Transmission dynamics of the COVID-19 epidemic in India, and evaluating the impact of asymptomatic carriers and role of expanded testing in the lockdown exit strategy: a modelling approach

Gupta, M.; Mohanta, S. S.; Rao, A.; Parameswaran, G. G.; Agarwal, M.; Arora, M.; Mazumder, A.; Lohiya, A.; Behera, P.; Bansal, A.; Kumar, R.; Meena, V. P.; Tiwari, P.; Mohan, A.; Bhatnagar, S.

Background: The coronavirus disease 2019 (COVID-19) has caused over 3 200 000 cases and 230 000 deaths as on 2 May 2020, and has quickly become an unprecedented global health threat. India, with its unique challenges in fighting this pandemic, imposed one of the worlds strictest and largest population-wide lockdown on 25 March 2020. Here, we estimated key epidemiological parameters and evaluated the effect of control measures on the COVID-19 epidemic in India and its states. Through a modeling approach that accounted for asymptomatics, we assessed the impact of lockdown relaxation and increased testing. Methods: We estimated the basic reproduction number and effective reproduction number at a national and state level in India after adjusting for imported cases and reporting lag using established statistical methods, using time-series data from 4 March to 25 April 2020. Using a dynamic SEIR-QDPA model fitted to data from India, we forecasted the size and temporality of the ongoing first wave while accounting for the interventions in place. We used the model to simulate lockdown relaxation under various scenarios to evaluate its effect on the size and temporality of the second wave. We also evaluated the feasibility of increased testing as a containment strategy after restrictions are relaxed and its impact on the epidemic size and resumption of socio-economic activities, while taking into account the changes in transmission dynamics brought about by asymptomatic carriers. Findings: The median delay from symptom onset to detection (reporting lag) was estimated to be 2{middle dot}68 days (95% CI 2{middle dot}00-3{middle dot}00) with an IQR of 2{middle dot}03 days (95% CI 1{middle dot}00-3{middle dot}00). The R0 for India was estimated to be 2{middle dot}083 (95% CI 2{middle dot}044-2{middle dot}122 ; R2 = 0{middle dot}972), while the Rt gradually down trended from 1{middle dot}665 (95%CI 1{middle dot}539-1{middle dot}789) on 30 March to 1{middle dot}159 (95% CI 1{middle dot}128-1{middle dot}189) on 21 April. 60{middle dot}7% of confirmed COVID-19 cases in our sample were found to be asymptomatic. We observed that delaying the lockdown relaxation increases the time to new rise in active cases after the relaxation in a linear fashion. If lockdown was reintroduced after a fixed relaxation period, the magnitude of the second peak could be reduced by delaying the relaxation and decreasing the duration of relaxation. These benefits were greater in case of a gradual relaxation as compared to a sudden lifting of lockdown. We found that detecting a higher proportion of cases through testing significantly decreases the total infections. This positive impact of testing progressively increased at higher transmission rates when restrictions were relaxed. We found that similar containment targets could be achieved by both, a combination of high testing and less social restrictions, and a combination of lower testing with intensive social distancing. Interpretation: The nationwide social distancing interventions in India since 25 March have reduced the effective transmission levels, though sub-threshold Rt remains to be achieved. If lockdown is to be extended, additional benefits for mitigating the second wave can be achieved if it is extended farther after the peak of active cases has passed. Intensive social distancing is inherently enough to contain the epidemic, however, testing will play a pivotal role in the lockdown exit strategy by impeding the epidemic growth enough to allow for a greater resumption of socio-economic activities, thus minimizing the social and economic fallout resulting from severe restrictions. Considering that asymptomatics play an undeniable role in the transmission of COVID-19, dependence on presence of symptoms for control strategies, behavioural changes and testing should be reduced.