The annual flu jab given to hundreds of thousands of people this winter provides only limited protection against the illness, researchers say.
More effective vaccines are "urgently needed" especially for the elderly and those with health problems who are at greatest risk, they add.
A review of 31 studies published over four decades found that the most widely used seasonal flu vaccine, known as trivalent inactivated vaccine, was only 59 per cent effective in healthy adults.
The swine flu vaccine introduced in response to the pandemic in 2009 was slightly better, giving 69 per cent protection among people under 65.
But the researchers say this is "not adequate" to protect a population against a future pandemic that caused serious illness and death.
In the UK flu jabs are recommended for everyone over 65 and for those of any age over six months with conditions such as asthma and bronchitis. But results show vaccination is even less effective in these groups.
A new kind of vaccine containing a weakened live virus and delivered as a nasal spray was shown to prevent flu in 83 per cent of children under seven, suggesting vaccinating children might in future be a better way of protecting the elderly, by reducing circulating flu viruses in the population.
But in the meantime the authors say in The Lancet Infectious Diseases that the present vaccination programme is "the best intervention available for seasonal influenza".
Michael Osterholm, of the University of Minnesota, who led the study, said: "The potential global effect of a severe pandemic suggests an urgent need for a new generation of more highly effective... vaccines that can be manufactured rapidly."
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