It varies depending on the vaccines in question. I can’t speak at all to military vaccine schedules specifically, but in general the answer depends on the vaccines in question.
TL;DR: Vaccines are spaced out when it is necessary to improve their safety and/or effectiveness. Clustering together is most commonly a pragmatic consideration.
Speaking to receiving multiple different vaccines, many can be given at once without impacting each others’ efficacy (in either direction). Childhood vaccination schedules are a good example of how we cluster immunizations that can be safely and effectively co-administered. This way we can ensure children receive the protection they need when they most need it and their body will best respond, without a pile of otherwise unnecessary office visits.
On the other hand, there are immunizations that we cannot give too close together, because in effect the immune system can’t respond fully to both without adequate time between them, so their effectiveness is reduced.
(Related: this is why we don’t just do all the doses of a multi-dose vaccine at once - your immune system needs time to respond. Think about it like training your body’s military - pile on all the drills at once and the end result is not going to be as good as giving the troops time to master each stage of training.)
As well, there are safety considerations. Some vaccines have shown to have increased risk of harmful side effects/reactions when given together, but are perfectly safe for use given enough time between them.
Hope this helps.
(Source: am MD. Disclaimer: am not immunologist or travel medicine expert - so I’ll leave the nitty gritty in more capable hands).
Edit: added TL;DR
It depends on the vaccines. We tend to not give multiple live attenuated vaccines all in one go but again, it depends on the patient as well as the vaccine.
Source: am PharmD student who just this morning, had a test in my Preventive Care class that was 90% about vaccines.
There is, and I cannot stress this enough to those who are uncertain, no evidence or proof to even suggest that taking ~~everything at once~~ multiple vaccines has a negative effect on the body. It could reduce the effectivity of some vaccines, which is why they’re often taken seperate.
The amount of pathogens our body is exposed to every day is staggering, and *if* we were adding anything to that we could say we were increasing the risk to our bodies. However, modern vaccines use *inactive* pathogens, harmless versions of more dangerous pathogens, to innoculate the user in a way that adds little to no risk at all.
It would be comparable to standing in a room, being bombarded by water balloons (pathogens), have a bucket of empty balloons (vaccines) thrown at you, and then argue that the empty balloons were just as troublesome.
Our bodies hardly notice the vaccines.
Edit: Of course there are exceptions, as with everything, but the problems these cause are negligable compared to the alternative. And vaccines are constantly improved.
I can tell you when I was deploying for Iraq in 2002 we were given the 3 step anthrax vaccine and the single dose live vaccinia shot. I want to say that the first anthrax and the smallpox were given together. Those are fairly unique to the military or maybe places like the CDC but even they have been proven fairly safe to be given to healthy adults. I did not have a family at the time but I want to say I remember that after those shots were given we weren't allowed to see civilians anymore because they were afraid of even the tiny remote chance of a spread of vaccinia to someone with immune system compromise. I know it wasn't REALLY part of your question but it might help spur more thought on the topic.
On the vaccine manufacturers Web site, they show lower effectiveness when combining vaccines.
So to answer OP's question, getting some of the vaccines in advance, and spacing them out over several weeks gives optimum immune response with minimal side effects.
If you are about to be deployed to a bacteria hot zone, the military rarely has the time so makes up for reduced effectiveness by over vaccinating.