Dominic Murphy and colleagues randomly selected 4,882 military personnel with a median age of 32 who had all been to Iraq since 2003. Before being sent to the war-torn Middle Eastern country, tetanus, typhoid and yellow fever vaccines were all routinely administered to military personnel.

The researchers used detailed questionnaires asking about the maximum number of vaccinations the soldiers had received in any one day in preparation for deployment. The researchers randomly selected and assessed the medical health records of 10 percent of the group in order to see if multiple vaccinations had resulted in any adverse health effects reported at the time.

The study, published in the British Medical Journal, found "significant associations" between service personnel recalling two or more vaccinations in one day and complaints of fatigue, common mental disorders and a variety of physical symptoms.

However, when the researchers looked at the medical records -- without exception -- there were no health differences in those who had received one vaccination and those who had received multiple vaccinations.