Vaccination part 6: how much is too much?

I took a short break from the series of vaccination posts, but I have a couple more topics to go over so we’re not quite done.

In Part 5, I talked about adverse event considerations with giving multiple vaccines at the same time. The other aspect of that is whether vaccine response can be poorer if we give a lot of vaccines at once.

There’s often concern about giving animals too many vaccines at once, with vague descriptions about overloading or exhausting the immune system, and it not being able to respond well because it’s being challenged with too many antigens at once.

Is there a real issue?

This is another concern around vaccination that became widely discussed despite a lack of evidence indicating that routine vaccine administration can overload the immune system.

The immune system is designed to handle multiple antigenic challenges. Right now, your immune system is detecting and reacting to myriad antigenic exposures. We encounter large antigen loads every day and have massive amounts of antigen permanently residing in and on us as part of our bacterial, fungal and viral microbiotas. It’s not a matter of one day we get exposed to a few antigens from a vaccine and then the immune system gets to take a break for the year. The vaccine antigens are a major antigenic load since we’re aiming to stimulate a strong response, but they are far from the only antigenic loads our bodies encounter.

The immune system is quite capable of handling many antigens at once and immune exhaustion from vaccination doesn’t appear to be a relevant concern.

Here’s an older but still relevant review of vaccination issues in kids that addresses the topic (Offit et al 2002). Based on estimates of circulating B-cell numbers, reproduction capacity, antigen loads and antibody production kinetics, they estimated that an infant has the theoretical capacity to respond to approximately 10,000 vaccines at once. Yes, it’s theoretical and based on various estimates, but even if it’s off by a lot, it still doesn’t raise concern for giving combinations of vaccines that we would consider in veterinary medicine.

While there are legitimate reasons to consider separating vaccines in some situations or limiting vaccine numbers, but those largely revolve around situations where there are known vaccine reaction issues that are being managed. For routine purposes, there’s no concern about a poor immune response from giving many vaccines at once.