WASHINGTON: US health officials said on Wednesday they are seeing a worrying pattern of serious bacterial infections in H1N1 flu patients, mostly among younger adults not normally vulnerable to them.

The pattern is typical of pandemics such as the current H1N1 pandemic but shows the need for patients and doctors to keep an eye out for the infections and treat them quickly, Dr Anne Schuchat of the US Centers for Disease Control and Prevention told reporters.

"We are seeing an increase of serious pneumococcal infections around the country," Schuchat told a telephone briefing.