After a long and painful month-and-a-half, confirmed cases of COVID-19 in India have been declining steadily for more than a month. Deaths have started to decrease as well. However, it is a long way down from an unprecedented peak of more than 400,000 daily cases, and the suffering will continue for weeks and months to come. It does, however, appear that the second wave of the novel coronavirus is abating.
‘R’ and determinants
A simple epidemiologic concept can be used to better understand the second wave and help plan for a potential third wave. The reproduction number — often referred as R — is the average number of new infections arising from one infected individual. R fluctuates over time during an epidemic. When R is greater than 1, infected individuals infect more than one person on average and we observe increasing cases. When it is less than 1, cases are declining. It is not a perfect statistic, especially when cases are low, but it does provide helpful insights into how an epidemic is changing.
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What led R to increase earlier this year resulting in a second wave? R depends on four factors, summarised by the acronym DOTS: the Duration a person is infectious; Opportunities infected individuals have to spread infection to others; the probability Transmission occurs given an opportunity, and the average Susceptibility of a population or subpopulation. Because each factor is required for increasing cases, reducing any of them to 0 would extinguish an epidemic. This is not practical right now anywhere in the world. However, we should work toward decreasing these factors such that R remains as low as possible. It is also critical to consider the effect new variants have had on each of these four factors.
Susceptibility, opportunities
Let us start with S — the proportion of the population susceptible to infection. Susceptible individuals lack immunity derived through prior infection or immunisation. Results from a national seroprevalence survey done in December 2020 and January 2021 indicate that roughly 25% of the population had antibodies (https://bit.ly/3v0WSWY) to the virus that causes COVID-19. Estimates were slightly different depending on geography. And some surveys showed substantially higher exposure to the virus. Nevertheless, there was still a substantial susceptible population in most parts of the country at the beginning of 2021. Susceptibility can be reduced through immunisation. By the end-March, however, less than 1% of the total population had received two doses of the vaccine. Taken together, the right conditions were set for a potential second wave at the beginning of 2021.
The next factor is the number of opportunities for transmission — or O in the DOTS. By January, there was a sense that India had made it through the worst of COVID-19. Many people were eager to get back to life and work, especially after a very challenging 2020. Social distancing had reduced and markets filled again with people. One salient characteristic of COVID-19 is that the disease is driven largely by superspreading, where many individuals are infected by a small number of individuals. Colleagues and I showed in research from Tamil Nadu and Andhra Pradesh (https://bit.ly/2T7Dpa9) that only 5% of infected individuals accounted for roughly 80% of all secondary infections. With this in mind, increased social mixing and large gatherings that took place in early 2021 also might have helped facilitate a second wave.
Transmission, duration
This brings us to T, or the probability of transmission. Not taking proper precautions can lead to increased transmission. There are no good national statistics on the proportion of people adhering to preventative measures. However, during my travels throughout India for work in early 2021, it was clear that few people were wearing masks while in public. In addition, new variants that are more transmissible have emerged. One new variant called B.1.617.2, or more recently known as the delta variant, is known to be much more transmissible — potentially twice as much (https://bit.ly/3pxqD0O) — than those circulating in 2020. This is demonstrated by the fact that it is the dominant variant in India and has emerged as the dominant variant in the United Kingdom according to data from there.
Finally, the last factor in the DOTS equation is the duration of infectiousness or D. Emerging evidence (https://bit.ly/3fZufpa) suggests that the duration of infectiousness could be slightly longer with some new variants. More research is needed to confirm this. However, this could help explain why some variants are outcompeting others and could have contributed to the increase in R in India earlier this year.
A third wave?
What does this mean for a potential third wave? First, we need well-designed seroprevalence surveys to understand how much of the population remains susceptible and where they reside. The Government has planned a seroprevalence study in June in the same 70 districts where the first three rounds were conducted. There also remain questions about waning immunity and the potential for reinfections, which would affect how we calculate the proportion of the population that is susceptible. The new variants also complicate this equation, as they are able to partially evade immunity developed through infection or immunisation.
Despite the need for more data, based on the existing evidence and out of an abundance of caution, we should anticipate that there could be a potential third wave.
Luckily, DOTS provides us with a framework for preventing or mitigating a third wave. We need to drive down the factors that contribute to R wherever possible. And we need to work even harder to do this, because the new variants have skewed the equation such that R can more easily be pushed to be greater than 1. Some regions have implemented lockdowns, which substantially reduce opportunities for transmission. These are temporary solutions and should be used to focus on slowing transmission and scaling up other interventions. Mass gatherings have also largely stopped, which should help reduce opportunities for transmission. This is welcome news and should continue after the second wave. We can reduce the susceptible population by substantially increasing immunisation coverage. Currently only 3% of the population has received both doses. The Government is working hard to procure additional doses that are desperately needed.
Mask use, ventilation
Transmission can be reduced through increased use of face masks and improved ventilation. Research from neighbouring Bangladesh (https://bit.ly/3vVDBrr) indicates that providing free masks together with community monitors can help improve adoption. Last, if the duration of infectiousness is indeed longer, isolation and quarantining guidelines should be revisited to minimise potential exposure to others.
The emergence of new variants means we need to take these interventions even more seriously. Connecting the DOTS, though, can help mitigate a third wave and the tremendous pain and suffering that have become all too common in recent weeks.
Brian Wahl, PhD, MPH, is an epidemiologist and faculty member at the Johns Hopkins Bloomberg School of Public Health and a member of the Johns Hopkins India Institute COVID-19 Response Task Force. He has worked on public health issues in India for more than a decade
