A sledgehammer to squash a flea?

Lockdown could be based on flawed calculations of the virus’ fatality rate, writes Simon Thornley

OPINION: As we gain more knowledge about Covid-19, it is vital to continually assess our response. As cases mount across the world, a picture is also emerging of the effect of the virus on populations.

Unprecedented social controls have been rapidly thrust upon us. The duration of the lockdown is uncertain. It is also unclear how much of a financial hit the country is willing to stomach.

We know this virus is serious, but exactly how serious? How does the casefatality rate, a measure of its importance, compare with other similar viruses?

As a rough guide, the US Centers for Disease Control and Prevention uses rates of between 0.1 to 2.0 per cent to determine how to respond to a new threat. This is clearly an important statistic to attend to, but it is easy to get this wrong.

The calculation is skewed, initially, because sicker patients are tested first, making the infection appear more serious. Also, in determining fatalities, uncertainty arises in patients who are otherwise sick with a limited life expectancy, who then test positive and subsequently die. Should they be labelled as Covid-19 deaths?

Remember swine flu in 2009? Initial estimates of case-fatality rates were about ten times higher than those calculated once the dust had settled. It turned out that swine flu, that year’s killer virus, was no more harmful than seasonal flu.

The Diamond Princess ship is one of the few examples of a closed population who were all tested for the disease. Seven deaths occurred in 700 testpositive patients, giving us a case-fatality rate of 1 per cent.

Remember, this was an elderly population. Calculations show that, if these rates were translated to a Western country’s overall age structure, the statistic would be 0.125 per cent (interval of plausible values: 0.025 per cent to 0.625 per cent), higher than normal flu (~0.1 per cent), but not by much.

The figures of up to 900 deaths a day in Italy are alarming, and so is the nation’s crude case-fatality rate of 9 per cent. However, a recent reanalysis of the deaths in Italy shows that only a small fraction were due to Covid-19 (3/355; 0.8 per cent). Many deaths were hastily labelled as Covid-19 related when they were not.

As the average age of those dying is 80, that is not surprising. This kind of seasonal epidemic of deaths has occurred frequently in the region for some years. If we recalibrate Italy’s death rate in light of the death record review, a better estimate of the casefatality is 0.07 per cent. This figure is lower than for seasonal flu (~0.1 per cent).

Another question is whether Covid-19 represents an added burden on top of usual seasonal viruses. After all, admissions to hospital and deaths occur at a background rate.

Time-series plots of overall deaths in European countries show surprisingly low rates for this time of year, even in heavily affected countries even in the over-65 age group. Italy has the most dramatic increase, but no higher than occurred during the same season two years ago.

Is a ‘‘lockdown’’ and closing the borders effective? Metaanalysis of social distancing measures for avoiding viral chest infections found that such an intervention was not strongly supported, since little evaluation of these policies had been done. Of all the preventive measures examined, improving hand hygiene had the best supporting evidence. It remains to be seen whether lockdown will ‘‘flatten the curve’’, but we don’t have strong evidence in favour.

Despite my scepticism, Covid-19 does pose a real risk to our health.

Sensible measures include better hand hygiene, good cough etiquette, and restricting large gatherings.

Limiting exposure for elderly, and people with chronic conditions, makes sense.

It is important that the public health response matches the threat posed to our health. We should keep abreast of developments, such as tests of immunity, so that we can return to normality quickly.

We don’t want to squash a flea with a sledgehammer and bring the house down.

I believe that other countries, such as Sweden, are steering a more sensible course through this turbulent time.