Why contact tracing is important

Tracking down close contacts of an infected person is vital to containing Covid-19.

Contact tracing, says scientist Shaun Hendy, is ‘‘our biggest weapon right now’’ against coronavirus.

It’s a phrase that has sprung instantly into the national lexicon – but what exactly is contact tracing, how does it work, and why is it important?

Contact tracing isn’t in Auckland University physics professor Hendy’s specialism – but he does understand how it works, primarily from being involved in the Ministry of Primary Industry’s work on tracing the M bovis cattle infection. You can’t, he notes, interview a cow about its movements – but you can ask a farmer where it’s been. The principles are the same.

How it works

Contact tracing means tracking down every person who has been in close contact with someone who is infected, or potentially infected. Those people can then be asked to self-isolate to contain the spread.

Until border security was increased and new arrivals asked to self-isolate for 14 days, people flying into New Zealand were able to resume normal life – so tracing their movements establishes who they may have passed the virus on to.

A Ministry of Health official will sit down for an intensive, one-onone interview with the infected person, and trace their movements back a fortnight – starting with the plane journey. The airline can provide contact details for passengers who sat within two rows of the infected person. Then they will recreate their diary from there, paying attention to any meetings lasting more than 15 minutes in a crowded room.

What the ministry is looking for is what it calls ‘‘close’’ contacts: anyone who has shared the same house during this time; spent two hours or more in the same room, bus or train; sat within two rows on a plane; or been face to face within 2 metres for more than 15 minutes ‘‘in any other setting’’. Anything less than this is a ‘‘casual’’ contact and at less risk.

Hendy understands the ministry is looking at rapidly growing its contact-tracing workforce and has been consulting MPI after its recent experience of working on two Queensland fruit fly incursions and the M bovis outbreak.

How hard the tracing work is depends on the movements of the person. What we can all do to help is start keeping diaries. If we get infected, handing over a detailed log of our movements and people we’ve met – and their contact details – it will save a lot of work.

What if you walked past an infected person?

‘‘It’s a much lower risk,’’ says Hendy. ‘‘The evidence seems to be that it’s [more common] from prolonged close contact. It’s droplet spread, so if someone is coughing, they could certainly put other people at risk, which is why even people who have got a cold it is worth staying home . . .

‘‘But generally public places, especially those which aren’t super-busy, the risk is relatively low. A park is low risk – a crowded train might be high risk.’’

Why don’t officials simply name the infected people?

Hendy says the harassment of some of the early victims of coronavirus in New Zealand illustrates the need to protect their privacy. ‘‘If people feel ashamed, they might conceal their symptoms or their movements – which is counterproductive. We want them to share that information freely: so there is a fine line to walk.’’

Instead, you may see a general appeal from the ministry to those who were in a specific cafe at a particular day or time to come forward.

Why does contact tracing matter right now?

All current cases in New Zealand can be tracked to a plane journey, so contact tracing can be a powerful tool. Once we begin to see ‘‘community transmission’’, where the infection cannot be traced to an international traveller, then Hendy says other measures will come into play.

This could be relatively soon. ‘‘Every day we don’t have that is a good day. But we have got to be prepared for the possibility we might see community transmission and, at that point, tactics change.’’ They could include closing schools and universities and asking people to work from home.

Hendy says the countries that have operated contact tracing best tend to be the Asian nations who developed sophisticated systems because of the 2003 Sars outbreak. They also have a population whose experience with a disease outbreak has led them to understand and cooperate, and also demand that the state runs effective response systems. The mainly Western countries who were unaffected by Sars, have ‘‘erred on the side of privacy’’.

What comes next?

Once community transmission takes hold, contact tracing becomes less effective – but there is still a similar tool that can be used.

The South Koreans have used cellphone network data to track people’s likely contacts. ‘‘From a privacy perspective, that is pretty invasive, so it is a method of last resort. But it can be effective,’’ Hendy says.

That sort of ‘‘big data’’ can be used to build up a more general picture which would help officials decide specific suburbs or demographic groups they ought to be targeting for testing.

For example, during the operation to track the movement of invasive Queensland fruit fly in the Auckland suburb of Grey Lynn in 2015, cellphone data was used, not to identify individuals, but to establish typical movement patterns to see which neighbouring suburbs might need spraying with insecticide.

Hendy says Taiwan and Singapore have used this method to effectively slow down the transmission of coronavirus.

How do you measure success?

With contact tracing for the M bovis outbreak, the infection curve has been flattened, proving its efficacy. ‘‘Flattening the curve’’ is about trying to space the demand for medical treatment so it does not overwhelm the system. ‘‘This is buying us time. We need to keep that curve down for as long as we can – it helps slow the spread, which is what we have to do. While we can stay in this contact tracing phase, this is our best weapon.’’

Again, says Hendy, it’s the Asian nations who have had the best success with contact tracing – whereas the US and Europe were quickly overwhelmed. So far, he says, we’ve been lucky – and contact tracing seems to be helping us ride that luck.