Stressing that concerns are most likely a false alarm, federal health officials have begun investigating the possibility that there could be significant complications from H1N1 vaccine.
A somewhat elevated rate of Guillain-Barre Syndrome, which can result in paralysis and death; Bell's palsy, which can lead to facial paralysis; and thrombocytopenia, which causes a low level of blood platelets, has been detected in the latest analysis of data.
The data is collected by five of the networks used by the government to monitor people inoculated against swine flu.
It is too early to know if the vaccine increased the risk of the conditions or if there is some other explanation, officials stressed.
A unanimous vote was made by the Health and Human Services Department's National Vaccine Advisory Committee to follow up on the findings.
"We're at the first step of determining whether there is a problem," Guthrie S. Birkhead, the chair of the committee, said, the Washington Post reports. "There's a lot more work to determine whether there is."
Further analysis, Marie McCormick, the leader of a subcommittee of experts of the National Vaccine Advisory Committee, could prove that the problems stem from other factors, the Washington Post reports.
If the Guillain-Barre Syndrome link is confirmed, the committee calculates, at most, the vaccine could be responsible for one extra case per one million vaccinations.
"From everything we know right now, the influenza vaccine, including the H1N1 vaccine, is very safe, and it's much riskier to get influenza than the influenza vaccine," Anne Schuchat of the federal Centers for Disease Control and Prevention told the Washington Post.