These swine flu deaths do not mean it's 1968 all over again

Are we going to have a mild outbreak of H1N1, or a fearsome dose of H3N2? America is the place to look for answer

• 
• • Hugh Pennington
  • guardian.co.uk, Tuesday 14 July 2009 17.36 BST
  • Article history

The deaths of a six-year-old girl and a Bedfordshire GP have raised new fears about swine flu. Both Chloe Buckley and Dr Michael Day had been infected with the virus, but Chloe appeared to be fit and well before falling ill, and Dr Day, though retired, was still active – and was the first healthcare worker in Britain to be infected. Three questions are now being asked: is the virus getting more virulent; where is the pandemic going; and what will happen next?

The driving force behind recent planning has been the fear that a new virus might evolve and spread worldwide to cause a pandemic with mortality rates like those seen in 1918-19. It is certain that this swine flu is far less virulent, but the experts are debating which of the common seasonal flu viruses it resembles. Is it mild like the H1N1, or slightly stronger like the H3N2? Opinion is divided, but there is no strong evidence that it is nastier than the average winter flu. Neither virus has been newsworthy for decades, but they kill thousands every winter. The precise number is unknown, because in contrast to the present situation with swine flu, most cases are not investigated virologically. A recent robust statistical study estimated that in the non-pandemic years of 1979-2001, seasonal flu killed an average of 41,400 people every year. Swine flu has been active in the US since April, and the death toll, as of 10 July, is 211.

Because swine flu is new, it is wrong to expect that the behaviour of flu viruses in the past will be a reliable guide to its development. And in any case, history has not been generous with experience. We have only three modern pandemics to go on: 1918-19. 1957, and 1968.

The best guide for the UK is to watch what is happening in the US; our virus is about six weeks behind theirs. On 8 July the Health Protection Agency reported 9,718 cases in the UK; to date there have been 17 deaths. Similar figures were reported from the US on 1 June (10,053 cases and 17 deaths), when the US moved from daily case counts to a weekly system relying on the reporting of illnesses to primary care providers; we did the same a month later.

Also similar is the uneven geographical distribution of cases. In the US, Wisconsin has topped the case numbers for many weeks, followed by Texas and Illinois. In the UK, the West Midlands, London and Scotland account for the majority of cases. Such distribution, caused by relatively intense but local outbreaks, is typical for flu. Of the deaths in the US, about 25% had been in people without underlying health problems, and events in the UK are beginning to mirror this pattern. And as in the UK, most US infections have been in the five-to-14 age group.

Seasonal flu in most healthy individuals is relatively mild, and volunteer studies have shown that a significant minority of those infected have no symptoms at all. However a small minority of people with no predisposing risk factors have a hard time, and a few die. Some of these develop a lethal viral pneumonia, perhaps made much worse by an over-vigorous immune response. Others have an overwhelming pneumonia caused by bacteria that have invaded lungs damaged by the virus. In a few it is not clear how the virus killed them.

The extremes of age predispose to lethality. Swine flu is following this pattern, with one striking exception – it is sparing those aged over 65. There is some very preliminary evidence that some in this age group have a degree of immunity because in years past they had been infected with a distantly related virus. Time will tell. It is expected that the current outbreaks will run out of steam as the summer progresses, but will then start up again when the cold weather returns. Events in Australia (so far 6,787 swine flu cases, with 18 deaths) and other countries in the southern hemisphere that have just started their winter flu season are being keenly monitored as guides of things to come here.

So we are lucky in the UK in that other countries are ahead of us. Just as well. Over the years flu has been difficult to second guess, mainly because it evolves rapidly – and because evolution, for all practical purposes, is a random process. Never has Sam Goldwyn's aphorism been more apposite: "Making predictions is difficult, particularly about the future."

Just like the seasonal flu over the last decade, swine flu is usually mild – but it can kill, even the healthy. The risk is very low – but swine flu parties are a bad idea. Far better to wait for the vaccine. It is coming.