SAN FRANCISCO -- It's a story that has been making headlines across the United States: Did Californians develop herd immunity after possibly being exposed to novel coronavirus last year?On Thursday, a fellow at Stanford's Hoover Institute released the theory suggesting that what was perceived to be a nasty and early flu season in California was actually COVID-19 spreading undetected throughout the community.Our sister station KGO-TV in San Francisco sat down with Dr. Alok Pate l for more insight on what herd immunity means and what it would take for it to happen in California and the United States."Herd immunity is basically a percentage of the entire community who would need to be immune to an infection so the infection couldn't spread from person to person. And so when we look at coronavirus, we talked about that number around two to three, that's a reproductive number of how many people can be infected from one case. Now, with coronavirus, we're suspecting that we would need about 50% to 60% of the population to be immune to it to achieve that herd immunity so it couldn't spread like wildfire. We would need millions and millions of Americans to actually get either infected or vaccinated to get immune to create herd immunity that's a huge number. And we'll probably have a huge number of casualties and deaths. If you had infections in the millions.""I think the latter is possible. A lot of people have mentioned that you know, we may have had COVID-19 here a few months earlier than we suspected it could have been spreading. But again, I would have to go back to the numbers, and really look at the fact that we've had so many cases, pop up all over the country, and you know people are dying they're getting hospitalized or getting extremely sick. And before we were actually testing for COVID-19, those would come up as mysterious cases. So I would believe that local public health experts in positions still would have been saying like hey we're having a lot of people hospitalized for something, and none of our tests are coming back positive and so that never came up. Now it's possible in the past somebody may have died and they would have said you know this is pneumonia or respiratory failure, and they weren't exactly calling it COVID-19. But again, if we say that 50 to 60% need to be infected at herd immunity. That's hard for me to believe right now in the United States. We have about 19,000 deaths and 500,000 cases, that's a little bit more than 3%. We're going to be talking about 150 million Americans infected. Or better yet, getting them vaccinated which would actually do this in a safe way, get that herd immunity.""We theorized that with this virus, like many others. You can get infected, you'll build up antibodies, over time, and those antibodies will protect you from reinfection. Now, because this is a novel virus, scientists don't yet know what it is defined as immunity, or even how long it lasts or how long it builds up. Those are the things that we really need to learn, hopefully from these serologic courses antibody test. It was just FDA approved on April 1, we need to see a lot more testing with antibody tests because that'll really tell us how many people may have been exposed in the past, and how widespread this disease is among people who may not have had really serious symptoms or may not have been hospitalized."