Federal health officials are investigating the first hints of possible significant complications from the H1N1 vaccine, but stressed that the concerns will probably turn out to be a false alarm.

The latest analysis of data has detected what could be a somewhat elevated rate of Guillain-Barre 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 was far too early to know whether the vaccine was increasing the risk of those conditions or whether there was another 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.

Marie McCormick, who led a subcommittee of experts, said there was a good chance the indications of problems could disappear with further analysis. Even if the link with Guillain-Barre syndrome is confirmed, the committee calculated the vaccine at most could be causing one extra case per 1 million people vaccinated.

Even if the possible risks turn out to be real, officials stressed that the danger of the flu remain far greater.