The Chief Medical Officer Sir Liam Donaldson said he remained "cautious" about the current fall in the number of new swine flu cases
The number of new cases of H1N1 swine flu in England and Scotland has fallen significantly, latest figures show.
England recorded an estimated 30,000 cases last week, compared with 110,000 the week before. In Scotland estimated numbers fell from 1,500 to 1,050.
The Health Protection Agency said there was no sign that the virus was mutating into a more lethal form, or developing resistance to drugs.
The total of swine flu-related deaths in England and Scotland stands at 40.
In all, 530 patients were admitted to hospital in England last week, down on the previous week's total of 793.
Wales is also reporting falling but widespread levels of "influenza like illness" with an estimated 2,670 new cases compared with 4,410 last week.
In Northern Ireland there were 16 new cases last week, up from 10 the previous week, bringing the total number of confirmed cases to 83 as at noon on 5 August.
HIGHEST FLU RATES IN ENGLAND
Wakefield District 188 GP consultations per 100,000
Tower Hamlets 163
Milton Keynes 123
North Tyneside 119
South Birmingham 117
Officials have always predicted rates of infection would fall away in the summer before a large surge in the autumn to coincide with the normal flu season.
England's Chief Medical Officer, Professor Sir Liam Donaldson, said it was very difficult to predict when a second wave would hit.
"It's guesswork really - we would anticipate that when the schools go back, at some point after that it would rise.
"We can't be complacent about this, we have to continue planning, we have to be ready for what happens in the autumn."
He added that the national pandemic flu service, launched two weeks ago to take the pressure off GPs, was "flexible enough" to scale up or down dependent on levels of swine flu.
Figures from the Department of Health suggest three-quarters of the general public are not very or not at all worried about swine flu.
Awareness is high, with 37% saying they heard a great deal about swine flu in the media in the past week.
Experts are currently looking at what is happening in the southern hemisphere to help predict what could happen in the UK over the winter months.
In the past week deaths in Argentina from swine flu have risen sharply and currently stand at 337.
There are also indications that Mexico may be now seeing a second wave of cases.
Meanwhile, the World Health Organization has announced that the first swine flu vaccines are likely to be licensed for use in the general population in September.
Several manufacturers have produced initial batches of a H1N1 vaccine and some clinical trials are already underway.
WHO director of vaccine research Dr Marie-Paule Kieny also sought to calm fears about safety of new vaccines.
She said the vaccines were based on "old and proven technology".
Although it has not yet been clarified who would be first in line for a vaccine, it is likely to be those who are most vulnerable, such as pregnant women and young children.
Some experts have raised concerns about the lack of safety data on flu vaccines in these groups.
In particular a very rare neurological condition called Guillain-Barre syndrome, which affected 500 people during a US vaccine programme against swine flu in 1976.
Dr Kieny said much was known about flu vaccines in these groups from seasonal vaccines given every winter and added that regulatory agencies would be monitoring for any signs of adverse reaction.
"The quality controls on today's vaccine are much better than they were 30 years ago," she added.
Regulators in the US and Europe have special plans in place to fast-track swine flu vaccines, some of which are based on conventional seasonal flu vaccines and some which use newer technology.
It comes as drug company, Baxter, has announced the production of the first commercial batches of its swine flu vaccine Celvapan.
The vaccine has been grown using cell culture, a much faster method than the traditional way of growing it in eggs.
Baxter is one of two companies contracted to provide pandemic flu vaccine to the UK, the other being GlaxoSmithKline, and both plan to start clinical trials this month.
One key part of the trials is to work out whether people need one or two doses of the vaccine.
Ministers have repeatedly said they expect to have enough doses for half the UK population by the end of the year.
Sir Liam said vaccine advisors were still finalising plans on who would get the first doses.