The Limitations of Contact Tracing: A Research Article

The whole is well worth reading. Here is an extract.

 http://www.sfu.ca/magpie/blog/fundamental-limitations-of-contact-tracing-for-covid-19.html

The fact that there is a critical value of the contact tracing delay beyond which contact tracing is not able to prevent a sufficient fraction of cases to bring COVID-19 under control  is an example of a tipping point: a value of a parameter where a system has qualitatively  different behaviour when the parameter is above or below it. If the delay τ is above its critical value, we have exponential growth in the number of cases; otherwise cases decline. There is another tipping point  for the coverage; increasing coverage (for example by expanding the definition of a contact to include more people, taking extra measures to insure compliance with self-isolation) could push R below 1 if τ is sufficiently short and other measures are in place. 

Now that vaccines are approved and are being widely deployed, and vaccinated populations are moving to reopen economic and social activities, testing and contact tracing will face new challenges. First, people will have a lot more contacts when they are working in person, socializing indoors and travelling more: effectively we are increasing the reproductive number back up towards pre-pandemic levels. 

Vaccination will counteract this rise, preventing a large fraction of infections and providing good protection against symptomatic disease, as infections in vaccinated individuals appear to be more often asymptomatic. The relative transmissibility per unit time is not known, but asymptomatic individuals will not know that they are infectious unless they are tested, so they will likely circulate (and have contacts) for longer times than symptomatic individuals have done under social distancing during the pandemic. This poses challenges for identifying an individual’s infector, though it does mean that if an individual is identified early in their infection, there is an opportunity to prevent a high portion of their onward transmission. It poses challenges for testing, as the contacts of a case may also be asymptomatic with higher probability due to vaccination.  This means that in order for contact tracing to succeed it may need to rely more on testing asymptomatic contacts, and its effectiveness may be undermined as we reopen by large numbers of potential contacts, reduced interest in testing, and assumptions that vaccinated individuals are not infected or infectious.  Contact tracing needs to be reassessed and adjusted once we know more about COVID-19 transmission in a largely immunized population.