Before announcements started rolling in late last year about vaccines for COVID-19, public health experts in the West had been increasingly touting adoption of strategies similar to the “cluster busting” used to stem the tide in Japan and South Korea.
This involves the inclusion of so-called backward tracing, through examining contacts that someone had with other individuals before infection rather than using only the standard forward tracing post-infection. Such a strategy is based on the characterisation of COVID-19 as a “superspreader” type disease, with the now widely cited statistic that just 10 to 20 per cent of cases are estimated to result in 80 per cent of transmission.
As this pattern means a large number of infections is linked to a relatively small number of initial clusters in locations ranging from restaurants to meat-packing plants and ski resorts, it may be more beneficial to trace backwards as well to find the original source.
Dr Akira Endo in the Department of Infectious Disease Epidemiology at London School of Hygiene & Tropical Medicine (LSHTM): “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.” – Read the full interview of Akira Endo
Taking this approach is not just about tracking the R number – or COVID-19’s basic ability to reproduce – but also the k number, which measures its dispersion. This reflects the issue highlighted that use of the R number alone as a measure for the disease’s spread in the West hid its “stochasticity”, or unpredictability.
Indeed, modelling carried out by LSHTM estimated that combining backward with forward tracing could double or even triple the number of cases averted. And in countries where backward tracing has been used, it has been cited as playing a key role in keeping infections down – though numbers of people infected in Japan and South Korea have risen over the past couple of months.
Vaccines on the scene
Now that vaccines have begun to arrive on the scene much faster than originally expected, hopes have dramatically increased that the world can return to a “new normal” sooner.
Yet although the latest developments have shifted the focus more towards vaccine efforts, researchers say that doesn’t diminish the value of investigating backward-tracing strategies. This is especially the case given the complications and potential delays associated with the emergence of new variants of the virus, while uncertainties remain about how quickly and successfully the vaccines can be rolled out, and approval requirements they face in different countries.
Samuel Scarpino, assistant professor in the Network Science Institute at Northeastern University in Boston, Massachusetts, says that while progress in vaccine rollout may be faster in some places such as Western countries, it could still take a couple of years to get vaccines out to many parts of the world.
This is because of the potential time taken to manufacture enough doses and potential logistical challenges, particularly in lower-income countries with less-developed health systems. Prof. Scarpino pointed out, for example, that the Pfizer/BioNTech vaccine needs to be stored at -70 or -80 °C throughout its lifecycle, which could hugely complicate the delivery of doses in some countries – although it can also be refrigerated at temperatures of 2 to 8 °C for up to five days.
And even though figures from three of the vaccine candidates at the forefront of the push – AstraZeneca, Pfizer/BioNTech and Moderna – suggest that together enough doses could be ready for more than a third of the world’s population by the end of 2021, there are concerns that vaccines could take several years to reach some parts of the world if doses are not distributed equitably.
So while we wait for the logistics to be sorted out and vaccines to be delivered on a wide scale, Prof. Scarpino says backward-tracing methods can be brought in as a further tool to help plug the gap, suppressing flare-ups and reducing the risk of disruptive additional lockdowns, as well as potentially improving targeting of high-risk people for vaccine campaigns themselves.
Even if the vaccines are effective fast and allow Western countries to get to the position of Japan and South Korea of eight or nine months ago, said Prof. Scarpino, cluster-busting could be used to help further reduce lingering infections.
Samuel Scarpino, assistant professor in the Network Science Institute at Northeastern University in Boston, Massachusetts: “If we took vaccines like these and layered that with case investigation, cluster-busting and mask-wearing, we may be able to get back to a kind of newer normal by the summer in a lot of places. All of these things might make the difference between us globally getting back to a new normal some time in the next 12 months and us never getting there for years and years.” – Read the full interview of Samuel Scarpino
He believes the science is there for such tracing, but it also requires the political will and coordination that has been lacking between the US and Europe, with too much focus on the more predictable type of model for influenza even though COVID-19 is a very different kind of disease. “It’s doable with the same tech as in forward tracing, it’s just that the kinds of data that get stored would have to be different,” he added.
Prof. Scarpino said that to further enhance tracing strategies through surveillance in a more passive way, methods can be used such as wastewater sampling to gauge the prevalence of COVID-19 in a community – work that his team is currently carrying out. “Our hope is to leverage some tools from artificial intelligence and machine learning to work with more minimally processed data to find COVID,” he said.
Looking backward and forward
Dr Akira Endo in the Department of Infectious Disease Epidemiology at LSHTM said he had seen countries increase their interest in combining backward with forward 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,” he said.
This includes the UK, where recent reports highlighted moves being made towards incorporating backward-tracing methods, in addition to the traditional tracking of the NHS Test and Trace app.
But Dr Endo said there were challenges to rolling out backward tracing, which may be more resource-intensive due to needing to interview people about historical contact with other people, and also for technical and privacy reasons. Resurgences of COVID-19 in many Western countries have made this more difficult too, as he points out that this type of tracing is more effective when the number of infected people is low. However, he thinks it could be incorporated more once cases reduce again.
Dr Akira Endo: “Looking into these methods has 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.”
He adds that using such strategies may be useful for optimal targeting of high-risk people in vaccination programmes, such as the type of “ring vaccination” used in the recent Ebola outbreak in the Democratic Republic of the Congo – though it requires further assessment of whether this could also be applied to COVID-19.
Dr Endo said that where backwards tracing has been used so far, much of it has been manually based, but that it could be incorporated in apps in future. However, given the issues that apps have had with gaining traction in this pandemic, this may be more applicable in future disease outbreaks with similar patterns of spread.
And certainly, the lessons we learn now could be invaluable not just for COVID-19, but also for battling against both future and existing infectious diseases with similar patterns. Dr Endo, for instance, notes the lessons that Japan applied to the current pandemic from tackling tuberculosis.
For the future, added Prof. Scarpino, “there will be another respiratory virus that finds itself in a crowded market and we’re going to have to stop that from becoming a pandemic again. We’re going to need all this stuff for the next one.”
• A scientist’s opinion : Interview with Dr Akira Endo about clusters & superspreading
• A scientist’s opinion : Interview with Samuel Scarpino about clusters & superspreading