OPINION: As the Covid-19 picture emerges, it is vital to continually assess our response. The virus was identified quickly and tests developed. We are acquiring knowledge about it at a great rate. As cases mount across the world, a picture is also emerging of the effect of the virus on populations, which, as an epidemiologist, is my interest.

Unprecedented social controls have been rapidly thrust upon us. The justification initially was not overloading intensive care facilities, but we have now moved beyond that to "flushing out the cases we already have". 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 case-fatality rate, a measure of the importance of the disease, 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 – very wrong.

Alvaro Barrientos/AP Italy has had a large number of deaths, but many may have been attributed to Covid-19 when they were not. Its overall deaths are much the same as during the flu season two years ago, writes epidemiologist Simon Thornley.

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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.

DEBORAH WINKLER Deaths among positive-tested passengers on the Diamond Princess cruise ship were at the rate of about 1 per cent. But this was among a largely elderly population.

So let's look at a couple of examples where more comprehensive testing has been completed. 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 test-positive 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 analysis of the deaths in Italy shows that only a small fraction were entirely due to Covid-19, occurring in people with no co-morbidities (3 out of 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 elderly populations living in this region for some years.

Another question to ask here is whether Covid-19 represents an added burden on top of usual seasonal viruses. After all, admissions to hospital, intensive care, 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, such as Germany, Spain, France and Italy, 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.

David Keyton/AP Swedes are continuing to gather in cafes, as the government has not yet ordered people to stay at home. Schools, kindergartens and sports clubs also remain open.

Is a "lockdown" and closing the borders even effective? Unfortunately, meta-analysis 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 that were examined, improving hand hygiene had the best supporting evidence. It remains to be seen whether lockdown will result in "flattening 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, ensuring good cough etiquette, and restricting large gatherings. Limiting exposure for the elderly, and people with chronic conditions, makes sense.

It is important that the public health response matches the threat posed to our health. It is important we 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.

* Simon Thornley is a senior lecturer and epidemiologist at the University of Auckland.