Dr Douglas Fleming, director of the RCGP research and surveillance centre, presented new data at the HPA conference.
His research team analysed excess winter hospital admissions for respiratory syncytial virus (RSV) or flu among UK children.
The researchers showed that, if vaccination were to reduce hospital admissions, it would need to be possible to provide vaccinations to children under six months of age.
Vaccination of children older than 24 months against flu or RSV is not likely to reduce hospital admissions, the researchers concluded.
Dr Fleming said that the finding, combined with other research he had conducted, suggested that vaccinating close contacts was the best way to protect young children against flu infection.
'If we are going to prevent influenza in these very young children, we have got to get over to household protection,' he said. 'I do not think it's just the siblings, it is the household.'
He said that parents and other adult relations were as likely to introduce flu into the house as a child's older siblings.
'Household protection of the older family members who are able to receive the vaccine is the policy I would favour,' he said.