Wednesday, 1 July 2020

Contact Tracing: Shoe Leather or Apps?


by Les May

IF WE are going to eliminate the virus which causes Covid 19 the only way to do it is to break the chain of transmission from one person to another. One effective way of doing this is to interview every infected person to find out who they have been in contact with, trace all the people named, contact them and determine if they are showing signs of infection. If they are, they too must be interviewed to determine their contacts, and so on. If they are not showing signs of infection they would be advised to ‘self isolate’, a.k.a. ‘go into quarantine’, and monitored daily. If they show symptoms during this period the business of contact tracing must start all over again.

The process of investigating identified cases and tracing contacts is well shown on the website of the US Center for Disease Control website, but the basic procedure is applicable to any public health system.


A more detailed explanation is given at:


This is what I call the ‘shoe leather’ approach to contact tracing. It has both advantages and disadvantages. Its major advantages are that it is ‘low tech’, a pencil and notebook is all that is needed, proactive in the sense that it is a public health initiative and does not rely on the infected person to initiate it, and it is infinitely adaptable, because the contact tracer can prompt the interviewee if necessary.

Its disadvantages are, it requires trained people to carry out the interviews, hence it is expensive and difficult to implement if the number of infections is high, it is relatively slow, the infected person may not remember all their contacts or fail to mention, for example, that they stood at a bus stop with other people.

It is in order to mitigate these disadvantages that technological solutions to the problem of contact tracing have been proposed. These necessarily involve smartphones. At which point one disadvantage of this approach becomes apparent, not everyone owns, or want to own, a smartphone. Nor would every smartphone owner want to allow it to be used in this way. This is not fatal to the enterprise; it only requires that about 60-70% of a population can or will allow this.

One of the things which may make people reluctant to allow their smartphone to be used for contact tracing is a concern for their personal privacy. There are however a number of points which people who have these concerns might like to consider. ‘Shoe leather’ contact tracing also carries risks to personal privacy, remember the pencil and notebook. Owning and carrying a smartphone poses even greater privacy risks. Users may not switch off, or know how to switch off, the GPS location facility on their phone. Even if they do, smartphones regularly ‘ping’ nearby mobile phone masts.  Both these can be used to obtain location data for a mobile phone owner.   Both may be misused for surveillance of individuals, but locations are too crude for contact tracing.

So called contact tracing ‘apps’ make use of Bluetooth hardware which is available on most smartphones.  This signal is of much lower power which restricts the detection range to other smartphones in the immediate vicinity, hence the term ‘proximity tracing’.

Proximity tracing applications send to, and collect from, other smartphones in the vicinity very short ‘nonsense’ messages which act as an identifier of the phone. These are changed frequently to prevent tracking by a third party.  This exchange only happens if the phones, and hence their owners, are sufficiently close, say less than 2 metres, for a sufficient length of time.  This assessment is carried out by the ‘app’, not the phone owner, and the identifier of the nearby phone is then logged. It is in measuring the strength of the Bluetooth signal and hence estimating the distance between phones and their owners, that ‘apps’ seem to run into trouble.

At this point all the logged data is on the user’s phone.  To be useful in alerting other smartphone users that they have been close enough, for long enough, to an infected individual to be considered a contact, some means must be found for bringing all logged identifiers together and alerting potential contacts. It is at this point that the potential for surveillance comes into play again.

The extreme surveillance by the state in China is well known, but other countries have implemented systems where there is a high risk of exposure to surveillance. The TraceTogether application used in Singapore requires users to share their contacts information with the authority which keeps a database that links identifiers to contact information. When a user tests positive, their phone sends all the identifiers it has logged over the past two weeks. The authority looks up the identifiers in its database, and contacts by phone or e-mail the people who may have been exposed to infection. This places a lot of information in the hands of the government.

For the potential for misuse of centralised information see:


For the potential misuse in some other countries see;


Apple and Google’s proposal is a more decentralised system which uses a database accessible to the public.  When a user tests positive, they can upload their private identifiers to that public database.  The database can be hosted by a health authority or on a peer-to-peer network; as long as everyone can access it, the contact tracing system functions effectively.  Peer to peer networks do not have a centralised server.  All users are equally privileged.

How the decentralised system works is here:



If I used a smartphone I would probably find this system acceptable from the privacy point of view if the code were ‘open source’ which would allow thousands of pairs of eyes all over the world to check it for potential privacy violations.

So far as I can ascertain the system which was proposed initially would be used in the UK was in the second rank so far as privacy is concerned; better than many, but still leaving something to be desired.  Now it seems to have been dropped altogether in favour of one using the applications interface (appi) proposed by Apple and Google, but will it be ‘open source’?


The downside of relying on technology to alert us to something that has already happened to us is that we will be lulled into a false sense of security about our present behaviour.  Rather like the man who fell off the Empire State Building and as he passed each window shouted “So far so good”.  Meeting as few people outside our own household and keeping at least two metres away from those we do meet is still the best way of reducing transmission and eliminating the virus. 

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