Covid-19: What you need to know today
Two questions. One, what is India’s Infection Fatality Rate for Covid-19? This is different from the case fatality rate, which is simply the number of deaths represented as a proportion of the number of cases. The infection fatality rate, or IFR, is the number of deaths represented as a proportion of the number of infected. Caveat: Death numbers everywhere in the world, directly because of the coronavirus disease, or indirectly caused by it, are being undercounted, but in the absence of accurate death data and records, it is pointless to try and guess by how much.
IFR is an important metric because it accurately measures the possible fatalities arising from a disease — and also the chances of dying from it. Measuring it, though, requires knowing the denominator — the number of infected people in a population.
This is why sero surveys, blood tests that look for Sars-CoV-2 antibodies (in this case), are important. Such surveys measure the prevalence of a disease in a population — or the infection rate. Since Covid-19 deaths are anyway recorded, it is then possible to calculate IFR.
There have been around 20 sero surveys in various parts of India covering close to 110,000 people. I decided to pick four of these, excluding ones with smaller samples. These four surveys, two in Delhi, and one each in Chennai and Ahmedabad, together covered around 78,000 people. The Ahmedabad survey showed that 17.6% of those studied had been exposed to the virus; the Chennai one 21.5%, and the two Delhi surveys 23% and 29.1%. The Ahmedabad and Chennai surveys were conducted in July; and the two Delhi ones in late June/early July and early August. Delhi’s third sero survey is nearing completion, and the results are expected next week. Based on these numbers, it can be assumed that at least 20% the population in large cities has been exposed to the disease.
What does that make Delhi’s IFR? There have been 4,567 deaths on account of Covid-19 in Delhi till Sunday night. A 20% infection rate in the population would mean four million infections. That translates into an infection fatality rate of 0.11%. This compares with a case fatality rate of 2.4% in Delhi. In Ahmedabad, the IFR is again 0.11. And in Chennai it is 0.13. All numbers are based on current population and the total number of deaths in these cities till Sunday night. Three cities in different parts of the countries, all boasting similar IFRs, is interesting, but also expected. Pandemics respect math (it’s also the reason I have repeatedly called out states pretending all is well on the basis of positivity rates that have not followed the trajectory of such rates around the world).
The second question, how common is a Covid-19 reinfection?
This is prompted by media reports that claim India’s first case of coronavirus reinfection has been identified in a Bengaluru hospital. There have been 27 million coronavirus infections around the world — but only two known and established instances of reinfection (both in late August). There are probably more, but establishing a reinfection requires sequencing the viral genome of the first infection in the patient, and then sequencing the viral genome of the second infection in the patient, and proving that the two are different. Doctors and scientists established this in the case of a Hong Kong man whose second infection was asymptomatic (it was detected because he was travelling) and a US woman whose second infection resulted in severe symptoms.
This doesn’t seem to have been done in the case of the Bengaluru patient — which doesn’t rule out a reinfection, but which also doesn’t rule out a residual infection or tests throwing up false negatives.
Scientists are very interested in reinfections because these have a bearing on vaccine development. If there are significant reinfections (two in 27 million aren’t) and if the immune system in these patients did work the way it is supposed to, then it makes the vaccine development process a bit more complex — it is always easier to develop vaccines for diseases where an infection results in lifelong immunity (or at least immunity for a significant period of time).