There’s confusion about what the Covid-19 numbers actually mean. Well, let’s make it simple. There are only two numbers that you need to understand. One, that the of the disease is one percent. That is, one out of every 100 people confirmed to have the novel coronavirus die. Two, it takes around three weeks for an infected person to become fatally sick.
Why are these two numbers helpful? First, from the data available, the number of deaths is unfortunately the only number that can be reliably known. Not that there aren't errors, but it’s the dataset with the fewest mistakes. On the other hand, counting the number of people with the disease can be a highly erratic exercise given that it depends on how many tests are done and how many infected people are actually symptomatic. Most countries, even rich ones such as the United Kingdom, aren’t testing enough and close to three quarters of those infected with the novel coronavirus are either asymptomatic or have mild symptoms.
But from data made available so far by countries that have carried out extensive testing, we can reasonably conclude that the case fatality rate of Covid-19 is around one percent and that an infected person grows fatally sick in roughly three weeks. The unfortunate situation onboard the cruise ship Diamond Princess is an ideal case study of a closed population infected by coronavirus. In spite of the mean age of its 700 passengers being above 50 – older people are more susceptible to Covid-19 – the is around one percent.
Since treatment for Covid-19 is largely supportive and not curative, and unless the virus mutates significantly, the fatality rate would not be greatly affected by the quality of healthcare available to the population. Nor would the three-week period from infection to death.
This is what it means: if a region records 50 deaths from Covid-19 today, the total number of infections was about 5,000 three weeks back. What is the number of positive cases today? That depends on whether the region was in lockdown for the past three weeks. If left unchecked, coronavirus infections double every five days, meaning that the total number of people infected until today is around 80,000.
Nearly 85 percent of them have already recovered, or were mildly affected or asymptomatic. They may thus not test positive for the virus. So, with good testing practices, 6,000-7,000 of the 80,000 infected people will show up positive today, and 50 will die. If the testing policy isn’t robust, however, the official statistics will likely show 50 deaths and around 2,000 positive cases.
Remember, though, that wider testing without public health interventions like social distancing and lockdown won’t serve the purpose of containing the spread of the virus.
If testing isn’t robust where you are, you can use these two numbers – fatality rate and three-week progression – to track the Covid-19 situation around you. If there was a death among the first few Covid-19 cases, it means 50-100 people had the disease around three weeks earlier, and it has likely spread to around 15 times more people now. It’s especially bad if the death was in the , that is, the person was below 60 years of age.
In countries where the case fatality rates are much higher than one percent – the UK, Holland, Sweden, Spain, Italy – they are only diagnosing seriously ill patients. It also means they have much greater caseloads than their official numbers show. And from this caseload in each country, a small percentage will end up clogging the ICUs over the next three weeks.
In Italy, this has happened already. It had only three confirmed infections in the first half of February. On February 20, a man in his 30s in Lombardy region was diagnosed with severe pneumonia caused by the novel coronavirus. He had not travelled abroad recently or had contact with anyone who had. It was an obvious indicator that the caseload in the community was very high by that time. One month later, Italy had 60,000 cases and around 6,000 deaths.
If you want to know whether a region is diagnosing Covid-19 properly, simply divide the official number of positive cases by 100. If the result roughly tallies with the number of deaths – like in Iceland, Germany or Australia – then they are testing properly and are in control of the situation, more or less.
If not, use the above rule of thumb to obtain a rough estimate of the actual number of infections. Countries that have tested widely – such as Iceland, South Korea, Germany – have found that a lot of younger people are acting as asymptomatic vectors for the older population.
Bimal Kumar is a surgeon with experience as a public health professional.