Many parents worry that receiving multiple vaccines at the same time is stressful for a child. They frequently tell us that they are worried about overwhelming their child’s immune system with so many different antigens, or foreign products. When we talk about stimulating the body to produce protective antibodies, they worry about overstimulating the immune system.
Fortunately, there is no evidence that this is true. On the most basic level, studies are commonly done to compare the antibody level when a vaccine is given alone to when it is given in combination with other vaccines. Studies are also done to see if the timing between different vaccines is relevant. For example, if vaccines are not given at the same time, do they need to be given a day or a week or a month apart? With very few exceptions, there is no difference in protection. This means that, from the body’s point of view, a single-dose vaccine produces the same level of protective antibodies as a combination vaccine, and the spacing between vaccines is irrelevant. (An exception to this rule with regard to live, attenuated viral vaccines needing to be at least four weeks apart was discussed earlier.)
On a broader level, the immune system is very flexible. You should remember that there are billions of B cell and helper T cell receptors ready to match almost any antigen in the world. The antigens in the vaccine are not catching the body unawares.
Finally, the volume of antigens in vaccines is not overwhelming. Even without vaccines, a baby is exposed to hundreds of new antigens on a daily basis, and the immune system is able to respond to all of them. Every new object that goes into the mouth provides several new antigens, whether it is food, a toy the dog slobbered on, or dirt from under the couch. And there are normal illnesses in the community to which the baby is exposed. Each new virus has several new antigens, and a strep throat has over twenty.
In contrast, the average vaccine has twenty to thirty antigens, well within the normal daily limit. It is true that the total of all the recommended vaccines at the two-, four-, and six-month well-child checkups is around 150 antigens. However, in comparison, that is the same order of magnitude as the normal daily exposure to antigens, and yet we don’t worry about “overwhelming” the immune system with normal daily activities.