Just how dangerous is swine flu?

There seems to have been a rhythm to the news reports about swine flu. "Biggest pandemic ever" and "Swine flu could kill 65,000 in UK" say headlines one day. "We're coping just fine" and "Less dangerous than the annual flu season" say the next day's papers. So how dangerous is swine flu really?

What do we know about death rates from swine flu?

At the time of writing, 29 people have died of swine flu in the UK. Obviously, every death is a tragedy for the family involved. But what does this tell us about how severe the virus is?

Researchers writing in the BMJ this week have pointed out good reasons for all the uncertainty about the dangers of swine flu.

The simplest way of calculating the danger posed by a virus is to divide the number of deaths by the number of people infected. The World Health Organisation is currently reporting about 100,000 confirmed cases of swine flu worldwide, and 429 deaths. That gives a death rate of roughly 0.5 percent.

However, the researchers point out several problems with this approach.

Is it less severe than we think?

Have we counted everyone who's been infected? People with a severe illness are more likely to get medical attention and laboratory tests to check for the swine flu virus. People who only suffered a mild illness might not see a doctor. This means that the virus might be less dangerous than it first appears, as people who are less seriously affected aren't included in the statistics.

During a pandemic, there are also likely to be changes in how doctors diagnose the illness. We've recently seen this in the UK. At first, people were diagnosed with swine flu using laboratory tests. As swine flu spread, testing everyone became impractical, and doctors now diagnose people based on their symptoms alone. Doctors might also start to focus only on the more serious cases as the virus spreads. All these factors mean that the number of reported cases isn't the same as the number of people who've really had swine flu. If only the severe cases get counted, the virus might seem more severe than it really is.

Another issue is the time delay between becoming ill and dying. Early in an epidemic, people may have caught the virus, but their illness won't have had time to run its course. Put bluntly, at a particular point in time there will be people who are going to die, but who are still alive at the time statistics are calculated. Mathematically, this type of bias is called censoring.

Censoring tends to have its strongest effect early in a pandemic when a disease is spreading faster and faster. It has the effect of making the illness look milder at first, then more serious later as the first wave of patients either die or recover. This happened in the SARS epidemic, and caused unwarranted fears that the virus was becoming more severe. So the fact we are now seeing more deaths doesn't necessarily mean the virus is getting more severe.

Is it more severe than we think?

A second problem is that flu can temporarily increase people's risk of dying of other problems, including a heart attack or stroke. Doctors might not make the link between these deaths and swine flu. This misses some of the people who've died because of flu, making the virus look less dangerous than it really is.

Another factor is the number of people infected during a pandemic. Assuming swine flu really does have a death rate of about 0.5 percent, that makes it slightly more dangerous than seasonal flu. However, most people have some immunity to seasonal flu, but not to pandemic flu. This means that pandemic flu affects far more people, so the potential number of deaths is higher.

So what's the answer?

Working out how severe swine flu is plays an important part in planning healthcare, and helping officials make decisions about whether to close schools and workplaces. At the moment, the World Health Organisation still describes swine flu as a 'moderate' illness, saying that most people recover without the need for hospitalisation or medical care. In the UK, the NHS has been warned to prepare for anything between 19,000 and 65,000 deaths. Although 19,000 sounds like a lot, in some years that many people die during normal flu outbreaks.

Researchers can use statistics carefully to make the best predictions possible. The conflicting tone of the news reports is down to the fact that, if we really want to know how dangerous swine flu is, the only thing to do is wait and see.

From:

Garske T, Legrand J, Donnelly CA, et al. Assessing the severity of the novel influenza A/H1N1 pandemic. BMJ. 2009; 339: b2840.