The annual arrival each summer of a fresh crop of surgical residents and interns is not necessarily linked to worse outcomes for patients, a U.S. study suggests.

Sometimes called the "July phenomenon," this period has long been thought to be a time of year when patients might not fare as well due to the influx of less experienced clinicians, the study authors note in the journal JAMA Surgery.

But when researchers compared data on emergency surgery outcomes for more than 1.4 million patients from 2007 to 2011, they found the people treated in July and August actually had slightly lower odds of death and complications than individuals who had operations from September through June.

"We were pleasantly surprised," lead study author Dr. Adil Haider of Brigham and Women's Hospital in Boston said by email. "Given that we were studying emergency surgery, where results are not always the most predictable, as many cases are done in the middle of the night and not in the most optimal conditions, we went into this thinking that emergency patients may be even more at risk."

While the study didn't have data on the experience levels and specialty training of surgical team members, or the amount of supervision residents received, it's possible that outcomes might have been a little better in the summer months due to more involvement from senior surgeons, Haider said.

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Many teaching hospitals are shifting away from an on-call rotation for surgeons that might, for example, lead a thyroid specialist to be summoned to perform an emergency gall bladder procedure in the middle of the night, Haider said.

Instead, more hospitals are relying on acute care surgeons who specialize in providing trauma care for all types of emergency surgical conditions and remain overnight in the hospital.

"There is always a senior surgeon in house providing care for these patients and hopefully helping and carefully supervising our new surgical interns," Haider said.

To assess the validity of the July phenomenon, researchers compared outcomes for about 250,000 patients treated early in new residencies - in July and August - to another group of almost 1.2 million people seen later - from September through June.

Overall, patients treated during the summer were 4 percent less likely to die and 2 percent less likely to have complications than people seen at other times of the year, the study found.

The patients treated early in the new residencies were also less likely to have certain conditions, including hernias, intestinal obstructions, vascular disorders and gastrointestinal bleeding.

Even though the findings provide evidence that the July phenomenon may be little more than hospital lore, it raises a bigger question about why patients might be doing worse in the winter months, Dr. Rachel Kelz of the Center for Surgery and Economics at the University of Pennsylvania in Philadelphia and colleagues wrote in an accompanying editorial.

Over the winter months, previous research has found that patients are more likely to have serious conditions like strokes, heart attacks and bloodstream infections, Kelz and colleagues note.

"The study findings are likely demonstrating the system when it works at its best," Kelz said by email. "This is an observational study so we can't really assess the cause of their findings."