With the Omicron-fuelled third wave of Covid-19 sweeping major cities in India, efforts are underway to estimate its potential size and impact.

Many state governments have drawn internal projections. Maharashtra expects double the number of cases recorded in the second wave, Chhattisgarh three times, West Bengal one and a half times, state officials told Scroll.in.

A mathematical model by scientists at the Indian Institute of Science and Indian Statistical Institute, based on data from South Africa, projects three scenarios for daily-case counts – 3 lakh, 6 lakh, 10 lakh – depending on varying assumptions.

What is proving difficult to estimate, however, is the scale of hospitalisations and deaths.

The only model which ventures into this territory has been prepared by the Tata Institute of Fundamental Research for the city of Mumbai. It predicts hospitalisations will be 50-70% of the peak seen in the second wave, while deaths will be 30-50%. Since the number of recorded deaths in Mumbai in the second wave between March till June were about 4,000, this translates into an estimated 1,200-2,000 deaths in the city in the current wave.

Simulated model for daily cases and deaths in Mumbai. Credit: Daksh Mittal/ Sandeep Juneja, Tata Institute of Fundamental Research

The one point of convergence across all estimates is that the third wave will peak faster – a prospect causing concern among health officials in some states as this could result in hospitals getting overwhelmed, even if a vast majority of cases are mild or asymptomatic.

Already, in Mumbai, where cases have grown by 33-fold in two weeks, 12% of the hospital beds are occupied. In Delhi, although hospital occupancy is currently low, the state government has asked private hospitals to reserve 40% of their beds for Covid care in anticipation of a rise.

Besides, the extreme virulence of Omicron means doctors and medical staff are at a higher risk of being infected. Staffing shortages are already looming in Bihar and West Bengal where hundreds of doctors are down with the infection. In Mumbai, a quarter of the resident doctors in a large public hospital cannot report to work because they are infected.

The burden on hospitals

The Omicron variant was first reported in South Africa’s Gauteng province on November 24. Within two weeks, the Covid-19 case counts in the country had surpassed previous records, with the variant spreading to 60 countries – a testament to its astonishingly high transmissibility.

But the silver lining is that several studies have found it causes mild disease among healthy and vaccinated people compared to the other variants of the coronavirus.

One study in Gauteng, for instance, found that those admitted to hospitals with Covid-19 were 73% less likely to have severe disease during the Omicron-led wave in November-December 2021, compared to the Delta wave in May 2021.

The study also noted that only 4.9% of Covid-positive people had to be hospitalised in Gauteng during the Omicron-led wave, compared to 13.7% in the Delta-led wave. But in absolute numbers, since the size of the Omicron wave was larger, this translated into 6,510 patients hospitalised, higher than the 4,574 seen in the Delta wave.

In India, this appears to be the main concern among officials: Omicron’s higher transmissibility could mean a much higher caseload in a much shorter time than the second wave. This, health officials fear, could overwhelm hospitals, especially in districts with low healthcare capacities.

Maharashtra expects cumulative infections in the third wave would be nearly twice the 45-lakh number reported in the second wave.“This time, our estimate is 80 lakh cases during the third wave,” said the state’s additional chief secretary (health) Dr Pradeep Vyas. He said that the cases are expected to peak and taper faster than previous waves.

Similarly, experts advising the Chhattisgarh government estimate the caseload in the third wave could be three times higher than that of the second wave. Their rationale: data from South Africa pegs the Omicron variant to be roughly thrice as infectious as Delta.

One of the experts said he expected the wave to be significantly shorter than the previous ones, likely somewhere between four to six weeks. A division with a population of 50 lakh could end up witnessing between 1-2 lakh cases in a week, he said. Even with a conservative 5% hospitalisation rate, this could translate into 12,000-37,500 patients requiring hospital care when the wave is at its peak, he added.

Meanwhile, in West Bengal, where cases are rising, especially in Kolkata, Dr Ajay Chakraborty, Director of Health Services, said the state had not prepared its own model. But, based on central government data, it estimates that the third wave could bring 1.5 times the cases recorded in the second wave. West Bengal saw 22,000 cases a day during the peak of the second wave. If this trend holds, the number may rise to 33,000-35,000 daily cases this time.

Two modelling exercises

Mathematical models, however, have thrown up mixed projections. A model by scientists at the Indian Institute of Science and Indian Statistical Institute, based on data from South Africa, draws up three scenarios, each taking into account a different fraction of the population susceptible to the new variant: 30%, 60% and 100%.

“Past infection, vaccination, affected by immunity waning” have a bearing on the size of the vulnerable population, the authors of the study note.

A 30% vulnerability assumption, according to the model, leads to a lower peak than the Delta wave: of slightly over 3 lakh daily cases. However, an assumption of 60% susceptible population would put the peak at over 6 lakh daily cases – a nearly 60% surge compared to the second wave. The worst case scenario of 100% vulnerability paints an ominous picture of nearly 10 lakh daily cases.

The model does not make any hospitalisation estimates.

Projections by Siva Athreya, Rajesh Sundaresan and team at the Centre for Networked Intelligence, Indian Institute of Science, and Indian Statistical Institute, Bangalore Centre

A Mumbai-centric model by the Tata Institute of Fundamental Research, however, throws up more granular projections for the city. It broadly estimates, with several caveats, that the city’s daily caseload will range within 12,000-16,000 cases between January 6-13 when cases are likely to peak. During the second wave, the highest single-day increase in cases was 11,206 on April 4.

The model, simulated by Daksh Mittal, a student at TIFR, is based on trends from South Africa and the United Kingdom. It assumes Omicron to be twice as transmissible as Delta while calculating the spread of cases, and factors in both reinfection and breakthrough infection among vaccinated people. The model is based on the assumption that 35% of the population are vulnerable to reinfection.

“Hospitalisation peak will be around 50-70% of the peak seen under delta,” said Professor Sandeep Juneja, from the School of Technology and Computer Science in TIFR. Since a rise in fatalities is seen a few days after the rise in infections, Juneja said their model shows fatalities in Mumbai will peak in the first week of February at around 30-50% of what the city saw under the Delta wave.

Vyas, the Maharashtra health official, said the hospitalisation rate this time is expected to be much lower than the Delta wave, but the peak number of cases is expected to exceed previous waves. “We don’t think oxygen supply will be under strain or hospital beds will fall short for critical care,” he said. “Looking at the way South Africa and France saw their latest surge, it seems that in Mumbai at least the peak will be seen by mid-January.”

This reporting was supported by a grant from the Thakur Family Foundation. Thakur Family Foundation has not exercised any editorial control over the contents of this article.