The chief medical officer has said the number of A(H1N1) cases are expected to rise in autumn, but found it "virtually impossible" to accurately predict the next wave of swine flu in the UK.
Some experts believe it will hit in September and October as schools and universities open after the summer break.
Sir Liam's comments come in the wake of figures released this week showing a decline in the number of newly diagnosed cases of swine flu, with about 11,000 people diagnosed with the infection in England last week.
The week before last saw an estimated 25,000 people contracting the infection, and data shows a drop in the number cases across all age groups and in most parts of England.
Sir Liam also said that currently, just one 10 people in England who report swine flu-like symptoms actually had the disease.
In England, 263 patients are being treated in hospital, of which 30 are in intensive care, down on 39 in intensive care out of the 371 in hospital last week.
Swine flu has claimed 54 lives until now, with over half having died in London.