LONDON - The declining wave of pandemic H1N1 flu is likely to be followed by new, unknown strains of seasonal flu which health authorities must watch carefully to devise protection measures, European flu experts said on Friday.

The European Centre for Disease Prevention and Control (ECDC) warned that flu viruses never stand still and said governments should not relax H1N1 flu vaccination programmes, but remain on guard for possible changes in the virus and new strains.

The historical pattern of human influenzas is that after pandemics, the world experiences a new mix of viruses, the ECDC's flu expert Angus Nicoll wrote in the Eurosurveillance scientific journal.

In a telephone interview, Nicoll said although signs from many parts of Europe and the United States suggest circulation of H1N1 is declining, it is still too early to say the pandemic is over.

He noted that the virus responsible for the last pandemic in 1968-70 became more easily transmitted between its first and second winter, so that there were more cases and deaths in the second winter (1969-70) in at least two European countries.

An earlier pandemic in 1957-58 also declined before Christmas 1957, but then came back to cause a rise in flu-related deaths in the new year of 1958.

In the current pandemic, new infections of H1N1 flu have fallen sharply in recent weeks and some governments have been left with an oversupply of vaccines ordered to protect their populations against the virus that emerged last March.

Uptake of the vaccine has been limited in some countries and advice from medical experts that one dose is enough to protect against the virus, rather than the two originally anticipated, means some governments have more than they need.

Latest data from the ECDC, which monitors disease in the European Union, show that H1N1 -- also known as swine flu -- has killed more than 11,600 people around the world, more than 2,000 of them in Europe.
Nicoll said pandemic H1N1 flu had not completely halted other flu viruses in recent months, but had been the predominant strain, meaning that its decline could open the way for a new mix of viruses known as inter-pandemic or seasonal flus.

He said governments should continue to urge people to get vaccinated against H1N1, since the shots were the most potent countermeasure for any human flu.

The rule with influenza, pandemic and inter-pandemic, is to maintain vigilance and expect the unexpected, he said.

Nicoll also said some H1N1 vaccines, which governments ordered from drugmakers like GlaxoSmithKline, AstraZeneca, Sanofi-Aventis, Novartis and Baxter, among others, may prove useful in warding off any new strains of seasonal flu that emerge in the wake of the pandemic.

Countries should see through what they planned to do, he said. And one of the good things about some of the vaccines that European countries are using is that they have adjuvants (or boosters), which it makes it much more likely that they can cope with a virus that changes.

(Editing by Mark Trevelyan)