Could the BBC's 'Survivors' scenario really happen?

YES! Of course it could, that is a stupid question. Anyway, the Times Online has a big article about the new show and its relation to reality:

"The sudden outbreak of a lethal virus that spreads quickly, killing millions, is a nightmare scenario that haunts many of us - especially those who have been watching BBC One's apocalyptic new drama Survivors.

Viruses have indeed wiped out 90 per cent of a human population. We've seen that in the Democratic Republic of Congo in 1976 with Ebola, an unspeakably nasty affliction caused by a haemorrhagic fever virus that has one of the highest case fatality rates of any human disease. It's one of a family of viruses that you definitely don't want to have, which also includes those responsible for Lassa fever and Marburg fever. But this event did take place in a very isolated area in a small population, with almost no access to medical help.

The lethality of these viruses limits their spread. People become so ill so quickly that they cannot travel far before dying. Ah yes, you say, (if you are the scriptwriter of Survivors), but what if someone boarded an aircraft immediately after infection but before symptoms appeared. Couldn't they spread it halfway across the world and kill 90 per cent of the population that way? No. Sorry to be a Survivors killjoy, but these viruses are pretty much stopped in their tracks by effective isolation and infection-control measures.

On the other hand, a candidate for a really good apocalypse virus is one that can be transmitted by droplet infection in the air, is infectious before a person has any symptoms and is not so lethal that people who have it can't spread it about by traveling. Something like flu, for instance. Pandemic flu is perfect. So would - could - pandemic flu wipe out 90 per cent of the population?

Categorically not, and for a number of reasons. One is logistics. You can't infect everyone at the same time, which means that we would have warning of potential infection and could prepare against it.

Avian flu poses a threat

The most likely (but by no means the only) pandemic flu candidate is H5N1, which causes avian flu in birds, such as chickens and ducks. At present, although human beings can be infected with avian flu and it has a high death rate (about 60 per cent), it is not spread from human being to human being. If it was, we would, thanks to the surveillance systems that the World Health Organisation has in place, know about it quickly, know where it was and could take counter measures. For instance, government might order mass vaccination with a pre-pandemic flu strain. This wouldn't be as effective as a vaccine specific to the particular strain, and would not necessarily prevent infection, but it is highly likely to prevent deaths.

But back to that scary old 60 per cent death rate with avian flu. Jonathan Van Tamm, Professor of Health Prevention at Nottingham University, did the numbers for me. “What this figure means is that 60 per cent of people who get ill with symptoms die, not 60 per cent of the population. It's a big difference.” To understand how many in a population might die from a new virus, you need to know the clinical attack rate, that is the proportion of the population that becomes ill with symptoms. Not everyone will be exposed to it and not all of those exposed are likely to develop symptoms and become sick.

If the clinical attack rate of a pandemic flu virus were, for example, 33 per cent (and not everyone would get it all at once of course, like in Survivors) and there was a case fatality rate of 60 per cent, 20 per cent of the population would die over the period of the pandemic, but this is unlikely.

As Professor Van Tamm says: “History tells us that, so far, we have never encountered a pandemic flu virus with a clinical fatality rate greater than 2.5 per cent (in 1918 it was 2.5 per cent; in 1957 and in 1968 it was less than 0.5 per cent). It's still an awful lot of people (280,000 in 1918) but nothing like the Survivors scenario.

A good plot line, and one that has been exploited in Survivors, is how panic and lack of preparation by government contributed to death rates. But here is the good news. As far as pandemic flu is concerned, which our Government regards as the No 1 threat facing the UK, Britain is among the best, if not the best, prepared countries in the world.

Lest you think this is hype, I took part in the G8 pandemic flu exercise in 2006, a simulation of a worldwide flu crisis in which all of the G8 nations took part. It was immediately apparent that this was the case. Most countries are woefully ill-prepared compared with us. If you want to check out the extensive and detailed plans for pandemic preparedness, they are on the Department of Health's website.

These plans have already been tested, in an exercise in January last year called Winter Willow. Lasting several days, it involved more than 5,000 people. It assumed an attack rate (those developing symptoms) of 30 per cent of the population. There were a number of lessons learnt, such as the need for clearer and more consistent advice on the use of antiviral drugs, face masks and the stocking of home supplies.

But there are other reasons why a flu pandemic is unlikely to mirror the Survivors outbreak. For instance, typically viruses do not retain their lethality. They are modified, with the passage through humans becoming more transmissible but less lethal with time.

Fit young men died within 24 hours

But perhaps the most potent reason is that we are not clones. Our immune systems are not all configured the same, our response to illness is highly variable, even when infected with the same virus. Typically, the old, sick and young would die, but even this isn't always the case. In the 1918 pandemic, fit young men died within 24 hours of infection, not from the virus but as a consequence of the “cytokine storm” unleashed by their healthy immune systems, which go into overdrive and attack the body while fighting the virus.

We are long overdue a flu pandemic. Expert opinion as to when it will happen varies between two and five years. It is certain that 90 per cent of us will not die. It is also certain that a flu pandemic would cause serious disruption to the economy, so, please, will everyone avoid hugging ducks in Asia - at least until we're done with this economic crisis."

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Interesting article, if not a little naive about the potential H5N1 has, or any biological agent for that matter. Ebola and Marburg have something like a 24 hour incubation period. There's no reason a hemorrhagic virus could not mutate (or be purposely developed, for that matter) that has a much longer incubation time.

When it comes to viruses there isn't some magical biological scale which keeps virulence and contagion in check. Usually its a game of red queen between the virus and its hosts for several generations before we see any kind of equilibrium.

Basically, believing that a virus can't have both a high mortality rate and a high infection spread is a belief based on hope and faith that trends in the past will stay the same.