to spread it to other people. Recognizing that chain of events, herd immunity or community immunity tries to break that cycle. With community immunity, the strategy is to make sure that there are enough people in the community who are either not able to catch the disease or not able to spread the disease so that the disease dies out. An individual comes in from the outside who is carrying the virus, but that virus can't find anybody in the community to actually infect. Whether that's through a natural immunity or through a vaccine, that's the idea of herd or community immunity. This is very dependent upon how contagious a disease is. Think of it from this perspective. If you can cough out three viral particles, and those three viral particles land on three different people, if I have a disease that only takes one viral particle to get them sick, then now I have three sick people. If I have a disease that requires three, so it's not as infectious, then none of those people are going to be sick. Herd immunity is dependent upon how easily a disease spreads. Measles is the greatest example of this. Riding the subway, 30 minutes after somebody with measles was in that subway car and coughed, you can get measles at that point. With that, let's move from herd immunity to pandemic versus endemic with Dr. Moy.
DR. MOY: Thanks, Dr. Kohl. Another set of terms that we don't really talk about in general are the terms endemic, epidemic, and pandemic. I just wanted to take a moment to review these terms with you. Endemic is the usual presence of a disease in a population within a geographic area. Examples of an endemic include malaria, syphilis, HIV and influenza. These are constantly occurring diseases in communities across the globe. Epidemic is an increase, which is sometimes sudden in the number of cases of a disease that have been normally expected in that same population area. In comparing and contrasting a disease that is endemic or one that is an epidemic, we can look at rabies. Since rabies is a constantly existing condition, a single bat who is infected with rabies would be an endemic rabies case. However, if you had an increase in the number of rabid bats in an area, so you had a large number, then you would get an epidemic. Another example of an epidemic that more people might be familiar with are cases of West Nile Fever. If you had a population of about 200 people in one city who were infected with this virus, which is transmitted by a mosquito bite, it would be an epidemic. A pandemic, which is a much larger epidemic, is spread over several countries or continents, and COVID-19 definitely fits the definition of pandemic. The other example we have of a pandemic in our history is the influenza pandemic of 1918. It lasted for at least two years and affected some populations across the globe; however, COVID-19 has affected populations in nearly all countries across the globe, having a far greater reach.
DR. KOHL: Thank you, Dr. Moy. We hope you have found this information helpful. Please listen to our two other podcasts that give more information about why we vaccinate and how to build immunity. With that, once again I’m Russell Kohl, chief medical officer and family physician at TMF Health Quality Institute. Keep up the good work, and don’t forget to keep making the world a better place. – END –