The figures seem stark: as many as 65,000 Britons may die over the next few months if a swine flu pandemic sweeps the country. And that may be just the toll from the first wave of the disease.

But health officials nervous of the stark headlines that will result from their statistical modelling stress this is the worst-case scenario, not a prediction.

There are too many imponderables: whether or not the nature of the virus changes, the level of accumulated resistance in the population, whether seasonal changes lead to an upsurge in levels of illness from the virus, and whether school closures over the summer holidays reduce the level of acceleration in the disease. Also, no one knows whether there will be a relatively brief wave of flu, or recurring ones, diminishing or otherwise.

Nearly one in three of the population may fall ill during the first wave of the illness, as many as one in 15 a week at its peak, with one in 50 of those cases ending up in hospital and up to one in 300 dying.

The 65,000 figure will shock, but remember it is only just over three times the "excess deaths" recorded from the bad winter flu season of 1999-2000, and around twice the number of UK deaths recorded in previous global flu pandemics of 1957-8 and 1968-70. It also pales by comparison with the scenarios worked out under pandemic flu plans based on assumptions about avian flu spreading from east Asia in recent years. Those suggested planning for an upper death toll figure of up to 750,000.

But levels of illness and death in certain areas and among young people may be higher than the national figures suggest.

And it is difficult to tell how reliable the any of the figures are, as experts from Imperial College London stressed earlier this week. Crudely comparing the reported number of deaths with the reported number of cases could lead to inaccuracies. Many people with swine flu are not visiting GPs and instead looking after themselves at home, so the number of people dying compared with the total number of cases could well be lower than current figures suggest.