By Richard Woodman

LONDON (Reuters Health) - New systems are urgently needed to ensure childhood vaccines are safe, a researcher urged on Monday.

Dr. Thomas Jefferson, who has been funded to investigate vaccine safety by the European Commission, said the clinical trials done before vaccines are launched lack statistical power to analyze the cause of rare severe events encountered when vaccines are used in large populations.

"At present the EU member states mainly use a mix of passive reporting and ad hoc studies for vaccine safety surveillance," he said in a statement. "The sensitivity of passive surveillance for the detection of adverse events is low and there are marked inter-country variations in the efficiency of safety surveillance.

"Ad hoc studies are usually retrospective, limited in size and prone to bias. The possible causal mechanisms of rare adverse events following immunization are difficult to interpret."

Jefferson, who coordinates vaccine work for the Cochrane Collaboration from Italy, told Reuters Health that vaccine safety evaluation was becoming even more difficult now that children were receiving several vaccines in the same jab.

"If you get six, seven or eight antigens and you get a temperature, which vaccine caused it?" he asked.

The Cochrane Collaboration, based in Oxford, England, has often criticized the lack of large randomized clinical trials in medicine generally, but according to Jefferson good research is even more important in the case of vaccines.

"It is not like giving drugs to treat patients with a disease. The aim is 100% coverage of children. You are experimenting on the weakest members of our society and they are healthy," Jefferson said.

He disclosed that pilot immunization registers are about to be launched in Sweden and Finland that will record when each person is vaccinated, the particular vaccine batch and any subsequent health outcomes.

Hopefully, these schemes could be extended across the European Union, though legislation would be required because of data protection and harmonization.

Other measures that would improve vaccine safety evaluation included internationally agreed definitions of adverse events and systematic reviews of the evidence, he said.