The latest analysis of data has detected what could be a somewhat elevated rate of Guillain-Barré syndrome, which can cause paralysis and death; Bell's palsy, a temporary facial paralysis; and thrombocytopenia, which is a low level of blood platelets, officials reported Friday. The data is being collected through five of the networks the government is using to monitor people who were inoculated against the swine flu.

Officials stressed that it is far too early to know whether the vaccine was increasing the risk of those conditions or whether there is some other explanation, such as doctors identifying more cases because of the intensive effort to pinpoint any safety problems with the vaccine.

Based on the preliminary report, the Health and Human Services Department's National Vaccine Advisory Committee, which has been charged with monitoring the vaccine's safety, voted unanimously to follow up on the findings. "We're at the first step of determining whether there is a problem," Guthrie S. Birkhead, who chairs the committee, said during a teleconference in which a subcommittee of experts presented its latest findings on the data. "There's a lot more work to determine whether there is."

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