Mathematical model predicts around 60% of population already exposed to virus, resulting in herd immunity
As on Friday, India has confirmed a little over 11 million infections since March 2020 of which 1,52,895 were active ones. Prof. Agrawal told The Hindu that based on the model, this would at most rise to 11.3 or 11.5 million infections by April 2021 — or about 3,00,000-5,00,000 new confirmed infections over the next 10 weeks (approx).
He and other scientists are working on a formal update of the model but were awaiting confirmed data on recoveries for the next two weeks, he said.
The current wave of infections — averaging 13,000-16,000 new confirmed infections a day since February 23 — was primarily being led by Maharashtra and wouldn’t last beyond “two-three weeks” in March, he said.
The reason, according to Prof. Agrawal, is because around 60% of India’s population had already been exposed to the virus and the country had reached herd immunity, or where the number of susceptible individuals were too few to allow the virus to exponentially grow.
Whether herd immunity levels have been reached is moot, because the Indian Council of Medical Research (ICMR) reported that only 21% of Indians were estimated to have been infected, as per its third national serological survey conducted from December 17 -January 8, 2021. The Super Model’s estimate is about thrice that number.
“The ICMR numbers are based on the numbers in whom antibodies have been detected. But we know now that antibody levels can wane but still be enough to confer immunity. Given the relaxations in movement since months, only 20% exposure cannot explain the confirmed symptomatic infections, active cases and recoveries that we now see,” he added.
In October, the ‘Super Model’ initiative had forecast that India’s caseload (of confirmed symptomatic infections) would top out at 10.6 million infections with less than 40,000 active infections by February end. However, current active infections are nearly four times the model’s estimates.
Travel, festival surge
Prof. Agrawal said the resumption of local train travel in Maharashtra and thereby increased contact between persons was a reason for the renewed spike over the past week.
“There was spike in Kerala because of festivals and increased movement and that has gone down. It appears to be a similar story in Maharashtra too,” he added.
COVID-19 was characterised by a large number of people who’d contracted the infection without manifesting symptoms. Moreover, those who manifested even minimal symptoms would quite likely quarantine themselves and limit contact with others. The mathematical model employed, said Prof. Agrawal, accounted for these factors and therefore instilled confidence in its projections.
“Vaccines and the continued use of precautions are critical to continuing this declining trend,” he added. The government has announced vaccinations for those over 45 with co-morbidities and those over 60 beginning March 1.
Scientists have expressed scepticism about India reaching herd immunity and point to Manaus in Brazil. The city experienced a surge in April and by October 2020, local researchers had estimated that three-fourth of the city had been exposed to the infection, conferring herd immunity. However January 2021 saw a fresh resurgence.
In a Lancet study, scientists speculated that this might be due to the actual number of people who contracted the virus in the first wave being over-estimated, or that immunity is temporary and re-infections, as well as being exposed to newer variants play a bigger role than is now anticipated. The ICMR has identified prominent variants — such as the UK variant, South African and Brazilian variants in India but has said recent spikes in Kerala or Maharashtra are not linked to them.
On Friday, scientists at the Institute of Genomics and Integrative Biology reported a case of re-infection in a person from Andhra Pradesh. The patient was harbouring a mutation in the coronavirus called N440K, identified last year in nearly a third of patients in the State and known to help the virus evade detection by the immune system’s antibodies.
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