There is no good science to back new American and Canadian policies of vaccinating children under the age of 2 against the flu, a new report claims.

However, the researchers stress, this doesn't mean the vaccine is ineffective in toddlers; it just means it needs to be studied further.

"In children below the age of 2, we could find no evidence that the vaccine works, but that does not mean that the vaccine doesn't work," said Dr. Tom Jefferson, lead author of the study, which appears in this week's issue of The Lancet. "The data is insufficient to draw any conclusion," said Jefferson, who works at Cochrane Vaccines Field, in Rome, Italy. "We need more research."

The researchers also did not find any convincing evidence that vaccines have an effect on death rates, hospital admissions, serious complications and transmission of the flu. Again, this doesn't mean that the vaccines are ineffective, just that there's no research to prove they work.

But before making vaccination of very young children a public health policy, Jefferson and his colleagues believe large-scale studies need to be conducted.

As the study authors pointed out, both the United States and Canada have already started vaccinating children under the age of 2 against the flu. Infants aged 6 months to 23 months were included in recommendations starting in the 2004-05 flu season. Historically, vaccination efforts have targeted people 65 and older.

U.S. federal statistics released earlier this month revealed that 57.3 percent of children aged 6 months to 23 months were vaccinated from September through December 2004. At the time, Dr. Julie Gerberding, director of the U.S. Centers for Disease Control and Prevention, called those results "wonderful" -- especially since a 2002 survey showed that only 7.7 percent of children in this age group were immunized.

The current review looked at the available data on the efficacy and effectiveness of the flu vaccine on children 16 years old and younger.

Efficacy meant any reduction laboratory-confirmed cases of the flu (children who actually got infected), while effectiveness referred to a reduction in symptomatic cases, which is measured by reductions in mortality, hospitalizations, school absenteeism, transmissions, and other indicators.

This analysis was part of a longer review, which should be released this summer.

The authors identified 25 comparative studies that evaluated the efficacy and/or effectiveness of flu vaccines in children aged 16 and younger. They looked specifically at flu, flu-like illness, hospital admissions, school absences, complications and secondary transmissions.

Overall, vaccines with live viruses had a 79 percent efficacy and 38 percent effectiveness in children older than 2, compared with no immunization or a placebo.

Inactivated vaccines had an efficacy of only 65 percent and, in children under 2, were similar in their effects to placebo. They had an effectiveness of about 28 percent in children over 2.

Vaccines seemed to be effective in reducing long school absences, but otherwise had little effect on hospital stays and other measures when compared with a placebo or no intervention. This information was based on a small number of studies, however.

Information for hospitalizations was similarly weak. "One small Italian study looked at length of hospitalizations and found no difference, but the number of children was very small, so you'd be wrong in concluding that it doesn't work," Jefferson said. "We just don't know."

The problem with ascertaining effectiveness, said Dr. Keith Powell, a member of the American Academy of Pediatrics' infectious diseases committee, is that kids may be hospitalized or miss school because of any number of respiratory infections. "The average kid has eight respiratory infections a year," he said. "Only one of those is going to be the flu."

"Unless we're doing something to diagnose and distinguish between the two, you wouldn't know if you just looked at hospital rates by age for respiratory illness," he added.

While Jefferson called for more research, Powell said he believes it is still a good idea to vaccinate kids aged 6 to 23 months.