A potential third wave would unlikely be as severe as the second wave, according to a modelling study by a team of scientists from the Indian Council of Medical Research (ICMR) and the Imperial College London, U.K. For it to be as devastating, at least 30% of the population who had been infected earlier must entirely lose their immunity, or an emerging variant of the virus must have a reproductive rate (R) over 4.5, that is, each infected person should be spreading to at least 4-5 others and these must occur almost immediately after the second wave ends.
The authors, who include Balram Bhargava, Director General, ICMR, and Samiran Panda, Head, Infectious Disease Unit, ICMR, suggest that these scenarios are implausible.
“..for a virus to cause a major third wave in the face of this pre-existing immunity, extreme scenarios for the abrogation of that immunity are required, or for that matter, for the transmission fitness of any novel virus,” they report in their article that appears in the in-house Indian Journal of Medical Research as a pre-print.
For their analysis, the scientists did not use — as most forecasting attempts — the reported daily case load numbers from previous months. Rather they chose a hypothetical R value computed from September 2020, when an estimated 7.1% of the population had been exposed to the virus.
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Thereafter, a second wave was simulated assuming a more infectious variant, and therefore a higher R value, and accounting for the fact that second wave saw a peak about four times higher than the first wave. Once this was done, and estimating the number of people who had been likely exposed to the infection, they then modelled a future third wave under several scenarios: a complete loss of immunity from previous waves; emergence of a variant that was more infectious and capable of thwarting immunity than the ones observed; the impact of moving out of lockdowns; and the role of a highly infectious variant in a scenario of a certain percentage of people getting vaccinated.
Their results showed that under all of these situations, the peak number of infections remained much lower than during the second wave, unless the R rate exceeded 4.5, or those previously susceptible exceeded 30%.
The emergence of a third wave could be significantly buffered by expanding vaccination, they contend. Illustratively, the authors calculate that were vaccines rolled out in a way to cover 40% of the population with two doses over a period of three months following the end of the second wave, it could reduce symptomatic incidence by around 55%.
Less than 20% of Indians have received at least one dose of the vaccine and only 4% are fully vaccinated. The Centre proposes to vaccinate all adults — about 94.4 crore — by the end of 2021.
Dr. Panda told The Hindu that while several factors needed to converge for a third wave as devastating as the second, complacency in observing physical distancing or mask usage could be dangerous. “Wearing masks consistently and correctly as well vaccination are critical to ensure that a third wave isn’t severe, even though the evidence, as we’ve calculated, shows that it is quite unlikely,” Dr. Panda said.
This is the first time that the ICMR’s scientists have attempted a formal forecast of a future wave. In spite of several modelling experts, no group estimated or warned of the severity of the second wave that saw new infections rise to nearly 400,000 a day. Currently, India’s second wave is on a decline, with about 50,000 cases being added daily. India, with over 30 million confirmed coronavirus cases and nearly 400,000 deaths, is the second most affected country after the United States.