 If you watched the early Republican U.S. presidential candidate debate, you may recall that the subject of vaccines was raised. Donald Trump, a reality TV star and billionaire, asserted that he believed that vaccines and autism are linked, and Ben Carson, a neurosurgeon-turned politician, while he didn't agree with that link, generously conceded that indeed, the modern vaccine schedule is too many, too soon for young children. They then turned to the ophthalmologist, Rand Paul, for his opinion. He also concedes that he's worried about how bunched up the vaccine schedule is. So we have a brain surgeon, an eye doctor, and a casino owner's opinion. And they're all equally wrong, and not just a little wrong, but profoundly ignorant. If we can talk about it without controversy in a nationally covered debate, among the people who could potentially run the U.S. executive branch, the part of our government that sets medical policy and administers the budgets of the CDC and National Institutes of Health, how widespread is this belief? In a phone survey of 1,600 parents with children under the age of 7, 25% believed that their child's immune system could become weakened as a result of too many immunizations, and 23% believed that children get more immunizations than are good for them, even among parents who rated vaccine safety and efficacy as high. I'd say this is a concern that a lot of parents have. If you know one, I hope you'll share this video with them, or post it on Facebook or Twitter. It's true that the number of vaccines has increased in recent years. In 1960, the recommended regimen included five vaccines and boosters, or second doses, for a grand total of eight shots by age two. That included smallpox, diphtheria, tetanus, pertussis, and polio vaccines. By 1980, some of these had been combined into a single shot, so that seven vaccines could be administered in a total of five injections. By 2000, the vaccine schedule included 11 vaccines administered in 20 total injections by age two, up to five in a single visit. It's an understandable concern that more vaccines and more shots might pose some risk for a child's immune system. Let's take a quick look at how vaccination works. It uses the adaptive immune system composed mostly of white blood cells called lymphocytes, which are categorized into T cells and B cells. Each individual cell reacts to exactly one foreign substance, what we call an antigen. It does this through an amazing shuffling of your DNA, your genome itself, into a combination of DNA bases that differs from the one you inherited from your parents. So each T or B cell recognizes one and only one antigen. But even then, it only recognizes a tiny part of that antigen, and multiple lymphocytes may recognize different bits of the same protein. So for example, the influenza virus has 18 unique proteins, so we would say it represents 18 potential antigens. But following vaccination or infection by influenza, there may be thousands of unique lymphocytes generated that recognize different bits of these 18 proteins. When an antigen is recognized by a B cell, that B cell grows into many thousands of copies of itself and generates lots of antibodies. Those antibodies stick to the foreign invader and light it up for the rest of the immune system to attack. Later, some of those antibody producing B cells become memory cells, remembering the most effective response to the same invader the next time it's detected. Is this capacity limited? Yes, absolutely. Your ability to continue to make new unique memory cells is limited. But you have to understand the kind of numbers involved. You have the potential to respond to between a billion and a hundred billion antigens at any given time. That's a one with between 9 or 11 zeros. Your body then makes about 2 billion new lymphocytes each day. Are we overloading that capacity with vaccines? Each antigen that is present in a vaccine, if it's effective, produces about a thousand specific B cells. Each vaccine contains approximately 100 antigens, and each antigen has about 10 parts that can generate unique responses. That means that every infant has the capacity to respond to 10,000 vaccines at any one time. The maximum dose in the current schedule is 11, or 0.1% of the total capacity of the immune system to respond. Is the antigen load increasing? No, in fact, it has decreased substantially since 1960. As a result of two things, one, we removed smallpox vaccine from the schedule as it was no longer needed due to global eradication, and it contained about 200 proteins. And two, we switched from vaccinations using dead pertussis bacteria, that's the causative agent and whooping cough, which contained at least 2,000 antigenic proteins, to a vaccine containing only a few proteins from that bacteria. In 1960, an infant by age two was exposed to about 3,000 antigens in their vaccine series. In the year 2000, that was reduced to only 125. What about children who are exposed to lots of vaccine antigens? We've tested the possibility that there's a correlation between vaccine status and autism spectrum disorders, or just general neuropsychological outcomes, and there's no correlation. But what if it's the number of vaccines, or the schedule of administration, too many too soon? This hypothesis has also been tested, most recently in 2013 by a group at the CDC. The infants exposed to the highest number of antigens before 24 months of age actually performed better on attention and executive function tests. In a separate study in 2010, children between the ages of 7 and 10 who had received vaccines on time, according to the current schedule, were compared to children who had received delayed or spread out vaccine schedules. The children on the schedule outperformed the delayed scheduled children in 12 out of 42 neuropsychological measures. And for the remaining 30, there was no difference between the groups. In no measure did delay have a positive effect. So if you are a parent watching this video, and you're swayed by the idea that maybe we overload our child's immune system with too many vaccines, I can assure you that this is not the case. Your child's body, shortly after he or she is born, will be colonized by good bacteria. An infant will likely have around 10 billion bacteria, representing a few thousand species. And each of those bacterial species will have a dozen or so antigenic proteins that your child's immune system will react to. Which means on the day you took your child to the physician for a vaccine, there was more challenge to their immune system in their diaper than there was in the syringe. And that's a good thing. We have an amazing immune system capable of incredible feats of immunity. Unfortunately, the pathogens that cause disease are likewise incredible and have learned to evade it. A vaccine is our way of using our human intellect to defeat nature's most clever strategies. As always, the best medical advice comes from a licensed physician. If you have questions or concerns, talk it over with your doctor and actually listen to their advice. I wish for you and your family good health. Thanks for watching.