Interview with Dr Akira Endo in the Department of Infectious Disease Epidemiology at the London School of Hygiene and Tropical Medicine.
Is it still beneficial to look into combining backward with forward contact tracing, as you describe in your recent paper, following the recent developments with COVID vaccines?
It’s very good news about the vaccines, though there’s still a need to make sure their estimated effectiveness is actually the case. But at the moment, it doesn’t invalidate the importance of backward tracing straight away, because even if there are vaccines, it takes a lot of time to disseminate them to the population until coverage exceeds a certain threshold for herd immunity. And if people are concerned about risks, that will be more difficult.
So in that time, we can combine these methods to suppress the infection, making vaccines, contact tracing and other measures complementary with each other.
One possibility with this would also be arranging some sort of “ring vaccination”– targeting vaccination to high-risk people, especially those who are in contact with cases. That was useful in the recent Ebola outbreak [in the Democratic Republic of the Congo]. However, it requires further assessment of whether ring vaccination could also be effective for COVID-19 because success in part relies on how quickly vaccines can induce protective immunity.
Looking into these methods has also been a good learning point for public health. In our study, we saw the relationship between the effectiveness of backward tracing and superspreading – or overdispersion – so I believe the information will be used in future outbreaks with similarly high overdispersion patterns.
Can this idea of combining the types of tracing, looking at both the R and k numbers, really take off in Europe during this pandemic?
The problem recently is that many European countries have had lots of cases, so the public health capacity to implement that kind of intense backward tracing has been lost. Contact tracing is basically most effective when the number of cases is low. If the number of cases goes very high, then even if you implement intense contact tracing, it would consist of only a small proportion of all cases – meaning the reproduction number will not reduce sufficiently.
Once the number gets low again, backward tracing could work as an alternative strategy to lockdowns – which have a large amount of detrimental effects – while vaccine programmes are being rolled out. More countries are now trying to design and prepare such tracing. Half a year ago, many public health guidelines for contact tracing only focused on forward tracing, but now many are changing so they can also focus on backward tracing.
What are the other challenges in implementing this kind of strategy, and are there technologies that can improve its success?
Backward tracing is kind of resource-intensive. Currently, much of this [in countries where it runs] is manually based: public health officials get in contact with cases and potential source cases, and interview them. That’s the main reason it cannot be rolled out easily and it’s limited to situations where the number of cases is low.
There are also technical and privacy issues, and if you want to use it for a wider range of countries there’s some cap on resources. However, with the development in ideas, countries could consider incorporating some backward-tracing functions into apps – but it may take some more development and discussion of whether it’s ethical or feasible within its functions.
Maybe an easier step before it’s implemented in an app is standardised electronic survey forms, instead of making phone calls to interview cases.
For an app, given the difficulties and that we’re still learning many intervention strategies, it may be more about preparation for the next pandemic rather than this one. It will be better if we can use backward tracing from the outset rather than developing it during an outbreak.