At the start of the month, I argued in my piece that the fastest scalable solution to India’s Covid-19 challenge was to employ digital technology for diagnosis and for contact tracing. Happily since then the Narendra Modi government has launched the Aarogya Setu app which does both, and the number of downloads has been climbing rapidly.
Now, I would like to spell out the logic of contact tracing and explain why one should think of digital contact tracing as conferring a new form of immunity – digital immunity.
Understanding the spread
Epidemiology 101. The simplest understanding of how Covid-19 spreads: each infected person in turn infects a number of others. An average value of this number is the infamous R0 – the basic reproduction number. For Covid-19, R0 appears to be roughly 3 in the course of life as it was prior to the pandemic. Over 13 rounds of infections, this would be expected to turn one starting case (“patient zero”) into 3, 9, 27…..and eventually 3^13 cases which is roughly 1.5 million cases.
As each new infection has to develop (“incubate”) for roughly a week before the patient can infect others, one should expect that over the course of 13 weeks since 1 January, we would be roughly where we are today worldwide – with about a million-and-a-half cases.
Currently, we are engaged in a vast, global exercise of reducing R0 by lowering contacts in daily life, better known as social distancing. If you don’t get physically close enough to others, you cannot give them the disease if you have it or contract it if they have it.
Limitations of lockdowns
As a thought experiment, perfect social distancing over a month should eliminate the disease – existing cases will reach their conclusions and no fresh ones will begin. But this thought experiment cannot be truly realised as it is too tough to implement. And so even in locked down geographies, such as India and New Jersey where I live, we are looking at restarting a less constrained life. This will lead to a situation in which some unlucky people will restart fresh chains of infection.
We would be spared this outcome if, magically, a vaccine were to become available today. The vaccine would not have to work for every single person nor would everyone need to be vaccinated. It would be sufficient for a bit more than two thirds of the population to become immune as a result of the vaccine, for at this point, less than one third would still be susceptible to the disease.
Each new “patient zero” would only pass on the disease, on average, to less than a third of R0 before vaccination, which is to say less than one, ensuring that the number of infections could no longer grow. We would have developed “herd immunity”.
Power of smartphones
This particular magic is not possible, but another trick very much lies in our hands – literally – is the ubiquitous smartphone. The smartphone can be used to develop, almost immediately, a new form of herd immunity – digital herd immunity or simply digital immunity. This may seem odd – a smartphone cannot make a given person immune to Covid-19, unlike a vaccination shot. But it can make the herd immune to developing an epidemic in precisely the same sense in which vaccination does.
The way this works is to turbo-charge the venerable public health measures of identifying exposed people who can then take measures to avoid infecting others while seeking treatment and rest for themselves. Contact tracing followed by quarantine in the jargon of the trade.
To see how this can be highly effective for Covid-19, consider a very simplified description of the course of the illness.
People who contract the disease will become contagious after 5 days. Of these (pessimistically) half will not develop any symptoms till they recover at the end of two weeks. The other half will develop symptoms after three days. Now in our thought experiment, a new patient zero who develops symptoms immediately self-isolates and is tested/treated. They are no longer a danger to the rest of the herd.
We are also quickly able to trace everyone who had problematic contact with the new patient zero over the past three days when they were contagious. And these people, who have not yet had a chance to infect others, also self-isolate, get tested and depending on the result resume normal life or get treated for the disease. They too are no longer a danger to the rest of the herd.
That leaves patient zeros without symptoms. They will, indeed, pass the disease on to 3 others, say. But of these, very likely, at least one will show symptoms. Once that person is identified, we work backwards and test their contacts and find patient zero. Then forwards to find others who had contact with patient zero. Isolate/test/treat – and we are done.
Bottom line: With accurate and quick contact tracing, patient zero cannot seed a new epidemic, exactly as if the population were vaccinated. Indeed, the analogy is even better. Not all contacts need to be traced. For parameters appropriate to Covid-19, me and my team estimate using a simple ‘branching tree model’ which suggests that as long as about 90 per cent of the contacts can be traced, no epidemic can grow. The herd has immunity.
My team that is working with me on the ‘branching tree model’ includes Vir Bulchandani, Saumya Shivam and Sanjay Moudgalya (all three are graduate students in Theoretical Physics at Berkeley, Princeton and Princeton respectively). I am also being helped by Shoibal Chakravarty, Fellow at the Ashoka Trust for Research in Ecology and the Environment, Bengaluru.
Digital solution: the way forward
Now for the digital part. As already shown in practice in various East Asian countries, smartphones can run apps that allow all of us to record the times and places where we had potentially problematic contact with others – periods when we were closer than 6 feet for more than 10 minutes for example. More precisely, they can record this information for all other people who also install and run the app – members of this mutual assistance society. If the membership of this mutual assistance society exceeds the digital herd immunity threshold – 90 per cent in our scenario – the herd now has digital immunity.
Getting to the digital immunity threshold in India involves two challenges. The first is to encourage widespread adoption of the Arogya Setu app among smartphone users.
The second is to include feature phone users in the same contact tracing system. The most immediate way to do this is by combining cell tower based location and time information, old fashioned interviews and broadcasting SMS warnings.
A more discriminate solution would be to distribute robust and inexpensive devices which have been adapted to only run the needed app with automatic access to mobile phone networks. These could, for example, be old donated smartphones. With some effort on all three axes, the problem can be solved.
Biological immunity to Covid-19 will take some time, but digital immunity is possible today.
The author is Professor of Physics at Princeton University and is working on India’s coronavirus challenges with a group of students, postdocs and faculty. Views are personal.
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