Flu jabs can be made more effective by changing the time of day they are given - mornings for men and afternoons for women are best - scientists believe.Synchronising the jab with the body's natural daily cyclical rhythm makes it more likely to offer good immunity, says the Medical Research Council team.The immune system gets sluggish as we age which explains why only a third of elderly people vaccinated get full protection from their winter flu vaccine.Rescheduling appointments may help.To test their theory, the researchers are using GP patients in Birmingham as guinea pigs.300 of them will be given morning or afternoon vaccination appointments, determined by their gender.Professor Janet Lord who is leading the research, said: "It's a major health issue trying to find ways to improve the vaccination response."We know that immunity goes down with ageing. But we may have found a way to counter that."She said it was a chance finding in about 150 patients that led them to the idea in the first place."A colleague discovered that vaccine response varies with the time of day."Men tend to have a better response in the morning, and women in the afternoon."We're not sure why, but we think it is down to hormones."Hormone levels in the body change throughout the day in a predictable, cyclical pattern. And the effect of this differs between men and women, the scientists suspect.Prof Lord and her team hope to get a definitive answer by studying at least 300 elderly patients attending for their routine flu vaccinations this winter and next."We've already made a start and hope to get enough patients on board to be able to see if such a simple, cheap measure of changing appointment times can make all the difference."