Now that coronavirus vaccines from Moderna (MRNA 0.17%) and the team of Pfizer (PFE -0.97%) and BioNTech (BNTX -2.55%) have started being distributed, many of our questions on the vaccine race have shifted into those concerning the vaccination challenge. An important one: Just how many people need to be vaccinated and protected against the coronavirus to reach herd immunity? Dr. Bruce Gellin of the Sabin Vaccine Institute joined Olivia Zitkus and Corinne Cardina of's Healthcare and Cannabis Bureau on a Dec. 18 episode of Fool Live to answer this question and more.

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Olivia Zitkus: I'd love to turn to that timeline and herd immunity idea. Herd immunity is when a critical mass of the population becomes immune to a pathogen. Ideally, through vaccination, not through infection and the spread dies out. I'll start here, if you could predict the timeline for when the US could reach herd immunity, what would be your most optimistic guess for what has to happen to get there?

Dr. Bruce Gellin: I'm going to hedge on this one because it's not about the calendar, but it is about the supply. I can give you some date, but if there aren't supplies to try to vaccinate people or if people aren't interested in getting vaccinated, then any of these projections are fanciful. The herd immunity is, now everybody has heard about that. A lot of people think it might be herd mentality. The herd immunity idea is that when there's enough people who are protected in a population because they're immune, either from having been infected from the natural virus or from a vaccine, from a virus standpoint, you are looking for people who you can infect. If more and more people in the community are protected, they have that shield in front of them or immunity, the virus has no place to go. That's the point when you start to see the virus will taper off, and essentially it has no place to go. Since it's only going to be infecting humans, that's for the herd immunity threshold. People throw around all kinds of numbers for that, from high numbers, from 70 to 85, some people think lower numbers. If you think about how many people are mixing in the population and agree that that affects transmission, the short story is it's going to take an overwhelming number of people, probably two-thirds of the population to be immune to start to see these effects. You can do the math. You hear all kinds of projections about supply, but just over the past day or two, we've heard about disruptions in the supply so you don't know. At the same time, there are other vaccines coming in that could increase that supply. It's a long way of saying, who knows. But if you want a date, I would say that by late spring, we should have a sense of where we are. That doesn't mean we're there, but I think we should at least have a sense, if not across the country, in communities how this is playing out to get a sense of how well this is happening. Again, learning both about the saturation of the population and the effects that vaccine might have on transmission. If it turns out that the vaccine stops transmission, that's a different story because we're going to then decrease the number of people who are spreading their virus. If a vaccine reduces your chance of getting sick but you're still infected and still have the ability to transmit asymptomatically, then that's a different problem where you still could have more virus in the community.