Death rates for patients brought into English hospitals on the first Wednesday in August were 6% higher than for those arriving a week earlier, researchers found.

The first Wednesday in August is traditionally when newly qualified junior doctors take up their positions in NHS hospitals in England.

This period has sometimes been cynically labelled the ‘killing season’, but studies have not proven this assumption to be true until now.

Researchers said the “statistically significant” trend emerged after factors such as age, sex, social and economic background and additional diagnoses were taken into account.

Mortality was higher for medical patients than those requiring surgery or suffering from cancer. For medical patients, death rates increased by 8%.

The study was unable to draw firm conclusions about what had led to the extra deaths. But the researchers said it was a widely held belief that early August is an “unsafe period” to be admitted to NHS hospitals, since this was when newly qualified junior doctors start work.

Dr Paul Aylin, from the Dr Foster Unit at Imperial College London, said: “We wanted to find out whether mortality rates changed on the first Wednesday in August, when junior doctors take up their new posts. What we have found looks like an interesting pattern and we would now like to look at this in more detail to find out what might be causing the increase.

“Our study does not mean that people should avoid going into hospital that week. This is a relatively small difference in mortality rates, and the numbers of excess deaths are very low. It’s too early to say what might be causing it. It might simply be the result of differences between the patients who were admitted.”

The researchers studied 299,741 patients who were admitted as emergency cases to hospitals belonging to 175 NHS Trusts between 2000 and 2008.

Over the nine-year period a total of 151,844 patients were admitted on the last Wednesday in July, and 147,897 on the first Wednesday in August.

Both groups were monitored for a week and the number of in-hospital deaths recorded.

A total of 2,182 patients died who were admitted on the last Wednesday in July, compared with 2,227 of those brought into hospital on the first Wednesday in August.

Hospital death rates normally fluctuate throughout the year, peaking in winter. But the study - the largest of its kind ever conducted - showed a relatively consistent pattern.

A similar trend, known as the “July phenomenon” has previously been observed in the US.

Reporting their findings in the journal PLoS One, the researchers concluded: “We have found that patients admitted on the first Wednesday in August have a higher death rate than those admitted on the last Wednesday in July in hospitals in England.

“There was also a statistically significant higher death rate for medical patients that was not evident for surgical admissions or patients with malignancy. If this is due to the changeover of junior hospital staff, then this has potential implications not only for patient care, but for NHS management approaches to delivering safe care.

“We suggest further work to look at other measures such as patient safety, quality of care, process measures or medical charge review to identify preventable deaths rather than overall early mortality to further evaluate the effect of junior doctor changeover.”

Dr Shree Datta, who chairs the British Medical Association’s Junior Doctor Committee, said: “This study has to be judged alongside many previous studies looking at mortality rates before and after junior doctors start their new jobs, which have not shown any differences. Clearly even a small increase in death rates is of great concern and we need further research to see whether this is a real effect or an anomaly.”

Patricia Hamilton, the Department of Health’s director of medical education, said: “As the report itself acknowledges, the reasons for the small increase in mortality rates are unclear and require further research.

“The vast majority of NHS patients experience good quality, safe and effective care.

“Patients should be reassured that junior doctors undergo rigorous training and they undertake direct clinical care in areas where they have been trained and assessed as meeting the required competency.

“While in training, junior doctors are closely supervised by a senior doctor and this ratio is often 1:1.

“Local hospitals must ensure that they responsibly manage the introduction of new junior doctors each August by providing appropriate senior cover and supervision.”