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In some cases, the combo shot does have less additives than the individual shots given separately, but not always. Obviously it is more at one time, and a good option if you were planning to give all those shots at once.
As for being ineffective...many children's immune systems have poor "memory", which means the shot provides immunity at first, but then fades over the weeks/months until the next shot, which "boosts" the immunity back up. So if you give a shot and wait way longer than the recommendation, the immunity could wear off. Not that the first shot is ineffective, just that the effectiveness is limited to a certain time frame. So exposure, say, six or eight months after the shot could be dangerous since immunity wears down, esp. in small kids.
After several reminders, most kids systems will "remember" and have a decent (although sometimes still imperfect) immunity. This is why it takes four shots of whooping cough vaccine to provide partial immunity in a child, but only one shot to give very stong immunity in an adult. This is also why they are now doing vaccine boosters for some illnesses in adults. Immunity is not always life long, and they now know many immunities fade over time.
So immanance to exposure is something you should consider in vaccines. Waiting to give shots can actually mean less shots over all, since shots in a mature system are more effective. Of course, some illnesses are very dangerous for children, and not for adults, so it is a balancing act either way.