New variant 2.5 times more infectious: TIFR-IISC model
MUMBAI: The new variant of the Covid-19 virus prevalent in the second wave is likely to be almost two to two-and-a-half times more infectious than the previous variant according to a statistical model by Tata Institute of Fundamental Research (TIFR), Mumbai, and Indian Institute of Science (IISC), Bangalore.
Allowing the virus to be two to two-and-a-half times more infectious than the virus strain seen last year best explains the observed Covid-related fatalities in Mumbai between February and April this year, finds the model. This was revealed in the latest report released on Monday. A simulation model, developed last year by scientists from the two institutes, projects that the R naught (R0) or reproduction number of the virus has increased from two-two-and-ahalf times to over three under the new variants in the non-slum regions, where the second wave was more severe.
R0 denotes the expected number of individuals a single person exposed to the virus infects in a city where everyone else is susceptible. This means, a person infected with the new strain of the virus is likely to infect over three people they come in contact with. “The increased R0 is an indicator that the new variant of the virus is much more infectious. This is in line with the rapid spread of the virus in the second wave and more number of people getting infected,” said coordinator of the simulation project at TIFR Sandeep Juneja, who is the dean of the institute’s school of technology and computer science.
The R0 for the overall city (slums and non-slum areas) is likely to be three-and-a-half to four-and-a-half. The R0 is higher in slums owing to a higher population density. “However, as the current infections in the city are largely in non-slums or high rises, the increase in R0 from non-slums is a better measure of the impact of more infective variants to the city,” said Juneja.
The model which makes this prediction mimics a city with a population of 13 million. It takes into consideration different population demographics, densities, household sizes, city movement restrictions and interaction of people with each other. The initial parameters that were chosen were such that the death rates matched the actual observations in last April. The simulations that arose from the parameters chosen were done for various scenarios such as population compliance, lockdowns and other restrictions, role of local trains, vaccinations.
Murad Banaji, senior lecturer in mathematics, department of design engineering and maths at the Middlesex University, London, who has been studying the epidemic in Mumbai, agreed that the mutant circulating in the city was more transmissible.
“The epidemiological evidence, the speed of spread in particular, does point to whatever variants are circulating in the city being more transmissible and/or more able to infect people who have previously been infected. The spread has been wider than we would expect, and this has undoubtedly resulted in more fatalities. On the other hand, I have not seen evidence that whatever variants are circulating in Mumbai are more lethal (causing more severe disease in those infected). Overall, I think we can be fairly confident that new variants have played a role in Mumbai’s latest wave, causing increased spread and hence fatalities; but I don’t feel I’ll be able to put numbers on this,” he said.
Pulmonary specialist Lancelot Pinto said that the current variants appear to be more transmissible. “While earlier one in five people in a family in close contact would contract the disease, now we seldom see that. Now almost all members of the family test positive for the virus. It does appear that the variant is more infectious. We are also seeing a rise in the close contact transmission rate,” said Pinto.
However, with the number of cases now dropping, the current reproduction number (Rt), which is an indicator of the infectiousness of the virus at a given point in time, is likely to have reduced. “Looking at the drop in cases, one can assume that the Rt in Mumbai could be less than 1, although our model doesn’t calculate Rt,” said Juneja. If Rt is below 1, an infected person infects one or less than one person on an average.
In yet another measure of the infectiousness and severity of the new strain of the virus, the TIFR-IISC model projects that the variant is likely to cause 1,500-2,500 additional deaths by September. The scientists arrive at this after plotting estimated fatalities for various scenarios, such as 2.5% of infected people infected with infectious strain on February 1 (two to two-and-ahalf times more infectious), with lockdown from April 15-May 15 and with vaccination effectiveness of 0.75. This was plotted against a scenario without infectious strain and without lockdown. Irrespective of the scenarios, the model predicts that the city may record a few thousand new infections each day in September, which correspond to a few hundred reported cases daily. “This is taking into account that there are no new variant or significant reinfections,” Juneja said.
Vaccine effectiveness
The analysis by the TIFR-IISC model as well as by Banaji point to the effectiveness of the vaccination drive in saving the lives of the elderly.
According to Banaji, as on April 25, up to 580 lives could already have been saved by the vaccination drive focussed on the elderly. “We have to remember that the estimate of 580 fewer fatalities is based on what we would expect to see if the elderly had continued to form 80% of the city’s fatalities, as was true from the start of the year to the end of March. But it is also roughly consistent with the drop in the delayed case fatality rate in the city. So we definitely have seen fewer recorded fatalities during April than expected based on the epidemic’s path up to the end of March,” said Banaji.
In the TIFR-IISC model, the reduction in fatality numbers due to the senior citizens vaccinated in March will start to show up around May. “If vaccination continues at a rapid pace, say more than 50,000 new persons daily, then by July it leads to significant reduction in fatalities. There are caveats. We don’t understand how infectious, virulent, and prevalent the variants virus are,” said Juneja