Flu jab worked in one in three cases
Last winter's flu jab worked in 34% of cases, according to a final report by Public Health England.
At one stage early in the season, it was estimated that the vaccine was stopping only three out of every 100 immunised people developing symptoms.
But the report said there had been a "shift" in the dominant circulating strains during the rest of the winter.
Prof Paul Cosford, from Public Health England, said its effectiveness had been "slightly lower" than usual.
Flu is a constantly shifting target making it difficult to develop a vaccine. It is why a new jab is needed each year.
Officials are concerned that the drop in the vaccine's effectiveness may affect uptake this coming winter.
Every year the World Health Organization picks the three strains of flu that are most likely to be circulating.
A flu vaccine normally works in 50 out of every 100 cases.
But one strain of flu mutated so significantly that the vaccine offered much lower levels of protection.
The strain in question, H3N2, was also a particular worry as it primarily kills the elderly.
Prof Cosford said: "Whilst it's not possible to fully predict the strains that will circulate in any given season, flu vaccination remains the best protection we have against an unpredictable virus which can cause severe illness and deaths each year among at-risk groups.
"These include older people, pregnant women and those with a health condition, even one that is well-managed."