WASHINGTON (Reuters) - US health officials said on Wednesday they are seeing a worrying pattern of serious bacterial infections in swine 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.

"That is the serious type of pneumococcal disease where the bacteria invades the blood and other internal sites." These so-called secondary infections can follow infection with a virus such asmillion high-priority people but companies are having trouble making and packaging vaccine. As of Wednesday, Schuchat said 61.2 million doses of H1N1 vaccine had been used or were ready to order.

She said surveillance so far had shown the H1N1 vaccine was as safe as the seasonal flu vaccine

"We don't see any problems at all," Schuchat said. Ten cases of a rare neurological condition called Guillain Barre Syndrome have been reported among people who got vaccinated, which is no more than would be seen among people not vaccinated, she said.

"So far, the vaccine data suggests that this is a safe vaccine," Schuchat said.

"We are expecting to see vaccination efforts really step up as we head into December," Schuchat added.

The CDC estimates swine flu has infected at least 22 million Americans and killed 3,900. The World Health Organization says the pandemic is moving from west to east and appears to have made one peak in the United States and the westernmost parts of Europe.

Globally, WHO estimates that 80 million doses of influenza vaccine have been distributed.