Many countries, including the UK, should reassess their seasonal flu immunisation programmes to include toddlers under the age of two years, according to Finnish researchers.
They monitored 631 children, aged nine and 40 months, of which 154 were fully vaccinated and the rest were not.
The study, published online in The Lancet Infectious Diseases, found 5 per cent of the vaccinated children contracted flu compared with 13 per cent of those unvaccinated.
No adverse effects were observed.
The authors said: “These results provide clear evidence for effectiveness of the inactivated influenza vaccine even in the youngest children during seasons with normal influenza activity.”