While the media is getting it wrong, a combination of data by the Ministry of Health and INSACOG could be better
It’s impossible to miss the slow build-up of strain on the Omicron version of the SARS-CoV-2 virus over the past three weeks. The news from Europe about the rise in cases and the severity of the disease is not good. Every day, national news reports of a number of cases that are slowly rising: 100 last week, 200 this week. So, is there any indication of the actual number of cases in India? Should one be concerned about the virus that has infected nearly 200 people in a land of more than 1.3 billion, and when many are vaccinated?
cause of error
Let me try to put the numbers in perspective. The reported cases of Omicron infection identified come from a genomic surveillance mounted by Indian SARS-CoV-2 Genomics Consortium (INSACOG), Genome sequencing is complex, and Only 38 laboratories across the country have the capacity to sequence such infectious viruses. Consequently, only a small fraction of infected individuals contribute virus samples for sequencing. The numbers reported by the media are numbers from this small sample showing infection by omicron variants. Therefore, it makes an error when the media reports it as the number of cases in the country.
a count
How can we do better? Let us think in terms of fractions or percentages. The Union Ministry of Health and Family Welfare (MoHFW) reports that in December, India conducted around 12 lakh RT-PCR tests a day, and about 0.5% test results were positive. So, in the first two weeks of December, around 17 million tests were conducted nationally, and around 80,000 people tested positive during this period. The number of genomes sequenced by INSACOG is probably 1% of these.
This means that out of 800 or so samples taken by the end of the second week of December, about 200 tested positive. If the virus genome sample to be sequenced was taken at random from the newly infected, one would be forced to conclude that there are about 20,000 omicron positive cases in India.
However, all public health agencies around the world have proceeded on the assumption that the Omicron variant originated more recently and is still being transported around the world. Therefore, the sequencing effort has been biased towards international travelers.
This means that the incidence of omicron infection will be somewhat lower. Could it be that only about 2,000 people are infected, In other words, about 2.5% of the cases? If the numbers were really that low, about 80% of virus samples would have been taken from travelers. The remaining 20% of samples are then unlikely to give an omicron positive result. But we know from news reports that at least about 10 cases are from people with no recent history of international travel.
Based on the numbers we get from the media and other public sources, we know that more than 5% of the cases seen in the first two weeks of December and certainly less than 25% are due to the Omicron version. Huh. The number of cases in this period is close to being somewhere between 5,000 and 25,000. Now that the total number of cases per day has started to rise, the possibility of a lower number has diminished.
improvement in estimates
Of course, these are very rough numbers. The agencies handling the data and the scientists running the statistical models will be able to highly refine these estimates and reduce the extent of uncertainty. If the number of genomes sequenced from infected travelers and others is given separately by INSACOG, and the date the sample was collected was tagged, it would be much easier for you and me to guess. However, there may be concerns about medical privacy that prevent the government and its agencies from making such details about the data public.
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One should also beware of other mistakes that may cause deliberate bias in sampling virus genomes. If international travelers visit Delhi and Mumbai more frequently, could the bias in sampling wrongly lead us to underestimate the speed of omicron’s spread to the rest of India?
Numbers will change every week. Is Omicron spreading faster than Delta, which gave India a second wave? If so, week by week, Omicron’s share of cases will increase, as it overtakes the delta version in infecting people. This has happened in other parts of the world and can happen here as well.
I have indicated here what kind of reasoning an aspiring media person or common man can use. If you make an informed decision about whether to invest your savings in fixed deposits or stocks, you routinely make more sophisticated numerical estimates. Looking at the numbers provided by MoHFW and INSACOG, it is possible for you to estimate your personal health risks from COVID-19, whether you live at home or work or travel on vacation.
Saurendu Gupta is a theoretical physicist at the Tata Institute of Fundamental Research, Mumbai. views expressed are personal
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