Let's start with this one: This is a myth. The human immune system is bombarded by antigens (essentially any foreign protein is an antigen) every minute of the day. If exposure to a vaccine antigen weakened or taxed the immune system, we wouldn't likely make it through the first year of life. Consider the principles in this CDC discussion of giving multiple childhood vaccines in combined doses: But an illness can make one vulnerable to a secondary infection, why is that different? Two reasons. In an active infection, the pathogen is actively multiplying. There can be billions of organisms your immune system is dealing with at a furious pace. In addition, the pathogens have multiple antigens. The immune system doesn't select one to address, it attacks all of them. And then there are toxins that some pathogens produce, also causing havoc in your body. Current vaccines, including flu vaccines, are extremely pure, containing single antigens in a small fixed quantity. One piece of evidence supporting the safe administration of multiple vaccines in single doses is the fact older tetanus vaccinations contained multiple different antigens. One dose of the older tetanus vaccination contained many more antigens than a current child would get with the whole series of vaccines they'd get on a single day now. And the second reason is local damage leaving the body open to invasion via the damaged cells. Influenza, for example, damages and incapacitates the cilia in the trachea, allowing invading organisms to collect in the lungs, in large enough numbers to cause infection. A vaccination, while occasionally causing local inflammation in the muscle, doesn't do so at the likely portal of entry of another pathogen.