Though the total number of coronavirus cases in India has crossed 10.1 million, the number of daily fresh cases has been steadily dropping from a peak of nearly 98,000 cases on September 16 to below 25,000 daily cases in the last few days; December 25 recorded 22,350 fresh cases. The steady drop in cases has been seen nationally without much of a spike since mid-September notwithstanding the festival season, winter and large gatherings, whether political or otherwise. The absence of a spike is all the more surprising as strict adherence to non-pharmaceutical interventions (NPIs) has been far from good. Does the steadily declining trend in daily cases and absence of a spike nationally then suggest that the first wave is slowly coming to an end in India?
“It would be inaccurate to infer at the national level while some of the north-eastern States have a cumulative test positivity rate above 8%. Even at national level, the cumulative test positivity rate is 6.1% as on December 23, which suggests we aren’t near the end immediately,” Dr. Giridhara Babu, Professor of Epidemiology at the Public Health Foundation of India, Bengaluru, and a member of the Karnataka COVID-19 Technical Advisory Committee says in an email.
The cumulative test positivity rate should be less than 5% to say that transmission has been contained. The reason: the seven day or monthly positivity rate only indicates active surge in cases as seen now in some north-east States, while the cumulative test positivity rate reflects the overall containment in transmission.
Note of caution
Gautam Menon, Professor of Physics and Biology at Ashoka University and co-author of COVID-19 modelling studies cautions in an email that test positivity is large in a number of States, including Kerala and Maharashtra, which were hard hit earlier as well. “This suggests that the disease is still spreading, although more slowly. I think this is mainly evidence that COVID-19 is no longer primarily spreading in more urban areas, so this should be expected to mark a qualitative change. In that sense, it can be thought of as an end to the ‘first wave’,” he says.
“The decline has been steady and prolonged, with the fall somewhat slower than the rise. Within this fall in national figures there are small regional spikes and that will continue to happen in the days to come,” says virologist Dr. Shahid Jameel, Director of the Trivedi School of Biosciences at Ashoka University.
Explaining that natural infection (both detected and undetected) has not reached the level to confer herd immunity and cause a slowdown in daily cases, Dr. Menon says: “My own guess would be that about 40–50% of urban India, in particular tier-1 and tier-2 cities has been infected at this point, while an India-wide number is more likely in the range of 30-40%. This is still below what might be a herd immunity threshold, although it would be consistent with a decrease in the number of daily cases, as is being seen.”
Dr. Jameel too feels only about 30% of people across India have been infected while positivity is somewhere in the range of 50% in large dense cities. “It is hard to say [if natural infection has come close to providing herd immunity] unless another more representative serosurvey is carried out,” Dr. Jameel says.
The only reasonable explanation for a drop in daily cases without any spike even after Durga pooja and Diwali is that besides other factors, the threshold for population immunity attributed to has been reached, inefficient testing strategy, better compliance to regulations or a combination of these, feels Dr. Babu.
“My only guess is that adherence to NPIs, although not ideal, was sufficient to slow down the spread and state governments did institute some reasonable measures to curb spread during festivals. Certainly, anecdotal evidence suggests that although crowds did build up in festival season, mask wearing was reasonably common,” says Dr. Menon. “The other possibility is that in regions which saw the largest crowds, as for example Kolkata's Durga Puja pandals, this was in the background of a large fraction of people with prior infections already, so this would have also hampered unrestricted spread.”
Explaining why daily cases in Kerala are still high while other States including Maharashtra, Tamil Nadu, Andhra Pradesh and Karnataka that had earlier reported high number of cases have seen a sharp drop, Dr. Menon says: “I would attribute this to the fact that the spread in Kerala started somewhat later than other regions and the fact that Kerala is fairly urbanised in comparison to most other states.”
Dr. Babu feels the sharp drop in Maharashtra and the three southern States is mostly because a reasonable proportion of people living in urban areas have been infected. He attributes the relatively high daily cases in Kerala to effective control of virus spread from the beginning. “This meant that a large proportion of people in Kerala remained uninfected in the initial stages. It is supported by the seroprevalence rate of 0.8% which is much lower than other States. It meant getting infected at some point,” Dr. Babu explains. “The tests done in Kerala are 2,08,100 per million, and is among the States with the best rates. It is because of this that the State is detecting more cases. Reporting more cases should not be seen as a problem.”
Though the number of cases appear to be dropping in the three southern States, the test positivity rate is still avove 5%. As on December 23 the cumulative test positivity rates in Karnataka, Tamil Nadu and Andhra Pradesh were 7%, 5.78% and 8%, respectively. “In addition to the numbers, testing the right persons (with symptoms), using the right test and improving the geographical coverage is important. Most tests are done in metros and tier1 cities with relatively less number in other areas. It is not efficient to increase tests in areas where the surge has already occurred in the past. This is a dynamic decision. Karnataka used the survey results to make these revisions in testing strategy for districts. Others states can follow similar strategy,” says Dr. Babu.
Dr. Babu attributes the over 180 cases reported in IIT Madras even as Tamil Nadu has reporting a steady decline in cases as an instance of an island of susceptible people in close contact settings and closed spaces serving as a potential cluster waiting to be triggered by introduction of infection.
“The solution to this is periodic testing at a density where you can pick infections early and isolate infected people. Many western universities have started doing this and various plans are available. At Ashoka University we have an elaborate plan to screen students as they come in for the coming semester,” says Dr. Jameel.