Both in the first and second wave, an increase in daily reported cases was first reported in Maharashtra, followed by Kerala and Delhi. These could be due to several reasons including greater integrity in testing and reporting data. Can a sharp and sustained increase in daily cases in these States serve as an early warning of an impending third wave? Dr. Tarun Bhatnagar, Senior Scientist at Chennai’s National Institute of Epidemiology, Dr. Gautam Menon, Professor of Physics and Biology at Ashoka University and Dr. Giridhara Babu, epidemiologist at the Public Health Foundation of India, Bengaluru explain the advantages and shortcomings of such an approach.
Considering that Kerala and Maharashtra have shown high integrity in testing and reporting cases, should a sustained spike in daily cases in these two States serve as a bellwether of a third wave?
Gautam Menon: We should not make the mistake of assuming that because there is more light under the lamppost, what we are searching for can be found there.
Given that these States have traditionally recorded well, it would be good to pay attention to them, but in a large and diverse country, a new variant of concern could emerge anywhere. We need to beef up testing and surveillance across India and not just in those States. It would be best if we looked for an unusual increase in cases, novel symptomatology and new variants across India, paying special attention to large and crowded cities, and not just to Kerala and Maharashtra. For example, Bengal is potentially another hotspot for a new wave to emerge, given the diversity of sequences obtained from there and Kolkata, its capital, is dense enough for a new variant to spread fast.
Will future spikes be first seen in Mumbai and Delhi or will tier-2, tier-3 cities serve as a better indicator of the beginning of the third wave?
Tarun Bhatnagar: New surge in cases would depend on the magnitude of susceptible population, population density, mobility and implementation of public health surveillance measures including testing and contact tracing. Duration of persistence of protective levels of immunity is another important parameter for the timing of another surge in cases. Published data indicates this to be for 7-12 months. Another critical factor is the distribution of variants with high transmissibility or ability for immune escape. Information on all these factors is needed to say where the third wave could be documented.
- In a large and diverse country, a new variant of concern could emerge anywhere.
- New surges can arise depending on magnitude of susceptible population, population density, mobility and implementation of surveillance measures.
- Duration of persistence of protective levels of immunity is another important parameter for the timing of another surge.
- Future surges in cases are expected in areas with low seraprevalence coupled with high population density and mobility.
- Tier-2 and tier-3 cities with relatively low case reporting during the earlier waves are more likely to be affected in the future.
GM: The Delta variant that appears to have been responsible for the initiation of the second wave across the west of the country originated in tier-2 and tier-3 cities of Maharashtra, spreading only later to Mumbai. Ideally, we should be sensitive to an anomalous rise in cases at the level of districts, since a new variant that has increased immune escape attributes could emerge anywhere.
Will the high seroprevalence recorded in Mumbai and Delhi diminish its ability to serve as a bellwether of the third wave?
TB: Future surge in cases is expected in areas with low seroprevalence coupled with high population density and mobility. This could happen even in localities within Mumbai and Delhi depending on the representativeness of the serosurveys done in these cities. However, tier-2 and tier-3 cities with relatively low case reporting during the earlier waves are more likely to be affected in the future. However, surveillance is key to early detection of such trends. Vaccination coverage and emergence and distribution of variants that are more transmissible or can evade immune response would also influence a fresh surge of cases.
Giridhara Babu: With increasing vaccination coverage and decline in the vulnerable population, any wave higher than earlier in these two cities might mean a higher reinfection rate and/or waning immune response. But yes, if these two cities have large outbreaks, the rest of the country will follow a similar trajectory as in previous two waves.
What could be the reasons for daily cases plateauing in Kerala?
GB: There might be two reasons, namely better testing strategy and transmission dynamics. One, the detection in Kerala is better than in other States. Although even Kerala can also do better in terms of the number of tests, their testing strategy is good. The districtwise and rural distribution of testing is higher and well distributed compared to other States. Therefore, they will continue to detect cases. The absence of detection in other rural hinterlands in India does not mean that there is no circulation. Two, it is possible that more spread might have occurred within primary and secondary contacts due to the contagious nature of the virus, poor isolation and quarantine efforts, resulting in a propagated outbreak in Kerala.
GM: There are no surprises in Kerala vis-a-vis the rest of the country in terms of new variants that are not seen elsewhere. My tentative guess is that despite Kerala's large numbers of cases, the multiplier between cases recorded and background unrecorded cases may be smaller than in the rest of the country. This means a reservoir of those who are still to be infected, an idea supported by the relatively low seropositivity found there, of about 10%, in a survey in March, a surprisingly low number. So the plateauing may simply be a network effect — those most at risk are likely infected already, but many are not and have been able to shield themselves so far, likely due to the stringent measures undertaken by government and precautions at the personal level. They are slowly getting infected, contributing to the cases, but not at the pace seen in the second wave, due to the restrictions in place.